LX470 AHC issues - pressure sensor 1998 Vs. 2002 (1 Viewer)

[suprarx7nut]

2001LC

Thank you for your reply and experience with this situation. Here is a full background of what I have done and maybe after reading this, something will trigger someone's memory from the ol Experience file that gets filled as we all get older!

replaced all 4 globes with new 6 months ago and a NEW accumulator due to a sheared bleed nipple, 3 months ago! Then I replaced a NEW pump assembly ( due to some failure not plugged screens or anything I came across on this forum). I started this journey because I wanted to replace ALL the strut bushings. So, I guess you could say I have a COMPLETE ...BRAND NEW AHC system. I did Run it dry when refilling and regret that, however....... it was working fine before I started this project. Now I cannot get the ACTIVE TEST to work after jumping the Diagnostic Tc -? (can't remember) in the pass. side diagnostic by the pump after pressing the DOWN button 5 times within 5 seconds of start-up. It doesn't go into 1762 fault mode until I press the UP switch. I am NOT getting the spec V of 1.3-1.8V from the PS, .. and that's the only parameter ( of 4 to test) that is NOT IN SPEC to the TSM.
My question for SUPRARX7nut, 2001LC, or anyone that is reading this ... Did anyone verify this Press Sen voltage when they experienced this problem with not getting the ACTIVE TEST to run?? I am waiting for another new PS from IMPEX of Japan and should have it this coming week. Until then, PLEASE feel free to list what wires or connectors or components, I need to check out.

I've never seen a confirmed bad pressure switch. Ever. I have helped countless people work through a persistent 1762 code. Every single time the solution is bleeding. There's air or gas in the system. That's it. Regardless of voltage at the pressure switch, I've never seen that actually be a problem.

I have also seen 1762 be triggered by other codes so if there's anything else active chase after all other codes first.

I've done a LOT of electrical troubleshooting on 90's Toyotas. Hundreds of hours across more vehicles than I can count. I've seen many instances where the EWD calls for a precise voltage and the sensor or pump in question is outside of spec, but works perfectly fine. I've seen it on brand new fresh out of the box components too. I'd take that only as a minor data point - certainly not a definitive pass/fail criteria.

My advice is to ignore the pressure switch and its wiring. The troublesome wiring in AHC is the section that runs along the frame on the driver's side near the rear wheel. That contains wiring for many components, but I don't believe it contains the pressure sensor circuit.

I don't believe the pump is fragile. It's a simple gear pump that typically runs for multiple decades of near constant use. One I'm pretty sure you just have air trapped. If you have air trapped in the system downstream of the pump, you can get 1762. It's an overly sensitive fail-safe built into the system that makes purging gas in the lines quite annoying.

 

[2001LC]

I agree with Andy (@suprarx7nut) advice. Look elsewhere!

You replaced the pump "due to some failure". What failure?

Have you read pressures. What were the FL, FR , Rear and Accumulator pressures/Mpa?, when working?
Would you have a screen short of AHC data stream?

Both Andy and I, have both found shorts (busted wires) in the wires under the body (undercarriage).

II see I used 2000 LX in above post #19, so I removed that page. As it has a different PS sensor. I pulled up the 2002LX C1762, DI-276.
#4 Check AHC relay.
#5Check for open and short circuit in harness---- , This is before #6 checking pressure sensor!

 

[suprasvobodea]

I've never seen a confirmed bad pressure switch. Ever. I have helped countless people work through a persistent 1762 code. Every single time the solution is bleeding. There's air or gas in the system. That's it. Regardless of voltage at the pressure switch, I've never seen that actually be a problem.

I have also seen 1762 be triggered by other codes so if there's anything else active chase after all other codes first.

I've done a LOT of electrical troubleshooting on 90's Toyotas. Hundreds of hours across more vehicles than I can count. I've seen many instances where the EWD calls for a precise voltage and the sensor or pump in question is outside of spec, but works perfectly fine. I've seen it on brand new fresh out of the box components too. I'd take that only as a minor data point - certainly not a definitive pass/fail criteria.

My advice is to ignore the pressure switch and its wiring. The troublesome wiring in AHC is the section that runs along the frame on the driver's side near the rear wheel. That contains wiring for many components, but I don't believe it contains the pressure sensor circuit.

I don't believe the pump is fragile. It's a simple gear pump that typically runs for multiple decades of near constant use. One I'm pretty sure you just have air trapped. If you have air trapped in the system downstream of the pump, you can get 1762. It's an overly sensitive fail-safe built into the system that makes purging gas in the lines quite annoying.

I agree with Andy (@suprarx7nut) advice. Look elsewhere!

You replaced the pump "due to some failure". What failure?

Have you read pressures. What were the FL, FR , Rear and Accumulator pressures/Mpa?, when working?
Would you have a screen short of AHC data stream?

Both Andy and I, have both found shorts (busted wires) in the wires under the body (undercarriage).

II see I used 2000 LX in above post #19, so I removed that page. As it has a different PS sensor. I pulled up the 2002LX C1762, DI-276.
#4 Check AHC relay.
#5Check for open and short circuit in harness---- , This is before #6 checking pressure sensor!

Gentleman,

The pump didn't "fail" per se, it was just 20 years old, took 20+ secs to go up from N-H, and after I refilled it( after replacing the strut bushing and anti-sway bar bushing &linkages, something happened and it was making a lot of loud, grinding noise and finally froze up ( It would NOT move when I applied 12V directly ) so I replaced it with a new one. I didn't have Techstream working on my laptop 4 months ago when I started to remove the struts/accumulator so I don't know the before pressures - unfortunately, but I have Techstream installed and working now! However, there is NO pressure in the system now because I cannot get the ACTIVE test to work!! That's "the Chicken-Egg dilemma"!! I cannot get pressure on the accumulator, to force oil to it or the globes to bleed them!!! This is my problem!............

I checked both the main relay( under the hood) and the motor relay by the AHC ECU under the steering wheel and they both function when the voltage is applied. There is also 5 V at the 1st or 2nd step/checkpoint for 1762 code diagnosis.

But don't know about a #5 _check for open and short in the harness. What is the process to do that .......specifically? If you guys are saying, "It is most likely the harness" then I want to check that but don't know which wires or where to start this, I have both an auto text light probe which can measure volts, and impedance with a sharp probe. As well as a Fluke DVM. I just need to know which wire connectors and wire colors to probe! Do you have a link or some PDF you can send me for a 2002 LX 470 that focuses on this area that your experience has told you to look at?
I just want to know WHERE to look on the vehicle and which wire harness bundle to check for a short in the harness. I live in Dallas TX so there is NO salt or corrosion like where I come from in Wisconsin! You mentioned previously about a harness on the driver's side, in the rear- can you send me a diagram or something as to where or what I should be looking for ? I DID check the voltage AT the pressure switch connector in the engine bay, AND back at the ECU connector per the TSM - and it was 0.505V at both locations. ( out of spec) Electrical diagnosing and troubleshooting are my weaknesses (I'm a Mechanical- Aerospace Engineer ) and mechanical issues I can easily, intuitively diagnose!@

I appreciate your help gentlemen, as I am soooooooooooooooooooo frustrated I can't think or reason about this problem anymore with a clear mind and it's been 2.5 months since I have driven or used my LX!

 

[suprarx7nut]

I appreciate your help gentlemen, as I am soooooooooooooooooooo frustrated I can't think or reason about this problem anymore with a clear mind and it's been 2.5 months since I have driven or used my LX!

That does sound frustrating. AHC is an annoying beast in that there's a lot that can theoretically go wrong. However, there's a very short list of things that we ever seem to see actually fail.

I'm saying very specifically that you should NOT chase down a wire problem of any sort. If the only code is 1762 I've never seen that be anything other than gas in the system.

I understand that active test refuses to run. That's a common annoyance and frankly a shortcoming by Toyota/Lexus, IMO. The fix for clearing gas is to run the pump via active test, but the system shuts it down if there's a small amount of gas in the lines... you know, exactly when you would want to run the protocol.

To put it simply:

Do you have only code 1762?

Yes: Then you have gas in the system. You need to purge the trapped gas.
No: Then chase after those other codes paying very close attention to any circuit that travels through the connectors at the rear driver side frame.

The fix for gas from other people I've dealt with has been to run the motor - using jumper wires if necessary - and bleeding out the lines. Active test is great if it actually runs, but it rarely does for me. Applying 12v directly to the motor (sometimes with alternating polarity to run the motor backwards) can help clear out debris, gel (from bad AHC fluid) or dislodge a stubborn gas bubble. When you bleed you should have absolutely perfect fluid only coming into your clear tube on the bleeder. Any tiny bubble is a sign you have more bleeding ahead of you. It can be very tedious.

 

[suprasvobodea]

That does sound frustrating. AHC is an annoying beast in that there's a lot that can theoretically go wrong. However, there's a very short list of things that we ever seem to see actually fail.

I'm saying very specifically that you should NOT chase down a wire problem of any sort. If the only code is 1762 I've never seen that be anything other than gas in the system.

I understand that active test refuses to run. That's a common annoyance and frankly a shortcoming by Toyota/Lexus, IMO. The fix for clearing gas is to run the pump via active test, but the system shuts it down if there's a small amount of gas in the lines... you know, exactly when you would want to run the protocol.

To put it simply:

Do you have only code 1762?

Yes: Then you have gas in the system. You need to purge the trapped gas.
No: Then chase after those other codes paying very close attention to any circuit that travels through the connectors at the rear driver side frame.

The fix for trapped air from other people I've dealt with has been to run the motor - using jumper wires if necessary - and bleeding out the lines. Active test is great if it actually runs, but it rarely does for me. Applying 12v directly to the motor (sometimes with alternating polarity to run the motor backwards) can help clear out debris, gel (from bad AHC fluid) or dislodge a stubborn gas bubble. When you bleed you should have absolutely perfect fluid only coming into your clear tube on the bleeder. Any tiny bubble is a sign you have more bleeding ahead of you. It can be very tedious.

Hello,

Yes, 1762 is the ONLY code that I have from Techstream. I also think that a direct 12V to the pump to FORCE it to run is the solution, However, INDOCRUISE who is also a huge contributor summarized in very specific detail WHY running the pump directly with 12V, will NOT work to build pressure in the system ( The ECU won't allow the solenoid valves in the accumulator or globes open!!) read in post below;

"AHC Inop after fluid change 2007 lx C1751 and C1762 - post #312 from INDOCRUISE..>>> (ref.)"​

Some notes on so-called ‘over-riding’ of the “fail-safe functions”:

• When the Height Control Operation Test (“Active Test”) does not work, it means a fault/DTC is preventing it from working and the fault and DTC must be cleared first before this test will work properly,
• This test is designed into the system to facilitate adjustments -- it is not designed to override fault conditions nor the "Fail Safe Function",
• When the Height Control Operation Test (“Active Test”) can work correctly, the ECU causes the AHC Pump to operate and also causes the Levelling Valves in the Control Valve Assembly to open as required to allow the vehicle to be raised or lowered,
• The “Levelling Valves” (one Front, one Rear) are ‘normally closed’,
• The vehicle cannot be raised or lowered unless the Levelling Valves are caused to open by the ECU – or unless the vehicle is lowered by dumping AHC Fluid from the bleeder screws,
• Operating the AHC Pump without opening the Levelling Valves (such as by direct 12 volt connection to the AHC Pump Motor) cannot raise the vehicle because there is no signal from the ECU to the Levelling Valves to cause them to open,
• The Solenoid Valve on the Height Control Accumulator is ‘normally closed’ -- the Solenoid Valve on a healthy AHC system only opens on a signal from the ECU to open (i) when the Height Control Accumulator is required to discharge at the start of a ‘raise’ to lift the vehicle, and (ii) when the Height Control Accumulator is being re-filled at the end of a ‘raise’ in readiness for the next raise,
• The Height Control Accumulator has no other function in the operation of the AHC system,
• The ECU will not signal to the Solenoid Valve to open at the Height Control Accumulator in the presence of a prohibitive fault/DTC which results in a “Fail Safe Function”,
• When the AHC Pump is caused to operate using a direct 12-volt supply against closed Levelling Valves and a closed Solenoid Valve on the Height Control Accumulator, then the AHC Fluid from the AHC Pump has nowhere to go other than some fluid forcibly bypassing/shortcircuiting internally within the AHC Pump sub-assembly, and with the pump pressure hammering on seals and valves. All that will happen is noisy cavitation within the AHC Pump. Unlike industrial gear pumps, there is no pressure relief valve to protect the AHC Pump. Instead, the system relies on ECU-controlled protections such as C1718, C1C1736, C1741, C1743, C1762, and C1763 to quickly shut down the AHC Pump under FSM-specified conditions."

"By the way, it is good to show the whole Techstream screenshot -- then it can be checked whether the "Main Relay Expectation" is "ON". This refers to the AHC Main Relay located at the ECU under the dashboard. If this relay is not working, operation of the AHC/TEMS is not possible. It also shows the number of Diagnostic Trouble Codes (DTCs) at the time of the test."


I think this is what broke my original pump and caused it to lock up... I was trying to force fluid, under extreme pressure against a CLOSED system, and no fluid left the tank and I was cavitating fluid inside the pump... Please let me know if this is wrong thinking on my part and INDOCRUISE..... But this does make sense. That's why I am hesitant to run the pump for more than 5 seconds to just check that it's working .( which it is, cause it's new!)

I think you're correct about the air, but I don't know how to force the pump to run AND get the solenoid valves to open so the system can actually pump oil to where I need it to go ?!????!!!

 

[suprarx7nut]

I think there's a lot of opportunities to get lost in the physics here.

Cavitation occurs when you're creating extremely low pressure in the pump - my understanding is that happens when you've got gas in the fluid or when you've got a restriction in the intake. Not necessarily that you have high system pressure. High pressure would help prevent cavitation.

Anyways, I think what people generally experience is that you've got gas in the system near the pump. Maybe you've got gel or other debris right near the intake screens. The code just means you're failing to create pressure when the pump runs. That's it. Given that, it's a low risk of overpressurizing anything by forcing the pump to run. Applying some pressure to the pump plumbing in both directions offers a chance to break that gas or debris free.

Cavitation definitely kills pumps, but it is a slow erosion process. Think in terms of days, not seconds.

I'm not sure how the valves operate during any of this, which is a gap in my knowledge and I trust @IndroCruise hands down knows it better than I do. However... I can tell you I've worked with dozens of people through mud, YouTube, reddit, phone consults and a few in person regarding this persistent 1762 and the fix has always been getting that pump to run a little and then bleed.

I think @BullElk had an interesting one recently, but I can't recall exactly what ended up working there. I think he tried just about everything and even had a few short lived fixed before landing on the final answer.

 

[BullElk]

I think @BullElk had an interesting one recently, but I can't recall exactly what ended up working there. I think he tried just about everything and even had a few short lived fixed before landing on the final answer.

Yea, thanks to help from @suprarx7nut we went thru everything possible to remedy a 62 code, fluid pressure malfunction. I finally replaced the couple O rings inside the pump and it came alive holding pressure again.

 

[2001LC]

HI

I have the same problem and after 60 hrs + of research, I will share my nightmare with you and everyone to help ease the pain and frustration. Suprarx7nut is correct on the Voltage specs for the pressure switch leads
Approx 5V at pin 1 of the pressure sensor to ground (pin 3).
1.48 to 1.85V signal from the pressure sensor (pins 2 and 3).

I get 5V at the ECU wire harness ( located by the driver's RH leg behind that metal plate under the steering wheel. I checked the V both at the pressure switch connected with a sharp probe (quite difficult to pierce the 18ga wire and not poke your finger holding it back but I did it! AND GOT 0.505 V!. This is NOT within the range of 1.48-1.85V when you depress the switch on the console,... so YOUR PRESSURE SWITCH IS SHOT AND YOU WILL NEED A NEW ONE! There is NO WAY to run the active test with this faulty pressure switch Voltage value at 0.5V !!

Here is the secret that has NOT been published in the TSM or by anyone on this forum before now, so you are the first one to get and hear this knowledge, please repost it to help every and anyone else with an AHC Lexus or LC that struggles with this issue.

<<<< If YOU RUN THE PUMP DRY WHILE FLUSHING OR CHANGING THE PUMP, REPLACING THE SHOCK BUSHING or REFILL THEM... anything that empties the reservoir while it's running,... IT WILL DAMAGE THE PRESSURE SWITCH! - AND YOU WILL HAVE TO BUY A NEW ONE. The cheapest I found it IMPEX out of Japan. They are USD 229 + shipping but that is a lot less than any US Lexus online parts shop. It takes a week or so to get but you will save $80-150 dollars on this one part ( Ask me how I know ..? )


I have gone through the REAR Hydraulic strut removal process and decided to cut access holes in the back cargo area to gain access to the 2 10mm bolts that are always rusted that hold the bracket over the top of the piston where the 2 gaskets are and it was THE best time-saving idea I had. I will be writing up a detailed, photo-supplied procedure that should help everyone who is too scared to tackle this job due to the other nightmare of accessing the top of those hydraulic "shocks " BTW >> ( they really are NOT shocks but actuators)

Feel free to contact me directly with any questions about this, or the Accumulator replacement, as I have done EVERYTHING now on my 2002 LX system and I want to help others!

I know all these things now after struggling for 3 months with trying to get the active test to work. It DID WORK after I bought a new pressure switch. but I didn't expect it to work and it drained a full tank of fluid. !!! I did NOT see it in time as I was in the vehicle ( big mistake! ) so I didn't know. I've checked the relays and fuses in the engine bay and the sidekick panel and all are working. The 4 step to check for code 1762 #4 is checking the pressure switch voltage and I get 0.5V when I press the UP-DOWN switch.
To bleed air from the Accumulator and globes, the PUMP MUST WORK first! I cannot manually get the pump to run due to the 1762 error! So how can I bleed if I cannot get fluid (and air) to travel to the accumulator and globes ....? This is my dilemma, so the only thing I know to do,it correct the only known problem ( out of spec V on the PS) and replace it again with a new one! If you know of ANYTHING else that could prevent an "ACTIVE TEST" from being performed, please offer any suggestions!


This is not the spec 1.48- 1.85V ... so by process of elimination, and by the TSM book ( not feelings or intuition...) that HAS to be the only reason it's still not working.! I have placed another order for a NEW PRESSURE switch from IMPEX of Japan and next week I will install it and, by God's GRACE and prayer..... it will work again. But I will be standing next to the reservoir and make sure IT DOES NOT EMPTY WHILE RUNNING!

Hello again!

Thanks for the prompt reply. I am puzzled like "The Grinch Who Stole Christmas "that the townspeople were signing in the morning after he just taken all their presents!
To answer your questions, you may make more suggestions that have worked for you in the past ( since you said you have a lot of experience on several models - I'm WILLING TO LISTEN & LEARN! I filled the reservoir with fluid to 1" from the top. After installing a brand new pressure switch, the pump started working when I pushed the up switch. I was inside the vehicle ( with the door closed!) so I couldn't see the reservoir. I used new fluid in the metal can and even poured it in an Elynmeyer Flask with a vacuum nipple and pulled the vacuum on the new fluid to remove all /any dissolved air! ( THERE WAS ALOT!! -believe it or not!) I recycle the fluid after purging air from the system as well to ensure COMPLETELY air-free oil.

The pump started working immediately after I pressed the UP switch and the tank went down steadily and emptied, I stopped the engine and pump. After it started making the cavitation sound, the 1762 code returned on my tech stream screen, and then the truck started falling and all the fluid that was pumped into the system ( plus a quart of additional) returned to the pump and overflowed out onto the ground. My LX is on the stops ... again!! As I indicated in my previous entry, I checked both relays, -they work and the fuses are good. Sooooooooooooo, what's up??! I even applied direct power to the pump with my automotive tester and it works and makes a nice quiet hum , however, I know that it will not pump any oil since the leveling and Accumulator solenoids are NOT open and it will just destroy the pump if you run it more than 10-15 seconds

My question is this,., Assuming that the pump gears may have air trapped in them... How do I remove any air IN the pump gears ... crack open the outlet fitting and let it drip for a few seconds ??

Next Question... Since I already know that the Pressure switch voltage is NOT IN SPEC (0.505) V, and the TSM says to REPLACE it if it is NOT in the range posted ... I need to start with this only parameter that is not per the TSM, so I just ordered ANOTHER PW and 1.5L can of Oil.

WHAT ELSE SHOULD I CHECK THAT IS PREVENTING THIS SYSTEM FROM DOING THE

2001LC

Thank you for your reply and experience with this situation. Here is a full background of what I have done and maybe after reading this, something will trigger someone's memory from the ol Experience file that gets filled as we all get older!

replaced all 4 globes with new 6 months ago and a NEW accumulator due to a sheared bleed nipple, 3 months ago! Then I replaced a NEW pump assembly ( due to some failure not plugged screens or anything I came across on this forum). I started this journey because I wanted to replace ALL the strut bushings. So, I guess you could say I have a COMPLETE ...BRAND NEW AHC system. I did Run it dry when refilling and regret that, however....... it was working fine before I started this project. Now I cannot get the ACTIVE TEST to work after jumping the Diagnostic Tc -? (can't remember) in the pass. side diagnostic by the pump after pressing the DOWN button 5 times within 5 seconds of start-up. It doesn't go into 1762 fault mode until I press the UP switch. I am NOT getting the spec V of 1.3-1.8V from the PS, .. and that's the only parameter ( of 4 to test) that is NOT IN SPEC to the TSM.
My question for SUPRARX7nut, 2001LC, or anyone that is reading this ... Did anyone verify this Press Sen voltage when they experienced this problem with not getting the ACTIVE TEST to run?? I am waiting for another new PS from IMPEX of Japan and should have it this coming week. Until then, PLEASE feel free to list what wires or connectors or components, I need to check out.

Gentleman,

The pump didn't "fail" per se, it was just 20 years old, took 20+ secs to go up from N-H, and after I refilled it( after replacing the strut bushing and anti-sway bar bushing &linkages, something happened and it was making a lot of loud, grinding noise and finally froze up ( It would NOT move when I applied 12V directly ) so I replaced it with a new one. I didn't have Techstream working on my laptop 4 months ago when I started to remove the struts/accumulator so I don't know the before pressures - unfortunately, but I have Techstream installed and working now! However, there is NO pressure in the system now because I cannot get the ACTIVE test to work!! That's "the Chicken-Egg dilemma"!! I cannot get pressure on the accumulator, to force oil to it or the globes to bleed them!!! This is my problem!............

I checked both the main relay( under the hood) and the motor relay by the AHC ECU under the steering wheel and they both function when the voltage is applied. There is also 5 V at the 1st or 2nd step/checkpoint for 1762 code diagnosis.

But don't know about a #5 _check for open and short in the harness. What is the process to do that .......specifically? If you guys are saying, "It is most likely the harness" then I want to check that but don't know which wires or where to start this, I have both an auto text light probe which can measure volts, and impedance with a sharp probe. As well as a Fluke DVM. I just need to know which wire connectors and wire colors to probe! Do you have a link or some PDF you can send me for a 2002 LX 470 that focuses on this area that your experience has told you to look at?
I just want to know WHERE to look on the vehicle and which wire harness bundle to check for a short in the harness. I live in Dallas TX so there is NO salt or corrosion like where I come from in Wisconsin! You mentioned previously about a harness on the driver's side, in the rear- can you send me a diagram or something as to where or what I should be looking for ? I DID check the voltage AT the pressure switch connector in the engine bay, AND back at the ECU connector per the TSM - and it was 0.505V at both locations. ( out of spec) Electrical diagnosing and troubleshooting are my weaknesses (I'm a Mechanical- Aerospace Engineer ) and mechanical issues I can easily, intuitively diagnose!@

I appreciate your help gentlemen, as I am soooooooooooooooooooo frustrated I can't think or reason about this problem anymore with a clear mind and it's been 2.5 months since I have driven or used my LX!

Hello,

Yes, 1762 is the ONLY code that I have from Techstream. I also think that a direct 12V to the pump to FORCE it to run is the solution, However, INDOCRUISE who is also a huge contributor summarized in very specific detail WHY running the pump directly with 12V, will NOT work to build pressure in the system ( The ECU won't allow the solenoid valves in the accumulator or globes open!!) read in post below;

"AHC Inop after fluid change 2007 lx C1751 and C1762 - post #312 from INDOCRUISE..>>> (ref.)"​

Some notes on so-called ‘over-riding’ of the “fail-safe functions”:

• When the Height Control Operation Test (“Active Test”) does not work, it means a fault/DTC is preventing it from working and the fault and DTC must be cleared first before this test will work properly,
• This test is designed into the system to facilitate adjustments -- it is not designed to override fault conditions nor the "Fail Safe Function",
• When the Height Control Operation Test (“Active Test”) can work correctly, the ECU causes the AHC Pump to operate and also causes the Levelling Valves in the Control Valve Assembly to open as required to allow the vehicle to be raised or lowered,
• The “Levelling Valves” (one Front, one Rear) are ‘normally closed’,
• The vehicle cannot be raised or lowered unless the Levelling Valves are caused to open by the ECU – or unless the vehicle is lowered by dumping AHC Fluid from the bleeder screws,
• Operating the AHC Pump without opening the Levelling Valves (such as by direct 12 volt connection to the AHC Pump Motor) cannot raise the vehicle because there is no signal from the ECU to the Levelling Valves to cause them to open,
• The Solenoid Valve on the Height Control Accumulator is ‘normally closed’ -- the Solenoid Valve on a healthy AHC system only opens on a signal from the ECU to open (i) when the Height Control Accumulator is required to discharge at the start of a ‘raise’ to lift the vehicle, and (ii) when the Height Control Accumulator is being re-filled at the end of a ‘raise’ in readiness for the next raise,
• The Height Control Accumulator has no other function in the operation of the AHC system,
• The ECU will not signal to the Solenoid Valve to open at the Height Control Accumulator in the presence of a prohibitive fault/DTC which results in a “Fail Safe Function”,
• When the AHC Pump is caused to operate using a direct 12-volt supply against closed Levelling Valves and a closed Solenoid Valve on the Height Control Accumulator, then the AHC Fluid from the AHC Pump has nowhere to go other than some fluid forcibly bypassing/shortcircuiting internally within the AHC Pump sub-assembly, and with the pump pressure hammering on seals and valves. All that will happen is noisy cavitation within the AHC Pump. Unlike industrial gear pumps, there is no pressure relief valve to protect the AHC Pump. Instead, the system relies on ECU-controlled protections such as C1718, C1C1736, C1741, C1743, C1762, and C1763 to quickly shut down the AHC Pump under FSM-specified conditions."

"By the way, it is good to show the whole Techstream screenshot -- then it can be checked whether the "Main Relay Expectation" is "ON". This refers to the AHC Main Relay located at the ECU under the dashboard. If this relay is not working, operation of the AHC/TEMS is not possible. It also shows the number of Diagnostic Trouble Codes (DTCs) at the time of the test."


I think this is what broke my original pump and caused it to lock up... I was trying to force fluid, under extreme pressure against a CLOSED system, and no fluid left the tank and I was cavitating fluid inside the pump... Please let me know if this is wrong thinking on my part and INDOCRUISE..... But this does make sense. That's why I am hesitant to run the pump for more than 5 seconds to just check that it's working .( which it is, cause it's new!)

I think you're correct about the air, but I don't know how to force the pump to run AND get the solenoid valves to open so the system can actually pump oil to where I need it to go ?!????!!!

I have had DTC C1762, that was related to undercarriage wire harness. I posted some of the work here. Which, also had DTC C1736 accumulator. It turned out to be corrosion inside a rear wire harness housing connector block. Which was changing the resistance, depend on moisture contend. Which was changing, on and give day. The changing nature of it, made very difficult to diagnose, as was a moving target.

I just had other case of AHC going into failsafe. It for sure had low pressure condition, that result in failsafe. But I did not find any DTC using BlueDriver nor have a CEL. Perhaps actual Tech Stream (TS) would have pickup a DTC, IDK!.
It was a DIY globes (aftermarket) install, where two procedural mistakes made by installer.
1) Globe O-ring came out of position, during install, unseen by installed. It for sure was loosing pressure at that one globe.
2) Under filled reservoir and ran dry, result in failsafe mode. He did NOT see, reservoir was out of fluid. Due to stains in the reservoir, it looked as if had fluid. I use a flashlight behind reservoir, to help me see actual level line. I corrected the issue, triple bleed/flushed and all good now.

Back to your C1762 @suprasvobodea:

With the added detail, you provided. I'm now thinking a short in wires, is not likely your issue(s). So let's table wire harness testing, for now.

One very big clue. Was running reservoir dry, after filling to with 1" of top/cap.

But can I get some more detail, before we getting into what I think is wrong and how to correct.

Please place your answers, next to ? marks. Changing color of your answer to green, will help me spot your answer easier.

To recap what you've posted, to see if I have a handle on time line and services, as I ask these question:

6 months ago. Replaced 4 globes & height accumulator with all NEW OEM?
Only reason stated for above R&R. A bust height accumulator bleeder. Was there and issue that prompted you to replace the globes?
Did not check pressure. No TS at the time.
I assumed you replaced the AHC fluid with new Toyota 2.5L tin, during above service?


No mention of adjusting height sensors or mention of past sensor lift. Did you?
Assume all stock. Since no mention being a built rig adding extra weight?
No mention of cross level?
No mention of fender to axle heights, Did you check this, if so what are they?
Did you get the fluid level graduation count, from H to L?


Drove for 2 or 3 months, AHC system worked. Only reported issues: Slow to raise ~20 seconds (spec max 15 sec.) from N to H. Any other issues?

Decided to replace bushing of shock (aka: Hydraulic ram) and stabilizer system. Removing all 4 shock to do so. No issue reported, as a reason globe R&R. So was this just a refresh?

Before installing shocks. Did you pre flush, fill and bleed? If yes give detail on procedure?

Also give detail on how installed shocks, and then lower to ground (took off jack stand) and then start up AHC?.

After install of shock. Upon starting and operating AHC. AHC vehicle raising "began, making a lot of loud, grinding noise and finally froze up".
I assume "froze up", means went into failsafe (AHC flashing OFF)?
Unclear if ran reservoir dry at this time?
Diagnosed, by jumping pump with 12Volts. "did not move" I assume this to mean vehicle did not raise, but pump did run?
So replaced the pump, since slow to raise and old anyway. Hoping to correct issue of "froze" with was this "NEW OEM"?
After which, AHC Systems failed to work properly. While raising went into failsafe, DTC C1762.
Diagnose PS sensor bad. So R&R PS sensor.
After filling reservoir to within 1" of top/cap. While raising, ran reservoir dry, AHC went into failsafe. DTC C1762.
Diagnose PS sensor bad again. Waiting on PS sensor, to replaced PS sensor again.

 

[suprasvobodea]

I have had DTC C1762, that was related to undercarriage wire harness. I posted some of the work here. Which, also had DTC C1736 accumulator. It turned out to be corrosion inside a rear wire harness housing connector block. Which was changing the resistance, depend on moisture contend. Which was changing, on and give day. The changing nature of it, made very difficult to diagnose, as was a moving target.

I just had other case of AHC going into failsafe. It for sure had low pressure condition, that result in failsafe. But I did not find any DTC using BlueDriver nor have a CEL. Perhaps actual Tech Stream (TS) would have pickup a DTC, IDK!.
It was a DIY globes (aftermarket) install, where two procedural mistakes made by installer.
1) Globe O-ring came out of position, during install, unseen by installed. It for sure was loosing pressure at that one globe.
2) Under filled reservoir and ran dry, result in failsafe mode. He did NOT see, reservoir was out of fluid. Due to stains in the reservoir, it looked as if had fluid. I use a flashlight behind reservoir, to help me see actual level line. I corrected the issue, triple bleed/flushed and all good now.

Back to your C1762 @suprasvobodea:

With the added detail, you provided. I'm now thinking a short in wires, is not likely your issue(s). So let's table wire harness testing, for now.

One very big clue. Was running reservoir dry, after filling to with 1" of top/cap.

But can I get some more detail, before we getting into what I think is wrong and how to correct.

Please place your answers, next to ? marks. Changing color of your answer to green, will help me spot your answer easier.

To recap what you've posted, to see if I have a handle on time line and services, as I ask these question:

6 months ago. Replaced 4 globes & height accumulator with all NEW OEM? YES- all NEW OEM
Only reason stated for above R&R. A bust height accumulator bleeder. Was there and issue that prompted you to replace the globes? -Yes- my LX was riding like a F -350 !!!! It was HORRIBLE rough and only 7 graduations ! - SO,... it was time
Did not check pressure. No TS at the time.
I assumed you replaced the AHC fluid with new Toyota 2.5L tin, during above service? YES. in metal can.


No mention of adjusting height sensors or mention of past sensor lift. Did you? DId NOT adjust or move ANY ride height sensors.
Assume all stock. Since no mention being a built rig adding extra weight? Everyting is stock .
No mention of cross level? I actually, DID adjust the torsions rods to level out theLEFT-RH difference , BEFORE i did any fluid -strut change replacement
No mention of fender to axle heights, Did you check this, if so what are they? YES- I need to look for the paper i wrote them down on but they were within the tolerance range. I know that means nothing as exact numbers are theonly thing that is important.
Did you get the fluid level graduation count, from H to L? YES- as i said previously, before doing any strut& Accumulator removeal, it was 7 graduations before replacemetn with OEM globes. It was 13 after repalcement .


Drove for 2 or 3 months, AHC system worked. Only reported issues: Slow to raise ~20 seconds (spec max 15 sec.) from N to H. Any other issues? I have NOT driving my LX in 3 + months - since i reinstalled the struts , accumultor, linkages bushing. I cannot get the suspension to raise so its drivable.!

Decided to replace bushing of shock (aka: Hydraulic ram) and stabilizer system. Removing all 4 shock to do so. No issue reported, as a reason globe R&R. So was this just a refresh? Yes- i replaced all the bushing in the struts - front and back .

Before installing shocks. Did you pre flush, fill and bleed? If yes give detail on procedure? No... didnt have Techstream working to do ANY testing before i opened up the system - UNfortunalty . I went thru my first hell - trying ....since last summer, to get techstream to load on my laptop. There are so many viruses afilitated with Techstream sellers and my Norton was blocking every attempt to load it, I finally found another source from this guru online - i forget his name , but i can find it if you want to know .

Also give detail on how installed shocks, and then lower to ground (took off jack stand) and then start up AHC?. OK, this was 4 months ago, but i ll do my best. After installing fronts, ( easy ones) i tackled the rears . I decide to cut 2 3/8 (60 mm) holes as close to the top of the reare struts as i could guess. THAT simple decision was the best one on the whole project. After intalling both new seals at the top, i lowered to ground. I filled the resevouir back up to 1 " and i got a 1762 code immeditaly . OF course i was shocked that this happened and after several weeks of diagostic advice and reading everything i could on this forum, i discoved the pressure switch V was one of the paramters to check after the AHC relays and fuses. Thats when i discovedc that voltage was 0.505V not the 1.4-1.8V per the TSM , so i ordered a NEW PS from IMPEX in Japan. After 2 more weeks, it arrived and i installed it. After filling resevoir , i started up and attemped to the ACTIVE TEST , it actually worked!! I was excited to see this thing rise first time in several months ! BUt was inside thetruck and didnt know the resevoirs was running out of fluid and then i heard the sound that i know it was empty and stoped the lift by pressing DOWN button and killed the engine. THis is where i am now ....

After install of shock. Upon starting and operating AHC. AHC vehicle raising "began, making a lot of loud, grinding noise and finally froze up".
I assume "froze up", means went into failsafe (AHC flashing OFF)? YES
Unclear if ran reservoir dry at this time? YES- after i installed a new PS,just decribed above , it ran dry !
Diagnosed, by jumping pump with 12Volts. "did not move" I assume this to mean vehicle did not raise, but pump did run? NO- opump did not run or turn. Im not sure why
So replaced the pump, since slow to raise and old anyway. Hoping to correct issue of "froze" with was this "NEW OEM"? No- it was the OLD pump that forze after i tried to run it directly 12V for over 3 minutes - thinking it would pump fluid into the accumulator - but as INDOCRUIDE said , none of teh solenloids are open unless the ECM tells them to open
After which, AHC Systems failed to work properly. While raising went into failsafe, DTC C1762.
Diagnose PS sensor bad. So R&R PS sensor.
After filling reservoir to within 1" of top/cap. While raising, ran reservoir dry, AHC went into failsafe. DTC C1762.
Diagnose PS sensor bad again. Waiting on PS sensor, to replaced PS sensor again. YES- that is the timeline of events. I am waiting for another new PS from IMPEX to re-install again.

However, I read that there could be a problem with the seal between the pump and the motor. when I ordered a new pump, I also ordered 3 o Rigns. The large one for the cover, the small one for the inside of the pump, and a third one - I can't remember, but will check tonight its part number and let you know. Im going to double-check these origins and take the pump assy apart from the motor and check this interface. Maybe there is an entering the connection between the pump and motor ..???

 

[2001LC]

So It's very likely your shock had/have air in them, when you installed. This may be why you ran reservoir down from full. Air may trap in inner cambers of shocks.

Detail matters!
I was hoping for step by step detail, from you. As so often people say they did this or that in very general terms as you did. Then I find procedurally, it was not done properly.
Why is this important. Because I strongly suspect, most all you issue, are from procedural errors. The reservoir would not have run dry otherwise. Sucking res dry, opening the all bleeder, dropping vehicle to the stops. Only take 2.5L out. Filling res., with 2.5L, fills to proper level in N. Unless full drained. Which can't be done unless shock completely drain, as I understand it. Dropping to bump stop, only get outer chamber of shock.

1st you replaced globes, without checking pressures. Ride quality is very reliant on pressure setting.
Grads test isn't very reliable, before flush and adjusting (setting pressure). I've had more than one, read 7 grades, then flush and adjust to then get ~9.

Cross leveling (CL) is a first step. Which I've found if never done before, always requires increasing LH T-bar tension (CW turns). If either LH or RH T-bar turns CCW. This would have likely increased pressure. If pressures to high, air in system, bad fluid, even low amperage to AHC motor. N to H time may increase/slow. With miles, age and loads carried. Rear coils and T-bar "sag". So we almost always find pressure high, if never adjusted.

Fender to axle measurement is not official, but good enough if no accidents beat/bent fenders. To say within spec. Well most think front in spec at 19 1/2". I find stock, never adjusted, factory set at 19 3/16". So If I see or hear; 19 1/2 frt, I also suspect a sensor lift.

So other than cross leveling and flushing. Not much can be done, without first checking and setting pressures.

You answer NO: "Drove for 2 or 3 months, AHC system worked. Only reported issues: Slow to raise ~20 seconds (spec max 15 sec.) from N to H. Any other issues? I have NOT driving my LX in 3 + months - since i reinstalled the struts , accumultor, linkages bushing. I cannot get the suspension to raise so its driv" I think a miss understanding and or statement. As you had R&R the globes & accu 6 months early. Than must have drove some for 2 or 3 months, before R&R the shocks. Then not been able to drive the past 3 or 4 months, since shock, then pump and then PS sensor.

You had gotten 13 grads 6 months ago, after globes. System must have worked to get reading on globes (13 grades). This matters, as it help Identify after what service, things really went bad (failsafe).

A) if globe rupture, they hold extra AHC fluid. This would explain running dry from a full (1" from cap) reservoir. System does not go into failsafe typically. But fluid level goes down.

But as I understand it, system worked until shock R&R. So it bring us to shocks, as first failsafe:

B) Air in shock. As pump first ran, fluid push air deeper into shock. This may explain the 2.5L tin (reservoir hold 2.5L, when filled ~1" from cap) in reservoir, but it ran dry. As fluid compress air into shock. Inner camber may stay dry or at least have difficult time, as it fills with air. Air will not compress as well or as fast as fluid. Pump keeps running, but pressure too slow to come up and to much time passes. C1762 DTC.

This air is hard to get out of, in inner shock camber. I've never had it happen. So just tossing out some thoughts:

One could try jacking up and lowering, to kind of pump the shocks. But likely the air just goes back and forth. Working bleeders and pump (tech Stream), while out of failsafe (Clear DTC). Which clearing DTC may give a few moments of run time. You can also work gates and pump from tech stream. But you may end up pulling HP fitting off top of shock, to bleed.

When ever I R&R a shock new or reusing. I pre-flush, bleed and fill. Before installing.
When pre-flushing a shock. We make sure to fully extend and compress twice in each direction. There's an inner camber we hit on second pull/or push. We first do this to expel all fluid. We then do, to suck in fluid, like using a syringe. Most shocks kind of stop short, as we extend. But if we pull hard, they extend further. On compression, we need to go all the way in.

I open and keep open, its bleeder. As I lower vehicle off jack-stands. This allow a path for shocks to push air/fluid out. I then close bleeder(s) and raise to only L. Bleed/drain frt, rear, accu. Fill and raise to L, then N., Bleed/drain fill raise to L then N than H. I may do one more bleed if any air seen last bleed/drain.

You may find this helpful:

I once had a front set of shocks, I was replacing bushing. Dare shocks were impossible to extend and compress by hand. So I installed, without HP line fitting connected to top. Hook-up a hose and large syringe as a funnel, to top HP fluid port. Used a jack under LCW, to raise and lower. Flush, filled and bleed shock in place this way.

It is also possible, you've leak(s),. This too, could result in pressure loss C1762. But I'd think a secondary issue. As bad leak you know. A small leak(s) you may not see.

Good luck.

 

[suprasvobodea]

So It's very likely your shock had/have air in them, when you installed. This may be why you ran reservoir down from full. Air may trap in inner cambers of shocks. I sucked oil like a syrnge by inserting it into a container and pulled the stut up intill it stoped. Then i used a funnel and filled it full befofre inserting back to the front. THen i used a jack to lift the LCA up and compress it so the fluid ozed out the top before sealing the cap back on with two 10 mm bolts.

Detail matters!
I was hoping for step by step detail, from you. As so often people say they did this or that in very general terms as you did. Then I find procedurally, it was not done properly.
Why is this important. Because I strongly suspect, most all you issue, are from procedural errors. The reservoir would not have run dry otherwise. Sucking res dry, opening the all bleeder, dropping vehicle to the stops. Only take 2.5L out. Filling res., with 2.5L, fills to proper level in N. Unless full drained. Which can't be done unless shock completely drain, as I understand it. Dropping to bump stop, only get outer chamber of shock.

1st you replaced globes, without checking pressures. Ride quality is very reliant on pressure setting.
Grads test isn't very reliable, before flush and adjusting (setting pressure). I've had more than one, read 7 grades, then flush and adjust to then get ~9.

Cross leveling (CL) is a first step. Which I've found if never done before, always requires increasing LH T-bar tension (CW turns). If either LH or RH T-bar turns CCW. This would have likely increased pressure. If pressures to high, air in system, bad fluid, even low amperage to AHC motor. N to H time may increase/slow. With miles, age and loads carried. Rear coils and T-bar "sag". So we almost always find pressure high, if never adjusted.

Fender to axle measurement is not official, but good enough if no accidents beat fenders. To say within spec. Well most think front in spec at 19 1/2". I find stock, never adjusted, factory set at 19 3/16". So If I see or hear; 19 1/2 frt, I also suspect a sensor lift.

So other than cross leveling and flushing. Not much can be done, without first checking and setting pressures.

You answer NO: "Drove for 2 or 3 months, AHC system worked. Only reported issues: Slow to raise ~20 seconds (spec max 15 sec.) from N to H. Any other issues? I have NOT driving my LX in 3 + months - since i reinstalled the struts , accumultor, linkages bushing. I cannot get the suspension to raise so its driv" I think a miss understanding and or statement. As you had R&R the globes & accu 6 months early. Than must drove for 2 or 3 months, before R&R the shocks. Then not been able to drive past 3 or 4 months, since shock, then pump and then PS sensor.

You had gotten 13 grads 6 months ago, after globes. System must have worked to get reading on globes (13 grades). This matters, as it help Identify after what service, things really went bad (failsafe).

A) if globe rupture, they hold extra AHC fluid. This would explain running dry from a full (1" from cap) reservoir. System does not go into failsafe typically. But fluid level goes down.

But as I understand it, system worked until shock R&R. So it bring to shocks, as first failsafe:

B) Air in shock. As pump first ran, fluid push air deeper into shock. This may explain the 2.5L tin (reservoir hold 2.5L, when filled ~1" from cap) in reservoir, but it ran dry. As fluid compress air into shock. Inner camber mat say dry or at least have difficult time, as it fills with air. Air will not compress as well or as fast as fluid. Pump keeps running, but pressure too slow to come up and to much time passes. C1762 DTC.

This air is hard to get out of, in inner shock camber. I've never had it happen. So just tossing out some thoughts:

One could try jacking up and lowering, to kind of pump the shocks. But likely the air just goes back and forth. If work bleeders and pump, while out of failsafe. Which clearing DTC may give a few moments of run time. You can also work gates and pump from tech stream. But you may end up pulling HP fitting off top of shock, to bleed.

When ever I R&R a shock new or reusing. I pre-flush, bleed and fill. Before installing.
When pre-flushing a shock. We make sure to fully extend and compress twice in each direction. There's an inner camber we hit on second pull/or push. We first do this to expel all fluid. We then do, to suck in fluid, like using a syringe. Most shocks kind of stop short, as we extend. But if we pull hard, they extend further. On compression, we need to go all the way in.

I open and keep open, its bleeder. As I lower vehicle off jack-stands. This allow a path for shocks to push air/fluid out. I then close bleeder(s) and raise to only L. Bleed/drain frt, rear, accu. Fill and raise to L, then N., Bleed/drain fill raise to L then N than H. I may do one more bleed if any air seen last bleed/drain.

You may find this helpful:

I once had a front set of shocks, I was replacing bushing. Dare shocks were impossible to extend and compress by hand. So I installed, without HP line fitting connected to top. Hook-up a hose and large syringe as a funnel, to top HP fluid port. Used a jack under LCW, to raise and lower. Flush, filled and bleed shock in place this way.

It is also possible, you've leak(s),. This too, could result in pressure loss C1762. But I'd think a secondary issue. As bad leak you know. A small leak(s) you may not see.

Good luck.

2001 LC

You have jogged my memory so let me start from the 1st step when I started working on the AHC System - the Globes and Accumulator replacement.

I replaced the 4 globes with a new, OEM and a NEW OEM Accumulator in the Fall of 2023. I had no issues with bleeding them & the accumulator after installing them. I did NOT drive the LX for several months as I said earlier, - I only drove it a few times, and then decided to do a complete job and change out the strut bushings after reading a post about that to get a NEW CAR ride! So, I ordered all the strut bushings and sway bar bushings in Jan 2024. I removed the front struts and replaced the bushings. I Sucked up fresh oil like a syringe by inverting the strut and dipping the top into a can of oil and did the syringe trick! Then I turned right side up & topped off with a funnel till full. After installing the lower bolt, I lifted the lower control arm with a bottle jack to raise the strut just high enough to push fluid out the top and then fasted the Cap with 2 new O-rings and flat washer, tighter up the 10mm bolts, and removed the support for CA. I did that for both fronts. THUS NO AIR IN THE STRUT! and saved a lot of time bleeding all that out!
Then I removed the 2 rear struts and did the same procedure. I sucked up fresh oil like a syringe by inverting the strut and dipping the top into a can of oil, then turned right side up & topped off with a funnel till full. After installing the lower bolt, Then I pushed Oil to the top by raising the rear axle with a bottle jack. Replaced the 2 O-rings with new ones and tightened the TWO 10mm bolts to hold the bracket on.

Then removed the jack stands and filled the reservoir with fresh fluid from the metal can. I started the engine and got the 1762 code, SPent several weeks trying to diagnose it and found out in the TSM from Lexus that the 1762 code had 4 steps to troubleshoot. The 2 AHC relays, and fuses, Test for 5 V from SHB -SHG at the ECU connector ( See picture ) Page DI 251. Then I followed the procedure for 1762 in the TSM ( See pictures ) I got 5V for the pressure sensor on page DI 251. Then I tested the Pressure switch both at the connector at the pump and on the Connector at the AHC ECU by the steering wheel and they both were 0.505 V. That is why I say the issue is with the PS,.... it's simply sending the wrong voltage to the ECU - thus the continuous fail-safe issue with the pump not working!

Here's my important question after reviewing your reply..........
]
BEFORE I get the new PS and install it,.... After I clear the 1762 code on Techstream and try the ACTIVE TEST again, should I open the bleeder on the accumulator so the pump can pump fluid to the accumulator and open one or both of the front globes bleeder and see if fluid with exit them while pressing the up switch ...?? Would this purge any air (if there) in the struts? You said that you think there is air INSIDE the struts, but I filled them with oil and compressed them BEFORE I put the cap and 2 new o-rings on them.

 

[suprarx7nut]

Good summary. Just interjecting some thoughts.

1. You exposed the system to air and then immediately had this problem. It's *very* common for people to have a 1762 right after a service involving shock changes. The fix is to flush (active test or manually wiring pump to run) and then bleeding.

2. You don't have code 1718, right? That code will flag if you've got a bad pressure sensor.

3. I'm not sure anyone on here has heard of any pressure sensor actually failing in the AHC system. Hundreds upon hundreds of people posting about AHC and I'm not sure that's ever come up as a verified problem. I've worked with a LOT of old Toyota sensors and electronics. I can't tell you how many times the tests in the EWD or TSM aren't quite perfect. They're a guide but not necessarily a bible. Keep in mind they were written for new cars under warranty and mostly exist to uncover manufacturing faults and basic order of ops. For AHC, the TSM failed us, IMO. The TSM being erroneous is a massive part - perhaps the single biggest reason - why AHC gets such a bad rap. If you're a mechanic in a shop following the TSM to a tee, you're doomed, IMO. It will run you through rarely helpful loops of tests when all you need is a bleed or a wire repair (in your case, it sounds exactly like air in the system).

4. Purging air from the rams/shocks should happen just by cycling them. The air rises to the top of the shock and then rushes out with fluid to the bleeder once you crack that open.

 

[suprasvobodea]

Good summary. Just interjecting some thoughts.

1. You exposed the system to air and then immediately had this problem. It's *very* common for people to have a 1762 right after a service involving shock changes. The fix is to flush (active test or manually wiring pump to run) and then bleeding. But….It won’t stop giving the 1762 code every time I start the active test!! Running the pump directly…. Won’t pump oil into anything!!!!. So that’s why I am stuck… & in an endless “do loop” !! So if it’s not electrical short, not a bad PS , and I do have air in system…. WHY Can’t I get the active test to work ?? WHAT AM I MISSING ?

I HAVE to run the active test to bleed system of air or I’ll NEVER Drive MY LX AGAIN ! What else should I try ???? I’m at the end of the road here in Dallas TX …

W

2. You don't have code 1718, right? right ! NO 1718 code ! That code will flag if you've got a bad pressure sensor.

3. I'm not sure anyone on here has heard of any pressure sensor actually failing in the AHC system. Hundreds upon hundreds of people posting about AHC and I'm not sure that's ever come up as a verified problem. I've worked with a LOT of old Toyota sensors and electronics. I can't tell you how many times the tests in the EWD or TSM aren't quite perfect. They're a guide but not necessarily a bible. Keep in mind they were written for new cars under warranty and mostly exist to uncover manufacturing faults and basic order of ops. For AHC, the TSM failed us, IMO. The TSM being erroneous is a massive part - perhaps the single biggest reason - why AHC gets such a bad rap. If you're a mechanic in a shop following the TSM to a tee, you're doomed, IMO. It will run you through rarely helpful loops of tests when all you need is a bleed or a wire repair (in your case, it sounds exactly like air in the system).

4. Purging air from the rams/shocks should happen just by cycling them. The air rises to the top of the shock and then rushes out with fluid to the bleeder once you crack that open.

 

[IndroCruise]

Hello,

Yes, 1762 is the ONLY code that I have from Techstream. I also think that a direct 12V to the pump to FORCE it to run is the solution, However, INDOCRUISE who is also a huge contributor summarized in very specific detail WHY running the pump directly with 12V, will NOT work to build pressure in the system ( The ECU won't allow the solenoid valves in the accumulator or globes open!!) read in post below;

"AHC Inop after fluid change 2007 lx C1751 and C1762 - post #312 from INDOCRUISE..>>> (ref.)"​

Some notes on so-called ‘over-riding’ of the “fail-safe functions”:

• When the Height Control Operation Test (“Active Test”) does not work, it means a fault/DTC is preventing it from working and the fault and DTC must be cleared first before this test will work properly,
• This test is designed into the system to facilitate adjustments -- it is not designed to override fault conditions nor the "Fail Safe Function",
• When the Height Control Operation Test (“Active Test”) can work correctly, the ECU causes the AHC Pump to operate and also causes the Levelling Valves in the Control Valve Assembly to open as required to allow the vehicle to be raised or lowered,
• The “Levelling Valves” (one Front, one Rear) are ‘normally closed’,
• The vehicle cannot be raised or lowered unless the Levelling Valves are caused to open by the ECU – or unless the vehicle is lowered by dumping AHC Fluid from the bleeder screws,
• Operating the AHC Pump without opening the Levelling Valves (such as by direct 12 volt connection to the AHC Pump Motor) cannot raise the vehicle because there is no signal from the ECU to the Levelling Valves to cause them to open,
• The Solenoid Valve on the Height Control Accumulator is ‘normally closed’ -- the Solenoid Valve on a healthy AHC system only opens on a signal from the ECU to open (i) when the Height Control Accumulator is required to discharge at the start of a ‘raise’ to lift the vehicle, and (ii) when the Height Control Accumulator is being re-filled at the end of a ‘raise’ in readiness for the next raise,
• The Height Control Accumulator has no other function in the operation of the AHC system,
• The ECU will not signal to the Solenoid Valve to open at the Height Control Accumulator in the presence of a prohibitive fault/DTC which results in a “Fail Safe Function”,
• When the AHC Pump is caused to operate using a direct 12-volt supply against closed Levelling Valves and a closed Solenoid Valve on the Height Control Accumulator, then the AHC Fluid from the AHC Pump has nowhere to go other than some fluid forcibly bypassing/shortcircuiting internally within the AHC Pump sub-assembly, and with the pump pressure hammering on seals and valves. All that will happen is noisy cavitation within the AHC Pump. Unlike industrial gear pumps, there is no pressure relief valve to protect the AHC Pump. Instead, the system relies on ECU-controlled protections such as C1718, C1C1736, C1741, C1743, C1762, and C1763 to quickly shut down the AHC Pump under FSM-specified conditions."

"By the way, it is good to show the whole Techstream screenshot -- then it can be checked whether the "Main Relay Expectation" is "ON". This refers to the AHC Main Relay located at the ECU under the dashboard. If this relay is not working, operation of the AHC/TEMS is not possible. It also shows the number of Diagnostic Trouble Codes (DTCs) at the time of the test."


I think this is what broke my original pump and caused it to lock up... I was trying to force fluid, under extreme pressure against a CLOSED system, and no fluid left the tank and I was cavitating fluid inside the pump... Please let me know if this is wrong thinking on my part and INDOCRUISE..... But this does make sense. That's why I am hesitant to run the pump for more than 5 seconds to just check that it's working .( which it is, cause it's new!)

I think you're correct about the air, but I don't know how to force the pump to run AND get the solenoid valves to open so the system can actually pump oil to where I need it to go ?!????!!!

Apologies offered to @suprasvobodea – my responses to requests on this Forum and in Personal Messages have been less than timely due to remote travels followed by illness, then some family issues and then other distractions.

Bottom line:

There are multiple issues here.

The fact that that the same erroneous low-voltage reading with two Pressure Sensors strongly suggests an open circuit somewhere -- meaning a broken wire.

If so, this will cause of a "fail safe mode" which is preventing the vehicle being raised in the usual way to enable bleeding to proceed.

Some futher notes a few other aspects of the AHC/TEMS system:

On air (or nitrogen) in the system ....

There is nothing to add to the long posts #307 and #312 (which also included parts lists of pump-related items and seals to replace) – see

AHC Inop after fluid change 2007 lx C1751 and C1762 - https://forum.ih8mud.com/threads/ahc-inop-after-fluid-change-2007-lx-c1751-and-c1762.1233088/page-16#post-14364800

and

AHC Inop after fluid change 2007 lx C1751 and C1762 - https://forum.ih8mud.com/threads/ahc-inop-after-fluid-change-2007-lx-c1751-and-c1762.1233088/page-16#post-15345982

Air in the system is, and remains, the primary cause of C1762 -- but not the only cause.

The concerns set out by @suprasvobodea are noted at posts #313, #314 in the same thread which also added that a new AHC Pump was installed.

Why is AHC Pump Replacement a Good Idea?

While opinions vary about the need for pump replacement, my opinion is definitely yes, just do it! -- and it has been done -- and it is smart when replacing 'globes' and/or 'shock absorbers' on an old vehicle.

Why bother?

It is suggested that because AHC Fluid is rarely changed in accordance with the Owner’s Manual, the fluid may be as old as these 1998-2007 vehicles (2002 LX470 in your case). If so, the fluid (which is a light mineral oil with proprietary additives)

• will be black, mainly due to oxidation,
• may contain air,
• inevitably picks up nitrogen which very slowly passes through the internal ‘globe’ membrane as ‘globes’ age,
• will have picked up atmospheric moisture (the system is not sealed -- open to air at the AHC Tank),
• will have picked up wear-and-tear debris from the around the AHC system over the years,
• will be sludgy and its properties as a hydraulic fluid will have degraded.

There are plenty of pictures of old AHC Fluid on IH8MUD showing the froth and bubbles which emerge (basically a phase change) when the fluid is released from high system pressure to atmospheric pressure in the bleeding tube and receiving container.

Air under operating pressures in the AHC system will be entrained or dissolved in the AHC Fluid and/or sludge rather than as 'bubbles'. The bubbles emerge when the fluid emerges to atmosphere at much lower pressure.

This is why, even though a great job may have been done of minimising air ingress during the parts change-over, it cannot be assumed that there is no air in the system. It will be there -- and it remains one of the primary causes of C1762 -- and necessitates repetitive and persistent bleeding to cure.

Good IH8MUD pictures also show how sludge accumulates, for example, between the AHC Pump sub-assembly and the outer cover. There are other ‘sludge accumulation’ points around the system where fluid flow similarly is low, slow or intermittent, such as the ‘shock absorber chambers’ which are not fully stroked by the piston, as recently mentioned by @2001LC .

Sludge accumulation is inevitable with age, and/or lack of good ‘hydraulic hygiene’ -- it was made worse by the ‘bad’ AHC Fluid batches in USA in one litre plastic bottles -- but these ‘bad’ batches are not the sole cause.

Inevitably, this degraded sludgy fluid arrives back at the AHC Tank -- and by the way, it goes directly to the tank courtesy of the Return Valve. The returning fluid does not to through the Pump. Then it is drawn from the Tank and pushed back in to the system by the AHC Pump, until given enough time and neglect, the strainers inside the AHC Pump, especially the Inlet Strainer, finally block -- at which point C1762 is inevitable.

Meanwhile, the AHC Pump has been operating in this degraded fluid.

Wear increases –

• at the gear teeth, and,
• where the hard steel gears are pressed into the side-walls of the softer cast-aluminium housing, and,
• especially where the hard steel gears are pressed onto the ‘floor’ of the softer cast-aluminium housing by the designed internal pressure within the pump acting on hard steel gear cases -- the purpose of which is to reduce fluid bypass across the top and bottom surfaces of the gears -- which is a fine and effective idea until it is eventually defeated by wear and scoring in the pump housing below the gears.

In addition, the Pump will slowly and progressively be ‘starved’ of fluid as the Inlet strainer slowly blocks -- until as mentioned, eventually DTC C1762 is triggered – full title: “Fluid Pressure Abnormality – Pump & Motor Does Not Supply Fluid”.

Long story short -- personal view: When opening and massively disrupting the AHC system and undertaking major, expensive replacements on these aged vehicle with unknown or doubtful AHC maintenance history, spend the additional USD100 and the additional half-hour replacing the AHC Pump as well, then close up the system, prime the new AHC Pump (or just let fluid settle into it) -- all in the interests of greater confidence in ongoing reliability and AHC ride comfort. So congratulations on getting this done already.

Further difficulties were reported by @suprasvobodea at posts #315 and #317 in the same thread.

It is also clear from posts #318, #319 and #320 in the same thread that there was some response from the AHC system but that DTC C1762 quickly returned.

At least it is known that the system will work when DTC C1762 is resolved -- bleeding out air or resolving whatever other cause.

Posts #14 and onwards by @suprasvobodea are noted, along with the very experienced and thoughtful replies which follow from @2001LC and @suprarx7nut at

LX470 AHC issues - pressure sensor 1998 Vs. 2002 - https://forum.ih8mud.com/threads/lx470-ahc-issues-pressure-sensor-1998-vs-2002.1267066/#post-15442867

On Levelling Valves ….

As @IndroCruise, I certainly wrote at Post #312 at

AHC Inop after fluid change 2007 lx C1751 and C1762 - https://forum.ih8mud.com/threads/ahc-inop-after-fluid-change-2007-lx-c1751-and-c1762.1233088/page-16#post-15345982

• The “Levelling Valves” (one Front, one Rear) are ‘normally closed’,
• The vehicle cannot be raised or lowered unless the Levelling Valves are caused to open by the ECU – or unless the vehicle is lowered by dumping AHC Fluid from the bleeder screws,
• Operating the AHC Pump without opening the Levelling Valves (such as by direct 12 volt connection to the AHC Pump Motor) cannot raise the vehicle because there is no signal from the ECU to the Levelling Valves to cause them to open,

There is an error in the detail offered here in the first bullet point. There is a Levelling Valve for each Gas Chamber a.k.a. ‘globe’ -- so there are four Levelling Valves in total in the Control Valve Assembly. I will edit the referenced post for that detail.

Not mentioned above but it is also worth noting that there are also four Gate Valves in the Control Valve Assembly -- one for each Gas Chamber a.k.a. ‘globe’. Relevantly, the Gate Valves are set “Normally Open” and remain so unless the vehicle is turning at the prescribed angle and at the prescribed speed. The effect is that when the vehicle is stationary -- such as in test conditions -- the fact that the Gate Valves are open means that the AHC pressures at Left Front and Right Front are equalised, and, the pressures at Left Rear and Right Rear are equalised. This means that the Front raises (or falls) equally on both sides. Ditto at the Rear. The LC100/LX470 system does not allow each corner to be raised or lowered independently, unlike some other vehicles. Also, there is no hydraulic connection at the Control Valve Assembly between Front and Rear -- so Front and Rear can and do operate independently (but not side to side).

The FSM extract below describes how the Levelling Valves are intended to work. In part, my opinion above is based on this extract.

I note that @suprarx7nut had a similar opinion at his post #250, as did @Moridinbg at his consecutive post #251, both here -- The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-13#post-14466603

The FSM description is that each Levelling Valve is “Normally Closed”. That much is quite clear. However, is there a further error here??

This extract can be taken to mean that the ECU must transmit EITHER an “Open” signal OR a “Close” signal to cause the relevant action by each of the Levelling Valves.

My questions to self are: Is this assertion correct? Or is it the case that the “Normally Closed” position of the Levelling Valves is established by a spring pressure, and, the “Open” position of the Levelling Valves is caused by a signal from the ECU causing the solenoids to be energised, in turn causing the Levelling Valves to open?? Close inspection of the diagram could suggest this.

If the Levelling Valves are spring-loaded to be “Normally Closed”, then maybe it could be expected that sufficient AHC Pump pressure -- such as by powering the AHC Pump directly from a 12 volt battery -- would cause fluid to force open the Levelling Valves, enable AHC Fluid to flow to the ‘shock absorbers’ and cause the vehicle to rise. It also could be expected that if a bleeder valve was opened in this condition, then bleeding would occur under pump pressure.

However, the earlier posts referenced above by @suprarx7nut and @Moridinbg suggest that this cannot happen -- that it is not possible to use direct 12 volt battery operation of the AHC Pump Motor and cause the AHC Pump to push past the closed Levelling Valves.

‘My’ 2006 LC100 is now in the hands of my son-in-law in Perth on the coast of Western Australia, whereas I reside about 4,300 kilometres (2,700 miles) distant in Brisbane on the East coast of Australia. If it were not for this ‘inconvenience’, I would happily experiment further to see how to open the Levelling Valves remotely, somehow.

All that said, I am more than happy to have any mistakes I may have made pointed out by anyone -- and I will correct them -- and hopefully that may provide @suprasvobodea with some of the reassurance he seeks.

On Pressure Sensor Test Voltages ….

I cannot contribute anything new on the dilemma of the consistently low reading of 0.505 volt reported by @suprasvobdea, equally when testing both the new and the old Pressure Sensor. I presume the tests were conducted exactly per FSM description, including with the engine running (not just with the ignition “ON”).

The fact that that the same erroneous low-voltage reading with two Pressure Sensors strongly suggests an open circuit somewhere -- meaning a broken wire.
This seems to be a likely cause of the "fail safe mode" which is preventing the vehicle being raised in the usual way to enable bleeding to proceed.

The hunt can be challenging. The discussion between @2001LC, @suprarx7nut and @Moridinbg provides a worthwhile review of their past experiences and gives some ideas on where to look -- starting at Post #249 through to Post #260 at this thread:

The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-13#post-14465654

A good Electrical Wiring Diagram (EWD) may be helpful -- a version can be found here at Post #420 by @usedname . There may be some differences between model years – but generally the AHC/TEMS circuits are very consistent. See:

The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-21#post-14909889

The relevant sections of the Workshop Manual may be helpful – and the LX470 FSM may be on hand. If not, the equivalent on-line version for LC100 (with AHC where fitted) is found here:
LC100 Workshop Manual - https://lc100e.github.io/
Scroll to
+ Repair Manual
+ DIAGNOSTICS
+ ACTIVE HEIGHT CONTROL SUSPENSION & SKYHOOK TEMS

 

[suprarx7nut]

It won’t stop giving the 1762 code every time I start the active test!! Running the pump directly…. Won’t pump oil into anything!!!!. So that’s why I am stuck… & in an endless “do loop” !! So if it’s not electrical short, not a bad PS , and I do have air in system…. WHY Can’t I get the active test to work ?? WHAT AM I MISSING ? I HAVE to run the active test to bleed system of air or I’ll NEVER Drive MY LX AGAIN ! What else should I try ???? I’m at the end of the road here in Dallas TX …

Active test commonly fails to run with air in the system. That's just how it works (which is frustrating).

What I would do: Wire the pump directly. Force it to run. You've got a backup, right? Let her rip. If you kill the pump (which I'm almost certain you won't) no biggie.

I'm telling you, the Active Test almost never works when you want/need it to. It's a great first step. If that won't run, you need to force the pump manually.

The TSM (or FSM if you like) is wrong. It will lead you astray.

Force the pump to run and flush. That's it.

The pump will move fluid around within itself and the plumbing near itself. Cracking the outlet line loose while the pump runs may also help (but be very careful of pressurized hydraulic fluid...).

There is no TSM-prescribed way out of this, haha. You need to color outside the lines a little here.

 

[suprasvobodea]

Active test commonly fails to run with air in the system. That's just how it works (which is frustrating).

What I would do: Wire the pump directly. Force it to run. You've got a backup, right? Let her rip. If you kill the pump (which I'm almost certain you won't) no biggie.

I'm telling you, the Active Test almost never works when you want/need it to. It's a great first step. If that won't run, you need to force the pump manually.

The TSM (or FSM if you like) is wrong. It will lead you astray.

Force the pump to run and flush. That's it.

The pump will move fluid around within itself and the plumbing near itself. Cracking the outlet line loose while the pump runs may also help (but be very careful of pressurized hydraulic fluid...).

There is no TSM-prescribed way out of this, haha. You need to color outside the lines a little here.

Indocruise and SupraX7nut,

I was thinking last night after I emptied and removed and disassembled the tank and pump motor to check the Large and small O_rings for the pump cover and the Seal between the motor and pump. I plan on refilling it today, by manually running the pump with 12V directly and opening the bleeder valve on the accumulator to remove the air in the lines after installing the pump assy. I wanted to try this BEFORE the new PS arrived from IMPEX later this week and see if what you guys are saying is true.... ( that you have never had or seen a PS go bad)
Remember, I said that I filled the reservoir within 1" , and started the car after installing a new PS. After clearing the 1762 code and doing the ACTIVE test, it started working. I got it to N !! ( About 3/4 of the reservoir ( 1.2 Liter) was input into the system!) I didn't notice that the tank was empty and heard it run dry and stopped the engine immediately. What happened next is what came to me last night.
I restarted the car and tried to run the pump again after filling it. But it would not work (probably due to the 1762 code), and then all the oil that was pumped into the struts and accumulator suddenly was now returning INTO THE TANK AND IT STARTED OVERFLOWING onto the ground! The question I have is..............

? How could this happen if the solenoid/gate valves/leveling valves are supposed to be closed? Maybe this is nothing or there is a perfect logical explanation for this to happen. But the oil that WAS pumped into the system, was pushed back into the tank and it overflowed all over the ground before I could stop it. ! THATS .. my question. Why didn't the oil stay in the system after I put more in the tank after it ran dry ? BTW- the LX is and has been on the stops, so it's really low to the gound .!

 

[IndroCruise]

Indocruise and SupraX7nut,

I was thinking last night after I emptied and removed and disassembled the tank and pump motor to check the Large and small O_rings for the pump cover and the Seal between the motor and pump. I plan on refilling it today, by manually running the pump with 12V directly and opening the bleeder valve on the accumulator to remove the air in the lines after installing the pump assy. I wanted to try this BEFORE the new PS arrived from IMPEX later this week and see if what you guys are saying is true.... ( that you have never had or seen a PS go bad)
Remember, I said that I filled the reservoir within 1" , and started the car after installing a new PS. After clearing the 1762 code and doing the ACTIVE test, it started working. I got it to N !! ( About 3/4 of the reservoir ( 1.2 Liter) was input into the system!) I didn't notice that the tank was empty and heard it run dry and stopped the engine immediately. What happened next is what came to me last night.
I restarted the car and tried to run the pump again after filling it. But it would not work (probably due to the 1762 code), and then all the oil that was pumped into the struts and accumulator suddenly was now returning INTO THE TANK AND IT STARTED OVERFLOWING onto the ground! The question I have is..............

? How could this happen if the solenoid/gate valves/leveling valves are supposed to be closed? Maybe this is nothing or there is a perfect logical explanation for this to happen. But the oil that WAS pumped into the system, was pushed back into the tank and it overflowed all over the ground before I could stop it. ! THATS .. my question. Why didn't the oil stay in the system after I put more in the tank after it ran dry ? BTW- the LX is and has been on the stops, so it's really low to the gound .!

? How could this happen if the solenoid/gate valves/leveling valves are supposed to be closed?

If I understand the reported sequence correctly:

1. New Pressure Sensor installed,
2. AHC Tank filled within 1” of the top,
3. Started engine,
4. Cleared existing DTC C1762,
5. Started Active Test,
6. Vehicle rose and green light for N indication appeared,
7. Vehicle rose to standard height (means “N” height) or more -- unclear whether light was flashing or solid, unclear what height vehicle had reached – “N” height? More? Less?
8. About ¾ of AHC Tank or about 1.2 litres was pumped into the system,
9. “Heard” AHC Pump “run dry” (not sure what this means – as highlighted in the attachment below, the AHC Pump will stop in 0.6 seconds if it runs out of fluid and DTC C1762 will be indicated),
10. Stopped engine,
11. Refilled AHC Tank,
12. Restarted engine -- but AHC Pump did not restart – or maybe ran for 0.6 seconds before DTC C1762 shut it down again???
13. Vehicle returned to bump-stops (means “LO” height or lower),
14. AHC Fluid returned from ‘shock absorbers’ to AHC Tank which had been refilled -- so an overflow was experienced

Firstly, just to be clear, and as fully described in an earlier post, the Gate Valves and Levelling Valves are different devices with different functions and operate in different ways -- please avoid confusion by NOT bracketing them together. “…. the solenoid/gate valves/leveling valves are supposed to be closed” is neither a correct nor helpful statement.

Secondly, the Levelling Valves ARE “normally closed” unless directed otherwise by the ECU – as described in the FSM extracts inserted in a previous post. [Whatever misgivings some may have experienced with the FSM, it is and remains the best documented guide available, aided and clarified by the reported experiences on IH8MUD and elsewhere].

You have experienced a repeat of DTC C1762, probably interrupted when the engine was turn OFF the first time, and repeated again when the engine was restarted.

This would certainly occur in the sequence described) and to go with this DTC you have experienced the “Fail Safe Function” prescribed in the FSM for this DTC and which is governed by the ECU -- see extract below -- specifically:

• height control is prohibited -- so the AHC Pump stops as directed by the ECU, and,
• the vehicle height is adjusted (by the ECU opening the Levelling Valves) to the standard height (meaning “N” height), OR,to the height of the lowest of the four wheels – and in this case the ECU has used inputs from the Height Control Sensors and determined that the relevant height is “LO” height and so the vehicle has been lowered accordingly, and,
• the height selected by the ECU for the purposes of this “Fail safe function” will be determined by the voltage signals coming from the Height Control Sensors to the ECU -- these in turn will be affected by the condition of the Height Control Sensors AND any slope on which the vehicle is parked.

The reported experience including the return fluid and the overflow is exactly as would be expected from the given description -- all part of the learning experience which we all have been through ....

 

[suprasvobodea]

? How could this happen if the solenoid/gate valves/leveling valves are supposed to be closed?

If I understand the reported sequence correctly:

1. New Pressure Sensor installed,
2. AHC Tank filled within 1” of the top,
3. Started engine,
4. Cleared existing DTC C1762,
5. Started Active Test,
6. Vehicle rose and green light for N indication appeared,
7. Vehicle rose to standard height (means “N” height) or more -- unclear whether light was flashing or solid, unclear what height vehicle had reached – “N” height? More? Less?
8. About ¾ of AHC Tank or about 1.2 litres was pumped into the system,
9. “Heard” AHC Pump “run dry” (not sure what this means – the AHC Pump will stop in 0.6 seconds if it runs out of fluid and DTC C1762 will be indicated),
10. Stopped engine,
11. Refilled AHC Tank,
12. Restarted engine -- but AHC Pump did not restart – or maybe ran for 0.6 seconds before DTC C1762 shut it down again???
13. Vehicle returned to bump-stops (means “LO” height or lower),
14. AHC Fluid returned from ‘shock absorbers’ to AHC Tank which had been refilled -- so an overflow was experienced

Firstly, just to be clear, and as fully described in an earlier post, the Gate Valves and Levelling Valves are different devices with different functions and operate in different ways -- please avoid confusion by NOT bracketing them together. “…. the solenoid/gate valves/leveling valves are supposed to be closed” is neither a correct nor helpful statement.

Secondly, the Levelling Valves ARE “normally closed” unless directed otherwise by the ECU – as described in the FSM extracts inserted in a previous post. [Whatever misgivings some may have experienced with the FSM, it is and remains the best documented guide available, aided and clarified by the reported experiences on IH8MUD and elsewhere].

You have experienced a repeat of DTC C1762, probably interrupted when the engine was turn OFF the first time, and repeated again when the engine was restarted.

This would certainly occur in the sequence described) and to go with this DTC you have experienced the “Fail Safe Function” prescribed in the FSM for this DTC and which is governed by the ECU -- see extract below -- specifically:

• height control is prohibited -- so the AHC Pump stops as directed by the ECU, and,
• the vehicle height is adjusted (by the ECU opening the Levelling Valves) to the standard height (meaning “N” height), OR,to the height of the lowest of the four wheels – and in this case the ECU has used inputs from the Height Control Sensors and determined that the relevant height is “LO” height and so the vehicle has been lowered accordingly, and,
• the height selected by the ECU for the purposees of this “Fail safe function” will be determined by the voltage signals coming from the Height Control Sensors to the ECU -- these in turn will be affected by the condition of the Height Control Sensors AND any slope on which the vehicle is parked.

The reported experience including the return fluid and the overflow is exactly as would be expected from the given description -- all part of the learning experience which we all have been through ....

View attachment 3616349

Okay, then if that’s what would be expected… then that ok . I’m going to manually run pump this afternoon to get fluid to the accumulator, before starting engine. This is BEFORE I get the new PS . Since everyone is saying they never encountered a failed pressure switch then I’d like to confirm that for my own belief as well.

However the TSM says that The pressure switch has to be a specified voltage and I’m far below that so my assumption is it’s sending a signal to the ECU not to work I have to work with that information because as you say ….the TSM is The printed documentation or as I call it “the Bible”. Everything else is…. opinion & experience

Should I try to raise the truck up on jackstands to take the load off the cylinders and the hydraulic lines before I activate the active test? Would that make any difference??

 

[2001LC]

"Devil is in the detail". You're a saint, of few words.

The detail of every step of service, order worked on events. Helps, paint a picture for all to see. Seems you've painting with broad strokes, in various threads and individual post. Seeing the whole picture for what it is, is difficult. I feel it's in parts my fault. As I gueston your assumption "running pump dry will damage PS sensor" in this thread. When you apparently had a working thread on your issue elsewhere.

Indocruise and SupraX7nut,

I was thinking last night after I emptied and removed and disassembled the tank and pump motor to check the Large and small O_rings for the pump cover and the Seal between the motor and pump. I plan on refilling it today, by manually running the pump with 12V directly and opening the bleeder valve on the accumulator to remove the air in the lines after installing the pump assy. I wanted to try this BEFORE the new PS arrived from IMPEX later this week and see if what you guys are saying is true.... ( that you have never had or seen a PS go bad)
Remember, I said that I filled the reservoir within 1" , and started the car after installing a new PS. After clearing the 1762 code and doing the ACTIVE test, it started working. I got it to N !! ( About 3/4 of the reservoir ( 1.2 Liter) was input into the system!) I didn't notice that the tank was empty and heard it run dry and stopped the engine immediately. What happened next is what came to me last night.
I restarted the car and tried to run the pump again after filling it. But it would not work (probably due to the 1762 code), and then all the oil that was pumped into the struts and accumulator suddenly was now returning INTO THE TANK AND IT STARTED OVERFLOWING onto the ground! The question I have is..............

? How could this happen if the solenoid/gate valves/leveling valves are supposed to be closed? Maybe this is nothing or there is a perfect logical explanation for this to happen. But the oil that WAS pumped into the system, was pushed back into the tank and it overflowed all over the ground before I could stop it. ! THATS .. my question. Why didn't the oil stay in the system after I put more in the tank after it ran dry ? BTW- the LX is and has been on the stops, so it's really low to the gound .!

You never ran dry and then overfilled. Likely reservoir wasn't dry. You "heard" a very common gown (not due to low fluid) we sometimes get, which is normal. IIRC, I mentioned that earlier.

If shock pre filled and no leak. Than you did not fill reservoir or it was not empty as you thought (heard). You stated you pre filled shock. But seems I now read you compressed shocks, before attaching HP fitting. I'm so confused. I had assumed you meant jack-up LCW until shock seated in upper frame enough to attach nut. Which may compressed shock just a tad, incidentally. You then torqued down top nut and had bottom bolt in place. Removed the jack, allowing full drop of LCA and full extension of shock. Which drop of LCA, would have (should have) only be a few MM at most. Topped shock with fluid, what should have been only a few drops needed and then attached HP fitting.

I, just had a case come in my shop. Where a DIYer was sure reservoir had fluid in bottom after running pump, when it went into failsafe. Reservoir was stained and just looked as if had fluid. I place a light behind his stained reservoir, found it empty. In addition one of his globes he'd just R&R, O-ring was out of position and leaking. All his globes were wet, as he had just R&R. But one area around a globe, just a little wetter than others.


"Can pump run, gates closed and pump fluid out through height accumulator bleeder?"
I've not tried it. But based on below diagram, YES! Try it! Just remember: Never put your body or limbs in position of danger under vehicle.

If pressure to high, because to little weight of vehicle on T-bar and coils. Raising with jack, taking weight off hydraulic (AHC system), will help.



Electrical Shorts: With them, often we can't clear DTC.

What I've seen with some shorts. DTC will often, kick system into failsafe instantly when key ON (before engine start), where they can't be cleared until short corrected. One exception to this, was the one I mention. Where it was intermittent and a resistance issue due to moisture in a wire housing connection block.

A short could be from when you replace the accumulator, remove wire housing connector block. Go back and check ever wire housing block you touched. Make sure no wires pulled, pins bent and that each plugged in until clicks/locks. The accumulator plugs would be one suspect, but then how could it have worked after R&R. Perhaps, intermittent fault. Which it did work, based on statements you made: i.e. The 2 to 3 months with two or three drives after it installed globes & ACCU. later mention: Time to raise 20 sec. But if a wire shorting, by just touching making intermittent contact or wire housing block not secure. You could get intermittent faults.The other wires you touched would have been at pump area, check them too. We also can't rule out moisture or rodents. Rodent eat wires, in moments, when vehicle parked.

In a link, I posted earlier in this thread. To a thread where I had a short in undercarriage wires and DTC C1762. You'll see much of wire routing in pictures. Look over all your wiring and wiring housing connector blocks. If no apparent damage or loose wire housing blocks. You may need to start checking each wires' continuity. Which simple means: a wire has not breaks, so resistance can be read.

A continuity test is a quick check to see if a circuit is open or closed. Only a closed, complete circuit (one that is switched ON) has continuity. During a continuity test, a digital multimeter sends a small current through the circuit to measure resistance in the circuit.

Place your multimeter into continuity in OMS to its tone setting. Touch one prob to the other, you'll hear tone. Now test each wire end to end. By disconnecting wire housing blocks at each end of harness, and inserting probes in select pins of wire of same color. Easier than exposing color of wire, is place prob at one pin in a wire housing block at one end of harness. The use other prob at other end of harness, touching each pin in a wire housing block. One will give a tone! No tone, means a wire dead.

You can also use needle point porbs, and pierce wire sheathing. Best to have a hepler.

The controller and accumulator wire are where I'd start. First disconnecting their wire harness connecting block. Test the control and accumulator themselve (not wire) first (see your FSM) for which pins. Then while those wire housing's blocks off, start testing wire to next wire housing blocks upstream on harness. They come from rear undercarriage, nearest rear bumper. IIRC there are 3 wire blocks up there. Two of the blocks have AHC wire, among others.

Notice that small inner chamber. That is the one I believe we get fluid from on the second full extension, when bench flushing.

Imagine when we open rear & front globe bleeders on one side, dropping vehicle to the bump stops. This push fluid out of main chamber, into line and out bleeder. We open accumulators bleed, get more. We suck remaining from reservoir. That totals ~ 2.5L.
Fluid remaining in system, is in the inner chamber of shock, small amount in main camber, the tube from bottom to top of shock where HP fitting connect, HP flitting, lines to controller valves, pump and lines down stream of it. This are also place air pocket can be. The shock is most difficult, to get air out.

 

[suprasvobodea]

"Devil is in the detail". You're a saint, of few words.

The detail of every step of service, order worked on events. Helps, paint a picture for all to see. Seems you've painting with broad strokes, in various threads and individual post. Seeing the whole picture for what it is, is difficult. I feel it's in parts my fault. As I gueston your assumption "running pump dry will damage PS sensor" in this thread. When you apparently had a working thread on your issue elsewhere.


You never ran dry and then overfilled. Likely reservoir wasn't dry. You "heard" a very common gown (not due to low fluid) we sometimes get, which is normal. IIRC, I mentioned that earlier.

If shock pre filled and no leak. Than you did not fill reservoir or it was not empty as you thought (heard). You stated you pre filled shock. But seems I now read you compressed shocks, before attaching HP fitting. I'm so confused. I had assumed you meant jack-up LCW until shock seated in upper frame enough to attach nut. Which may compressed shock just a tad, incidentally. You then torqued down top nut and had bottom bolt in place. Removed the jack, allowing full drop of LCA and full extension of shock. Which drop of LCA, would have (should have) only be a few MM at most. Topped shock with fluid, what should have been only a few drops needed and then attached HP fitting.

I, just had a case come in my shop. Where a DIYer was sure reservoir had fluid in bottom after running pump, when it went into failsafe. Reservoir was stained and just looked as if had fluid. I place a light behind his stained reservoir, found it empty. In addition one of his globes he'd just R&R, O-ring was out of position and leaking. All his globes were wet, as he had just R&R. But one area around a globe, just a little wetter than others. (My Globes are all dry - i thoguht of that.


"Can pump run, gates closed and pump fluid out through height accumulator bleeder?"
I've not tried it. But based on below diagram, YES! Try it! Just remember: Never put your body or limbs in position of danger under vehicle.

View attachment 3616250

If pressure to high, because to little weight of vehicle on T-bar and coils. Raising with jack, taking weight off hydraulic (AHC system), will help.



Electrical Shorts: With them, often we can't clear DTC.

What I've seen with some shorts. DTC will often, kick system into failsafe instantly when key ON (before engine start), where they can't be cleared until short corrected. One exception to this, was the one I mention. Where it was intermittent and a resistance issue due to moisture in a wire housing connection block.

A short could be from when you replace the accumulator, remove wire housing connector block. Go back and check ever wire housing block you touched. Make sure no wires pulled, pins bent and that each plugged in until clicks/locks. The accumulator plugs would be one suspect, but then how could it have worked after R&R. (Yes- it DID WORK ! untill it emptied the resevoir and i heard it make that noise! Perhaps, intermittent fault. Which it did work, based on statements you made: i.e. The 2 to 3 months with two or three drives after it installed globes & ACCU. later mention: Time to raise 20 sec. But if a wire shorting, by just touching making intermittent contact or wire housing block not secure. You could get intermittent faults.The other wires you touched would have been at pump area, (There are only 3 connectors at pump that i touched -1. Power to pump, 2,. RPressure sensor ( shows 0.5V at the connector) and 3. Temp connector. check them too. We also can't rule out moisture or rodents. Rodent eat wires, in moments, when vehicle parked.

In a link, I posted earlier in this thread. To a thread where I had a short in undercarriage wires and DTC C1762. You'll see much of wire routing in pictures. Look over all your wiring and wiring housing connector blocks. If no apparent damage or loose wire housing blocks. You may need to start checking each wires' continuity. Which simple means: a wire has not breaks, so resistance can be read.

A continuity test is a quick check to see if a circuit is open or closed. Only a closed, complete circuit (one that is switched ON) has continuity. During a continuity test, a digital multimeter sends a small current through the circuit to measure resistance in the circuit.

Place your multimeter into continuity in OMS to its tone setting. Touch one prob to the other, you'll hear tone. Now test each wire end to end. By disconnecting wire housing blocks at each end of harness, and inserting probes in select pins of wire of same color. Easier than exposing color of wire, is place prob at one pin in a wire housing block at one end of harness. The use other prob at other end of harness, touching each pin in a wire housing block. One will give a tone! No tone, means a wire dead.

You can also use needle point porbs, and pierce wire sheathing. Best to have a hepler.

The controller and accumulator wire are where I'd start. First disconnecting their wire harness connecting block. Test the control (What is "the CONTROLER " The AHC ECM connector with 3 connectors in it, by RH side next to the steering )? and accumulator themselve (not wire) first (see your FSM) for which pins. Then while those wire housing's blocks off, start testing wire to next wire housing blocks upstream on harness. They come from rear undercarriage, nearest rear bumper. IIRC (What is the IIRC ...??) there are 3 wire blocks up there. Two of the blocks have AHC wire, among others.

Notice that small inner chamber. That is the one I believe we get fluid from on the second full extension, when bench flushing.
View attachment 3615360
Imagine when we open rear & front globe bleeders on one side, dropping vehicle to the bump stops. This push fluid out of main chamber, into line and out bleeder. We open accumulators bleed, get more. We suck remaining from reservoir. That totals ~ 2.5L.
Fluid remaining in system, is in the inner chamber of shock, small amount in main camber, the tube from bottom to top of shock where HP fitting connect, HP flitting, lines to controller valves, pump and lines down stream of it. This are also place air pocket can be. The shock is most difficult, to get air out.

2001 LC

PLEASE SEE MY ANSWERS IN RED TO YOUR STATEMENTS/QUESTIONS ABOVE. What is IIRC ..?
Update after 2 hr of diagnostic fun :
I started doing the electrical troubleshooting per your suggestion
I disconnected the Accumulator 2-pin connector and checked impedance through all the intermediate connectors directly to the AHC ECU pin #24 (SLB) and pin 6 (SLAC) on the AHC ECU. I got 3 ohms on both leads from the connector back to the ECU so they are good! Also got 3 ohms on the 3 wires on the pressure switch connector back to the ECU. That is the only connector I touched besides the 3 pump connectors in the engine bay.
I think I found that gray connector in the DS rear axle on top of the frame, but it is even MORE inaccessible than the top of the strut that I cut a hole to access the top two 10 mm bolts and end cap! I too had to raise the rear with jack stands, as my LX is resting on the stops (and is as low as a Corvette now,). Please let me know if anyone can tell me HOW to access this large gray connector that rests on top of the frame rail on the driver's side I might try to disconnect it, but it may not be worth it. I don't think I need to go there, as the system DID WORK with a new PS after I installed it and I ran the reservoir to almost empty and heard that "noise " of an empty tank! and shut down the engine.
I cleared the 1762 code and as soon as I turned on the car and did the active test by pushing the DOWN switch, it immediately got 1762.

Im again past frustrated and ready to sell this damn thing. The electrical wire issue I tried ( between the Accumulator and ECU was good. ) I can't access the large gray connector on top of the frame on the Drives side rear wheel ( per 2001LC -body-off-frame pictures in post

The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-13#post-14465654

from 2001 LC is nice, but the truck's body is removed! Since my LX was working before I opened up the system and replaced everything, and I didn't touch anything electrical under the back rear vehicle ( just the midsection to remove the supply line from the accumulator to the pump attenuator by the PS frame- due to a rounded flare nut!!!! ).

If ANYONE reading this thread has any more specific ideas about specific connectors or impedance checks on specific wire harnesses per the TSM or your own experience, PLEASE let me know. I truly appreciate all the suggestions so far and have implemented them ( except for that large gray rear connector on top of the DS rear frame rail- as explained above)

All I can think of anymore ., is to wait till another new PS arrives, install it, fill the reservoir, start the LX with the ACTIVE TEST, and see if it will work again. I am truly at the end of my game here and don't know what to try to think to do anymore...... I wish one of you guys was close to Dallas TX. I would like to meet you first of all, and then tackle this problem, in the flesh until we beat it int submission.