LX470 AHC issues - pressure sensor 1998 Vs. 2002 (3 Viewers)

[IndroCruise]

Cograts! Which "Russian Guy" video are you referring to?

@Moridinbg is referrig to the video at my Post #139 this thread. See also the discussion and pictures at Post #132 this thread.

 

[Sameer]

I have been following this thread since a while now.

I acquired a lx470 98 almost a 9 months ago. It was in decent condition and was taken from a auction site with no details of prev owner. the car had been put on LC100 Shocks with the Globes missing.

I had the wild idea to restore this as i wanted to inflict pain on myself
so i gathered all the possible parts needed from Impex and local yards till i finally have all the parts since last week.

I have gotten the
1. New Globes
2. New bushing , O - rings (Shock top)
3. Used Shocks (apparently AHC shocks dont go bad)

The Car has the 1762 Death Code, and the whole system will have a ton of Air as the globes were missing and globe outlets open. The shocks hoses were open and just kept hanging with LC 100 shocks installed.

I am going to follow the narrative @suprarx7nut and others too stating that flush flush flush flush flush flus fls flushh .. untill the system becomes super clear.

There was a 50A AHC fuse which was blown which i replaced. and now im giving it direct 12v power via the battery terminal to push fluid around. There was success last night in getting fluid to come out of the accumulator bleed valve and dirt / old oil was out and now its cleaner fluid.

Next step would be to get the same bleed to happen at the other 4 globe bleeder location.

Few queries on this
1. Should i try the bleed with the engine on ? car in ON position ?
2. Will the solenoid in any part of the system prevent oil from going to the globes units across the car?

It was very interesting to read that the return valve seemed to be the biggest culprit in this re-build for @suprasvobodea and would love to understand what removed the dreaded error.

Thanks and its been great following this thread!

PS: attaching initial Techstream readings, It doesnt allow me any active tests / running the motor via techstream thus using straight 12v power

 

[medtro]

Hi @medtro. Sounds like we're in the same boat at the same time. My understanding of the repeat bleed is to leave the bleeder open until the last step. Here's my ongoing thread if it can be of any help.

AHC pump assembly - bench test NG

Hello All. Currently trying to diagnose a persistent DTC1762 on my 2003 LX470. Lots of diagnostic history I can share at some point, but right now I'd like to see if anyone has any input specifically on getting flow from the pump assembly. I've pulled the pump assembly and have bench tested it...

forum.ih8mud.com

Thanks for your comment and link.

Hope you are doing well @IndroCruise. Still waiting for your clarification..

 

[IndroCruise]

Thanks for your comment and link.

Hope you are doing well @IndroCruise. Still waiting for your clarification..

Apologies offered to @medtro -- I thought I had posted a response long, long ago -- before the distraction of extensive travels in Australia -- and before more recent travels in South America and Antarctica but without the LC100. Instead the response to @medtro was found in my Drafts folder during a clean-up!!

Anyway, it is posted now in case it is of assistance to someone, somewhere ....

First, it is necessary to go back to the whole of Post #132 – and some of the Posts before and after in this thread:
LX470 AHC issues - pressure sensor 1998 Vs. 2002 - https://forum.ih8mud.com/threads/lx470-ahc-issues-pressure-sensor-1998-vs-2002.1267066/page-7#post-15549367

This part of the thread is a very long discussion involving @IndroCruise, @2001LC, @suprarx7nut, @Moridinbg and others to assist @suprasvobodea with a systematic process, working downstream (top to bottom) from the AHC Tank and the AHC Pump, to identify the cause and fix his very frustrating experience with DTC C1762 causing a longlasting prohibition of AHC/TEMS operation.

As is well-known, DTC C1762 most commonly (but not exclusively) is associated with gas (meaning air or nitrogen) in the system -- either caused by air ingress while the system is open for other repairs, or, leaking seals within the system, or, from nitrogen from a defunct ‘globe’ or ‘globes’ needing replacement.

For this reason, after ‘globes’ have been tested and given satisfactory proof of their condition -- meaning much more than 7 graduations of level difference at the AHC Tank between “LO” height and “HI” height, and, after there is confidence in the AHC Pump performance, then multiple patient and persistent repetitive bleeding efforts of the AHC Fluid in the AHC/TEMS system usually is the next recommendation.

Repetitive bleeding usually is successful IF gas-in-fluid truly is the cause and IF the ‘globes’ do not have heavy and continuing nitrogen leakage.

It is worthwhile to eliminate the gas-in-fluid possibility before looking for more complex causes.

(Aside: Eventually, it turned out that a faulty AHC Return Valve, not gas-in-fluid, was the cause in the @suprasvobodea case, as described in earlier posts in this thread. A later post in another thread linked below by @LX470YYC has provided further excellent pictures and explanations of finding and fixing AHC Return Valve blockage issues -- which likely are becoming more common as AHC-equipped LC100/LX470 vehicles age and especially where AHC Fluid changes have been neglected for long periods of time .…

AHC pump assembly - bench test NG

Hello All. Currently trying to diagnose a persistent DTC1762 on my 2003 LX470. Lots of diagnostic history I can share at some point, but right now I'd like to see if anyone has any input specifically on getting flow from the pump assembly. I've pulled the pump assembly and have bench tested it...

forum.ih8mud.com

-- see pics at Post #13 and onwards in this thread.
This also may answer a subsequent question by @Sameer at Post #182 this thread).

Anyway, if all is good otherwise (meaning the AHC/TEMS systems, especially 'globes', Pump, Return Valve, wiring harness, connectors, etc), but air is still trapped somewhere in the AHC system, then the bleeding process can become “stuck” or frustrated due to repetitive appearances of DTC 1751 and/or DTC C1762.

These DTC’s are triggered in response to the yellow-highlighted “Detecting Conditions” as defined in the FSM extracts attached below.

The point is that if either of these DTC’s occurs, then the ECU causes the Levelling Valves in the Control Valve Assembly to resume their ‘normally closed’ position. This means that air-polluted AHC Fluid cannot pass the Levelling Valves and cannot arrive at the Bleeder Valves. The effort to 'flush' the system cannot succeed with this restriction.

The aim then remains somehow to get out of this ‘vicious circle’ and to remove the trapped compressible air causing a delay in the AHC Pump achieving the required flow and pressure performance. This delay initiates DTC 1751 and/or DTC C1762, and in turn causes the ECU to put the AHC system into "fail safe function" -- which prohibits AHC and TEMS operation.

How to overcome this?? The following process was suggested in previous post #132 in this thread and is clarified below:

With a bleeder open, pressure will not be developed and DTC C1762 will persist. The idea is to

• cycle through repetitive efforts, in which the distant bleeder screw is opened to allow Fluid and as much Air as possible to escape before the 0.6 seconds cut-off is initiated by the ECU,​
• then close the bleeder screw, clear the DTC, and attempt to build AHC pressure in repetitive attempts:

-- clear the DTC,
-- press UP switch with furthest bleeder open (driver side rear),
-- bleed,
-- receive DTC C1762,
-- close the bleeder valve,
-- clear the DTC,
-- press UP switch with furthest bleeder open (driver side rear),
-- bleed,
-- receive DTC C1762,
-- close the bleeder valve,
-- clear the DTC,
-- press UP switch with furthest bleeder open (driver side rear),
-- bleed,
-- receive DTC C1762,
-- close the bleeder valve,
-- clear the DTC,
-- etc, etc

Repeat over and over and over again -- this is a tedious "inching" process which attempts to increase the AHC system pressure and raise the vehicle little by little, recognising that the DTC C1762 will be activated repeatedly until sufficient pressure is developed.

Air in the system is the most common cause of DTC C1762 -- it occurs because pressure cannot be developed quickly enough (within 0.6 seconds) and DTC C1762 is triggered.

With a bleeder open, pressure will not be developed and DTC C1762 will persist. The idea is to keep the distant bleeder screw open to allow Fluid and as much Air as possible to escape, then close the bleeder screw and attempt to build pressure in repetitive attempts.

If better solutions to overcome the problem of being "stuck-in-C1762" have been found by anyone, then please post -- it would be most welcome!!

 

[2001LC]

@IndroCruise what's your thoughts in this incidents. Why did rear fail?
(Spoiler alert. My guess, valve in controller stuck. Possible due to pressure differential between front with air in fluid and good fluid in rear.)

I just had a strange incident. Where rear end, drop to LOW and bouncy.

I'll note it had been raining a bit, over the few days prior to incident. OAT between 28F and 70F. This 07LX w/175K miles, was parked outside and not driven, during rain. Found a napkin in DS door card pocket, damp. It also had not been driven much over the past 12 months. Sat parked, for ~3 months at a time. But driven more often in the weeks before incident.

Cold start and drove 1/2 mile. Noticed rear end bouncy. Returned home and parked. Drove another LX, for the day.
Notice, rear end sitting low, when I returned. Look like rear in L front in N.

1. Placed the LX, on jackstands. Open rear bleeders. Fluid only has ~10K miles 5 yrs and look good. No practicals seen suspend in fluid in jar. Expected, to see dark fluid with many practicals (globe bladder material). I just dump it in my waste oil can.
2. Removed the rear globes They looked normal (silver cross) inside and could not press in blader. Expected to see black inside (Blown).
3. Bleed Accumulator. Fluid darker, not clear like rear. I just dump it in my waste oil can. Regretted not examining closer.
4. Bleed front DS bleeder. Fluid very foamy (air). Saved and examined. Took some time for foam/air to settle out. Clear as would expect for 10K miles.
5. Drew fluid from reservoir. Clear as would expect for 10K miles.
6. Note: Let both front and accumulator fluids, settle in jars. Poured off all but bottom inch from jars. Mix and examined: Minior number of practical visible.
7. Reinstalled old rear globes.
8. Close bleeders
9. Filled reservoir with from fresh 2.5L can.
10. Started engine, doors close, AHC in L. Cracked open bleed both rears and DS front, one at a time. Minor air bubble rears, as expected with globes R&R. No air bubble in front, as expected.

System functioning normal.

Why was front fluid so foamy!
Likely a very very minor FR shock weep. Was sucking in air over last 10K miles.

Note on pressure:
DTC C1340 (CDL circuit) only. Cleared has not returned.
After flush: Frt 7.7Mpa, Rear 6.3Mpa, Acc not noted.
1 3/4 CW each T-bar. Temp (T-22F) sensor disconnected.
Frt 6.9Mpa, R 5.8Mpa, Acc 0 T-22F after short drive.
Plug in Temp sensor.
Frt 6.3Mpa, R 5.7Mpa, Acc 10.4 Mpa, T122F

Ride, a bit stiff in comfort. Feels as if riding to much on T-bars, too little on AHC. As pressure indicate, it would.

Side note:
I typically find Mpa's do not change. Once temperature sensor reconnected. In this case I saw a 0.6 Mpa drop after adjusting pressure and reconnecting Temp sensor.

Not bad looking fluid, for 10K miles.

 

[IndroCruise]

@IndroCruise what's your thoughts in this incidents. Why did rear fail?
(Spoiler alert. My guess, valve in controller stuck. Possible due to pressure differential between front with air in fluid and good fluid in rear.)

I just had a strange incident. Where rear end, drop to LOW and bouncy.

I'll note it had been raining a bit, over the few days prior to incident. OAT between 28F and 70F. This 07LX w/175K miles, was parked outside and not driven, during rain. Found a napkin in DS door card pocket, damp. It also had not been driven much over the past 12 months. Sat parked, for ~3 months at a time. But driven more often in the weeks before incident.

Cold start and drove 1/2 mile. Noticed rear end bouncy. Returned home and parked. Drove another LX, for the day.
Notice, rear end sitting low, when I returned. Look like rear in L front in N.

1. Placed the LX, on jackstands. Open rear bleeders. Fluid only has ~10K miles 5 yrs and look good. No practicals seen suspend in fluid in jar. Expected, to see dark fluid with many practicals (globe bladder material). I just dump it in my waste oil can.
2. Removed the rear globes They looked normal (silver cross) inside and could not press in blader. Expected to see black inside (Blown).
3. Bleed Accumulator. Fluid darker, not clear like rear. I just dump it in my waste oil can. Regretted not examining closer.
4. Bleed front DS bleeder. Fluid very foamy (air). Saved and examined. Took some time for foam/air to settle out. Clear as would expect for 10K miles.
5. Drew fluid from reservoir. Clear as would expect for 10K miles.
6. Note: Let both front and accumulator fluids, settle in jars. Poured off all but bottom inch from jars. Mix and examined: Minior number of practical visible.
7. Reinstalled old rear globes.
8. Close bleeders
9. Filled reservoir with from fresh 2.5L can.
10. Started engine, doors close, AHC in L. Cracked open bleed both rears and DS front, one at a time. Minor air bubble rears, as expected with globes R&R. No air bubble in front, as expected.

System functioning normal.

Why was front fluid so foamy!
Likely a very very minor FR shock weep. Was sucking in air over last 10K miles.

Note on pressure:
DTC C1340 (CDL circuit) only. Cleared has not returned.
After flush: Frt 7.7Mpa, Rear 6.3Mpa, Acc not noted.
1 3/4 CW each T-bar. Temp (T-22F) sensor disconnected.
Frt 6.9Mpa, R 5.8Mpa, Acc 0 T-22F after short drive.
Plug in Temp sensor.
Frt 6.3Mpa, R 5.7Mpa, Acc 10.4 Mpa, T122F

Ride, a bit stiff in comfort. Feels as if riding to much on T-bars, too little on AHC. As pressure indicate, it would.

Side note:
I typically find Mpa's do not change. Once temperature sensor reconnected. In this case I saw a 0.6 Mpa drop after adjusting pressure and reconnecting Temp sensor.

Not bad looking fluid, for 10K miles.
View attachment 3908697

Good afternoon @2001LC (updated about 2:45pm on Sunday afternoon May 25, 2025 in my part of the world - 'shock absorber' pic added later).

I hesitate to suggest anything given your knowledge and experience with 100 series but in return for your extensive contributions to this IH8MUD forum since 2007 I offer below a few thoughts in response to your request.

There is nothing here that you do not already know -- but the information and attachments may help others who come this way ....

The observations and measurements in your post #185 on this 2007 LX470 at 175,000 miles are noted, including:

“I just had a strange incident. Where rear end, drop to LOW and bouncy”.

Height Control Sensors, Connectors/Junctions and Harnesses/Wiring:

I would assume that already you would have considered the condition of the Height Control Sensors and possible wiring harness and/or connector faults, including near the hot parts of the exhaust system, and also including the connector at the Sensor itself, and also including the infamous BI1, BI2, BI3 junctions located at the chassis behind the LHS Rear Quarter panel, and other connectors and Ground Points along or above the chassis rails, such as BD1 and BD2.

As you know, an intermittent height fault can result from imperfect Height Control Sensors and circuits. The Sensor and/or circuit faults may not throw DTC’s 1711, 1712 or 1713, provided that the FSM-defined voltage and timing ranges are not exceeded at the AHC ECU -- such as when the voltage signal from a Sensor circuit remains within the specified range of 0.3 volts to 4.7 volts but does not correctly match the height of the vehicle – for example, due to poor condition or contact of the brushes or spring connectors inside a Sensor, or faults and erratic resistance in the carbon track inside the Sensor which may not be visible but causes a wrong voltage signal for the actual height. Fault conditions can occur within the FSM-specfied ranges and cause aberrant vehicle behaviours but not show as a DTC.

Alternatively, intermittency often means damage somewhere in the wiring or connectors which is causing a poor connection to “make or break” an intermittently open circuit as the vehicle moves and vibrates, or, water ingress into a junction connector is causing an intermittent short circuit. (Rain and interior dampness have been mentioned in Post #185, although outside air temperatures are warm at 28 degF to 70 degF and should have a drying effect).

AHC Pump and Return Valve:

DTC C1751 and DTC 1762 have not appeared. The AHC/TEMS systems appear to be working so there does not appear to be concerns about internal wear of the AHC Pump nor concerns about full or partial blockage of the internal strainers within the AHC Pump, nor concerns about correct operation of the separate Return Valve assembly -- any of these items usually would cause one or both of these DTC’s to appear.

The only DTC mentioned is C1340 (for the CDL circuit -- resolved -- and it is unclear whether this can affect the AHC/TEMS systems).

AHC ‘globes’:

The description of the ‘globes’ and the AHC Front and Rear pressures as reported seem to be reasonable. I have never understood exactly why disconnecting the Temperature Sensor at the AHC Pump per FSM instructions seems to make a difference in AHC pressure readings on some vehicles and but not on other vehicles. I would assume that a satisfactory test of ‘globe’ condition has been done by observing the difference in graduations at the AHC Tank between “LO” height and “HI” height at, or close to, FSM-specified AHC Front and Rear pressures and when tape-measured hub-to-fender distances are as recommended at “N” height setting. This also tests that the return function of the Return Valve is working as it should.

Height Control Accumulator:

Darker than usual AHC Fluid may indicate only that this Accumulator has not been fully bled for a long time and its fluid content may be old, and/or, it may simply indicate that the Accumulator has not seen much use. This Accumulator is activated only when a height change “LO” to “N” or “N” to “HI” is initiated at the centre console switch. It is inactive at all other times during vehicle operation. The vehicle is reported as not being used much and it has been stationary for long periods of time.

AHC Fluid and the Hydraulic Circuit:

AHC Fluid condition seems to be very good at the Rear and does not suggest nitrogen leakage from a failed ‘globe’ or ingested air at the Rear. It does have to be said that fluid polluted with air or nitrogen anywhere in the system eventually will return to the AHC Tank and then the polluted fluid will be sent by the AHC Pump throughout the AHC/TEMS system. Air and/or nitrogen in the AHC Tank may dissipate slowly to atmosphere but air and/or nitrogen within the AHC/TEMS system remains “locked in” behind the closed Levelling Valves in the Control Valve Assembly (unless the ECU determines from the Height Control Sensor signals that height adjustment up or down is required). The contents of the Height Control Accumulator remain "locked in" behind the closed Solenoid Valve unless a height raise or a height drop is initiated at the switch on the centre console.

NOTE: Any “locked in” air is under high AHC system pressure and is diffused in the fluid, not as visible bubbles. This is a ‘change of state’. The bubbles only emerge when the fluid is released to atmospheric pressure. This is much the same, but more intense, than gas in a sealed bottle of soda water. The bubbles are not visible until the cap is removed and the fluid is exposed to atmospheric pressure.

Initially in the case of this vehicle, foamy AHC Fluid was observed on exit when bleeding at the Front and meant either air or nitrogen was present. This was eliminated by adding new AHC Fluid and bleeding.

Can air really be drawn in past worn internal ‘shock absorber’ seals as suggested??

This would mean that at some point between full compression and full rebound of the ‘shock absorber’, the fluid pressure inside the ‘shock absorber’ becomes less than atmospheric pressure.

Can this really happen??

If it does, then should this situation be expected and reported as a common occurrence on 18 to 27 years old 100 series vehicles with AHC??

Air could be drawn in past a defective seal on the small driveshaft between the AHC Pump Motor and the AHC Pump, or, at a defective seal at the outer case of the overall AHC Pump Assembly Part Number 48910-60012, or, at a defective seal where the AHC Tank joins the overall AHC Pump Assembly Part Number 48910-60012. All of these situations are on the low pressure (intake side) of the AHC Pump sub-assembly Part Number 48901-60010.

Particulates in AHC Fluid accumulating from debris in the AHC/TEMS systems along with gel formation and long term oxidation and degradation of AHC Fluid is likely to be MUCH more common in AHC/TEMS vehicles than is widely realised -- especially when good ‘hydraulic hygiene’ and regular fluid changes (using only new correct AHC Fluid Part Number 08886-01805) have been neglected over the years. This has been demonstrated recently with increasing recognition of problems caused by fouling of the Return Valve by particulates carried in old AHC Fluid. The returning AHC Fluid DOES NOT pass through a strainer and DOES NOT pass through the AHC Pump gears on its way back to the AHC Tank. Instead, the sludge accumulates in low pressure or low flow parts of the hydraulic circuit. The picture of the surrounds of the AHC Pump sub-assembly behind the outer cover is a good example. Other examples are the ‘unswept’ regions of the ‘shock absorbers’, long AHC pipes, rarely used Height Control Accumulator, etc.

If any doubts remain about air in the system, then it may be worth devising a method and pursuing a ‘deep bleed’, such as variations on the idea discussed with @medtro in Post #184 in this thread -- or any other idea which allows the Levelling Valves to remain open (maybe by applying the test voltage to the Levelling Valve solenoids) AND while the bleeder valves also remain open while operating the AHC Pump (directly from the battery if necessary) to give continuous flow of air-free fluid. The aim would be to flush and clean out as much of the system as possible.

Control Valve Assembly and Levelling Valves and Gate Valves:

Personally, I have never had the courage to open and disassemble a Control Valve Assembly to see what can be understood about the ECU-controlled Levelling Valves beyond the FSM diagrams below:

Even if none of the relevant DTC’s 1731, 1732, 1733, 1734, 1736 have been received, it may be worthwhile to conduct the relevant tests and especially test the continuity of the circuits shown below and in the attachment at Page 46:

As you have suggested, it is conceivable that particulates could cause obstructions or mechanical interference within the Levelling Valves in much the same manner as is known to happen elsewhere, such as in the Return Valve mechanism. In such a case, these electrical tests will not reveal DTC's about these mechanical issues unless an obstruction has caused a Levelling Valve solenoid coil to burn out!!

Similar problems within the Gate Valves also could be possible. These "normally open" valves control side-to-side fluid flow (and therefore pressures) until certain speeds are reached when the Gate Valves are closed by the ECU to better resist body roll. This does not appear to be relevant nor a cause of the symptoms described.

If all other possibilities have been eliminated and it is considered that obstructed Levelling Valve(s) are the most likely problem, then devising a method to hold the Levelling Valve(s) in the open position with an applied test voltage, then flushing fluid or compressed air through the valves, may displace the obstruction. This might best be done on a work bench after removing the Control Valve Assembly.

A broken or weakened spring(s) in the Levelling Valve(s) will not be repaired by attempts to flush and clean.

Or it may be simpler to replace the entire Control Valve Assembly with a new unit or a Second Hand unit obtained from someone who has abandoned the AHC system. As the provenance of a Second Hand unit would be unknown, pre-testing before installation would be wise. IMPEX offers a new OEM Control Valve Assembly (Toyota/Lexus Part Number 48940-60010) for USD300.89 plus delivery plus taxes.

 

[2001LC]

Good afternoon @2001LC (updated about 2:45pm on Sunday afternoon May 25, 2025 in my part of the world).

I hesitate to suggest anything given your knowledge and experience with 100 series but in return for your extensive contributions to this IH8MUD forum since 2007 I offer below a few thoughts in response to your request.

There is nothing here that you do not already know -- but the information and attachments may help others who come this way ....

The observations and measurements in your post #185 on this 2007 LX470 at 175,000 miles are noted, including:

“I just had a strange incident. Where rear end, drop to LOW and bouncy”.

Height Control Sensors, Connectors/Junctions and Harnesses/Wiring:

I would assume that already you would have considered the condition of the Height Control Sensors and possible wiring harness and/or connector faults, including near the hot parts of the exhaust system, and also including the connector at the Sensor itself, and also including the infamous BI1, BI2, BI3 junctions located at the chassis behind the LHS Rear Quarter panel, and other connectors and Ground Points along or above the chassis rails, such as BD1 and BD2.

As you know, an intermittent height fault can result from imperfect Height Control Sensors and circuits. The Sensor and/or circuit faults may not throw DTC’s 1711, 1712 or 1713, provided that the FSM-defined voltage and timing ranges are not exceeded at the AHC ECU -- such as when the voltage signal from a Sensor circuit remains within the specified range of 0.3 volts to 4.7 volts but does not correctly match the height of the vehicle – for example, due to poor condition or contact of the brushes or spring connectors inside a Sensor, or faults and erratic resistance in the carbon track inside the Sensor which may not be visible but causes a wrong voltage signal for the actual height. Fault conditions can occur within the FSM-specfied ranges and cause aberrant vehicle behaviours but not show as a DTC.

Alternatively, intermittency often means damage somewhere in the wiring or connectors which is causing a poor connection to “make or break” the circuit intermittently (open circuit) as the vehicle moves and vibrates, or, water ingress into a junction connector is causing an intermittent short circuit. (Rain and interior dampness have been mentioned in Post #185, although outside air temperatures are warm at 28 degF to 70 degF and should have a drying effect).

AHC Pump and Return Valve:

DTC C1751 and DTC 1762 have not appeared. The AHC/TEMS systems appear to be working so there does not appear to be concerns about internal wear of the AHC Pump nor concerns about full or partial blockage of the internal strainers within the AHC Pump, nor concerns about correct operation of the separate Return Valve assembly -- any of these items usually would cause one or both of these DTC’s to appear.

The only DTC mentioned is C1340 (for the CDL circuit -- resolved -- and it is unclear whether this can affect the AHC/TEMS systems).

AHC ‘globes’:

The description of the ‘globes’ and the AHC Front and Rear pressures as reported seem to be reasonable. I have never understood exactly why disconnecting the Temperature Sensor at the AHC Pump per FSM instructions seems to make a difference in AHC pressure readings on some vehicles and but not on other vehicles. I would assume that a satisfactory test of ‘globe’ condition has been done by observing the difference in graduations at the AHC Tank between “LO” height and “HI” height at, or close to, FSM-specified AHC Front and Rear pressures and when tape-measured hub-to-fender distances are as recommended at “N” height setting. This also tests that the return function of the Return Valve is working as it should.

Height Control Accumulator:

Darker than usual AHC Fluid may indicate only that this Accumulator has not been fully bled for a long time and its fluid content may be old, and/or, it may simply indicate that the Accumulator has not seen much use. This Accumulator is activated only when a height change “LO” to “N” or “N” to “HI” is initiated at the centre console switch. It is inactive at all other times. The vehicle is reported as not being used much and it has been stationary for long periods of time.

AHC Fluid and the Hydraulic Circuit:

AHC Fluid condition seems to be very good at the Rear and does not suggest nitrogen leakage from a failed ‘globe’ or ingested air at the Rear. It does have to be said that fluid polluted with air or nitrogen anywhere in the system eventually will return to the AHC Tank and then the polluted fluid will be sent by the AHC Pump throughout the AHC/TEMS system. Air and/or nitrogen in the AHC Tank may dissipate slowly to atmosphere but air and/or nitrogen within the AHC/TEMS system remains “locked in” behind the closed Levelling Valves in the Control Valve Assembly (unless the ECU determines from the Height Control Sensor signals that height adjustment up or down is required). The contents of the Height Control Accumulator remain "locked in" behind the closed Solenoid Valve unless a height raise or a height drop is initiated at the switch on the centre console.

View attachment 3913678

NOTE: Any “locked in” air is under high AHC system pressure and is diffused in the fluid, not as visible bubbles. This is a ‘change of state’. The bubbles only emerge when the fluid is released to atmospheric pressure. This is much the same, but more intense, than gas in a sealed bottle of soda water. The bubbles are not visible until the cap is removed and the fluid is exposed to atmospheric pressure.

Initially in the case of this vehicle, foamy AHC Fluid was observed on exit when bleeding at the Front and meant either air or nitrogen was present. This was eliminated by adding new AHC Fluid and bleeding.

Can air really be drawn in past worn internal ‘shock absorber’ seals as suggested??

This would mean that at some point between full compression and full rebound of the ‘shock absorber’, the fluid pressure inside the ‘shock absorber’ becomes less than atmospheric pressure.

Can this really happen??

If it does, then should this situation be expected and reported as a common occurrence on 18 to 27 years old 100 series vehicles with AHC??

Air could be drawn in past a defective seal on the small driveshaft between the AHC Pump Motor and the AHC Pump, or, at a defective seal at the outer case of the overall AHC Pump Assembly Part Number 48910-60012, or, at a defective seal where the AHC Tank joins the overall AHC Pump Assembly Part Number 48910-60012. All of these situations are on the low pressure (intake side) of the AHC Pump sub-assembly Part Number 48901-60010.

Particulates in AHC Fluid accumulating from debris in the AHC/TEMS systems along with gel formation and long term oxidation and degradation of AHC Fluid is likely to be MUCH more common in AHC/TEMS vehicles than is widely realised -- especially when good ‘hydraulic hygiene’ and regular fluid changes (using only new correct AHC Fluid Part Number 08886-01805) have been neglected over the years. This has been demonstrated recently with increasing recognition of problems caused by fouling of the Return Valve by particulates carried in old AHC Fluid. The returning AHC Fluid DOES NOT pass through a strainer and DOES NOT pass through the AHC Pump gears on its way back to the AHC Tank. Instead, the sludge accumulates in low pressure or low flow parts of the hydraulic circuit. The picture of the surrounds of the AHC Pump sub-assembly behind the outer cover is a good example. Other examples are the ‘unswept’ regions of the ‘shock absorbers’, long AHC pipes, rarely used Height Control Accumulator, etc.

View attachment 3912988

If any doubts remain about air in the system, then it may be worth devising a method and pursuing a ‘deep bleed’, such as variations on the idea discussed in Post #184 in this thread -- or any other idea which allows the Levelling Valves to remain open (maybe by applying the test voltage to the Levelling Valve solenoids) AND while the bleeder valves also remain open while operating the AHC Pump (directly from the battery if necessary) to give continuous flow of air-free fluid. The aim would be to flush and clean out as much of the system as possible.

Control Valve Assembly and Levelling Valves and Gate Valves:

Personally, I have never had the courage to open and disassemble a Control Valve Assembly to see what can be understood about the ECU-controlled Levelling Valves beyond the FSM diagrams below:

View attachment 3912984

View attachment 3912985

Even if none of the relevant DTC’s 1731, 1732, 1733, 1734, 1736 have been received, it may be worthwhile to conduct the tests and especially test the continuity of the circuits shown below and in the attachment at Page 46:

View attachment 3912986
As you have suggested, it is conceivable that particulates could cause obstructions or mechanical interference within the Levelling Valves in much the same manner as is known to happen elsewhere, such as in the Return Valve mechanism. In such a case, these electrical tests will not reveal DTC's about these mechanical issues unless an obstruction has caused a Levelling Valve solenoid coil to burn out!!

Similar problems within the Gate Valves also could be possible. These "normally open" valves control side-to-side fluid flow (and therefore pressures) until certain speeds are reached when the Gate Valves close to better resist body roll. This does not appear to be relevant nor a cause of the symptoms described.

If all other possibilities have been eliminated and it is considered that obstructed Levelling Valve(s) are the most likely problem, then devising a method to hold the Levellung Valve(s) in the open position with an applied test voltage, then flushing fluid or compressed air through the valves, may displace the obstruction. This might best be done on a work bench after removing the Control Valve Assembly.

A broken or weakened spring(s) in the Levelling Valve(s) will not be repaired by attempts to flush and clean.

Or it may be simpler to replace the entire Control Valve Assembly with a new unit or a Second Hand unit obtained from someone who has abandoned the AHC system. As the provenance of a Second Hand unit would be unknown, pre-testing before installation would be wise. IMPEX offers a new OEM Control Valve Assembly (Toyota/Lexus Part Number 48940-60010) for USD300.89 plus delivery plus taxes.

View attachment 3912993

View attachment 3912994

Thanks @IndroCruise . I can see you spent a great deal of time on this. Thank You! I'll go through line by line, see if it "rings any bells. Currently the system is working fine.

Other than flushing. I did lift with a jack, which would move hieght sensors and drawn some fluid through gate valve. Perhaps is was a rear sensor going bad or a stuck valve, that lift benefitted.

 

[2001LC]

Good afternoon @2001LC (updated about 2:45pm on Sunday afternoon May 25, 2025 in my part of the world - 'shock absorber' pic added later).

I hesitate to suggest anything given your knowledge and experience with 100 series but in return for your extensive contributions to this IH8MUD forum since 2007 I offer below a few thoughts in response to your request.

There is nothing here that you do not already know -- but the information and attachments may help others who come this way ....

I'm learning more, each day. When it comes to AHC, I think you, have the best handle on the system.

The observations and measurements in your post #185 on this 2007 LX470 at 175,000 miles are noted, including:

“I just had a strange incident. Where rear end, drop to LOW and bouncy”.

Height Control Sensors, Connectors/Junctions and Harnesses/Wiring:

I would assume that already you would have considered the condition of the Height Control Sensors and possible wiring harness and/or connector faults, including near the hot parts of the exhaust system, and also including the connector at the Sensor itself, and also including the infamous BI1, BI2, BI3 junctions located at the chassis behind the LHS Rear Quarter panel, and other connectors and Ground Points along or above the chassis rails, such as BD1 and BD2.

As you know, an intermittent height fault can result from imperfect Height Control Sensors and circuits. The Sensor and/or circuit faults may not throw DTC’s 1711, 1712 or 1713, provided that the FSM-defined voltage and timing ranges are not exceeded at the AHC ECU -- such as when the voltage signal from a Sensor circuit remains within the specified range of 0.3 volts to 4.7 volts but does not correctly match the height of the vehicle – for example, due to poor condition or contact of the brushes or spring connectors inside a Sensor, or faults and erratic resistance in the carbon track inside the Sensor which may not be visible but causes a wrong voltage signal for the actual height. Fault conditions can occur within the FSM-specfied ranges and cause aberrant vehicle behaviours but not show as a DTC.

Alternatively, intermittency often means damage somewhere in the wiring or connectors which is causing a poor connection to “make or break” an intermittently open circuit as the vehicle moves and vibrates, or, water ingress into a junction connector is causing an intermittent short circuit. (Rain and interior dampness have been mentioned in Post #185, although outside air temperatures are warm at 28 degF to 70 degF and should have a drying effect).

I forgot to mention. I did raise to H, than disconnect the rear hieght sensor wire housing block, from sensor. Which raising with AHC to H, moving sensor arm by doing so and with floor jack and disconnecting wire housing block from sensor. May have jostled the contacts, both of wires and internal contacts. Which that may indeed be, where the fault was/is. If fails/falls again, i will inspect height sensor closely.

I regret pulling globes and flushing, before just seeing if raising and lower corrected.

If issue happens again. I will be pulling and inspect rear hieght sensor, first thing.

AHC Pump and Return Valve:

DTC C1751 and DTC 1762 have not appeared. The AHC/TEMS systems appear to be working so there does not appear to be concerns about internal wear of the AHC Pump nor concerns about full or partial blockage of the internal strainers within the AHC Pump, nor concerns about correct operation of the separate Return Valve assembly -- any of these items usually would cause one or both of these DTC’s to appear.

The only DTC mentioned is C1340 (for the CDL circuit -- resolved -- and it is unclear whether this can affect the AHC/TEMS systems).

I've no idea, why C1340 was present. I do not think, it was affecting the AHC. As always, I cleared the codes, and watch to see what returns. I've rechecked for DTC, non present.

AHC ‘globes’:

The description of the ‘globes’ and the AHC Front and Rear pressures as reported seem to be reasonable. I have never understood exactly why disconnecting the Temperature Sensor at the AHC Pump per FSM instructions seems to make a difference in AHC pressure readings on some vehicles and but not on other vehicles.

It may be FSM typo, or maybe it's concerned with condition of very high fluid temp. Or could it be, the AHC CPU uses the info to adjust pressure info it receives, to account for temperatures. I've not seen any info, why the system even has a temperature sensor! Have you?

I do know, in the VGRS systems. It monitors steering VGRS actuator temperature. That if its temperature gets to high, VGRS goes into fail safe. Which it locks at same ratio as non VGRS steering. The systems does this, without any notice to drive or setting off any DTC. One would have to be watching data stream in tech stream, to even know this happens. Or check if system working, by counting steering wheel turns (~2 1/3) from lock to lock.

It may be that if:

AHC CPU sees, temperature of AHC fluid to high. It locks out pump, maybe even control valves. Until temperature back (cooler) within operating range. Again, may be a case, where we don't know it is even happening. Where it a behind the sine function. Possible, FSM wants sensor disconnect. To make sure system doesn't go into fail safe as we check and adjust pressure.

I would assume that a satisfactory test of ‘globe’ condition has been done by observing the difference in graduations at the AHC Tank between “LO” height and “HI” height at, or close to, FSM-specified AHC Front and Rear pressures and when tape-measured hub-to-fender distances are as recommended at “N” height setting. This also tests that the return function of the Return Valve is working as it should.

Yep. 11 grads.

January 2020 I saw 10 grads. But I'll note, that fluid Temperature was 64f, at time of count.
Now (May 2025) I'm seeing improvement, with it at 11 grads. But this time, I check when temp at 122F. Which I have notice grad counts, do slightly increase at higher temps. Also splitting hairs, when dealing with just one grad.

I've also found count changes with balance of system. In that, let say a system has never been flushed or adjusted, Counting grad's, I almost always see a low count. Flush and adjust, count almost always comes up.

It surprised me, I needed 1 3/4 CW turns, to get to 6.9Mpa this time (May 2025). Perhaps I did get air in front, back then. But I really felt, I did not. I actually double flush, back then ( 5 years, 12K miles ago). Perhaps I should have driven to settle before adjusting T-bars for pressure.

I should also note. Front Pressure change from 6.9 to 6.4Mpa, after driving to settle. It was on this drive, where I stopped and reconnect temperature sensors. I've not readjusted T-bar yet, to bring front up to 6.9. It could be, vehicle just settle. I may be giving back the 1 3/4 CW turns, to get my 6.9Mpa front. Which may also affect rear reading.

A little strange:

I found my RR fender to axle hieght low. RL at 20 1/8, RR at 19 7/8" (fronts: FL & FR 19 3/16) I'll note: in 2020 I measured by hand. In 2025, I used the tool sold on mud, that centers on wheel by slipping over 3 lug nuts. I've found the tool, is subject to variation in measurement, depending on orientation. That my hand measure is more accurate. I'll also note the fender to axle, which I'm very accurate and consistent in procedure.
As always, fender damage. One day I'll get myself a measure stick, and check at FSM points/method.

I found (in 2025) rear pressure at 5.7Mpa, with full tank of gas. 5 years ago (in 2020), I found rear at 6.4Mpa with ~ 2/3 tank of gas.

I'll be rechecking heights, pressures and adjust again soon. Currently it's riding fine, but does feel like pressure a bit low in front. That is more weight on T-bars, reducing dampening (stiff ride).

Height Control Accumulator:

Darker than usual AHC Fluid may indicate only that this Accumulator has not been fully bled for a long time and its fluid content may be old, and/or, it may simply indicate that the Accumulator has not seen much use. This Accumulator is activated only when a height change “LO” to “N” or “N” to “HI” is initiated at the centre console switch. It is inactive at all other times during vehicle operation. The vehicle is reported as not being used much and it has been stationary for long periods of time.

Perhaps when I double bleed 5 years ago. I didn't double bleed accultor.

I actually, have been using AHC height function often. I hate door dings. So when I park in a public lot. I raise to H. This is to get typically door ding hit, onto plastic side cladding.

I also periodical take to low, just to exercise (move fluid) system.

AHC Fluid and the Hydraulic Circuit:

AHC Fluid condition seems to be very good at the Rear and does not suggest nitrogen leakage from a failed ‘globe’ or ingested air at the Rear. It does have to be said that fluid polluted with air or nitrogen anywhere in the system eventually will return to the AHC Tank and then the polluted fluid will be sent by the AHC Pump throughout the AHC/TEMS system. Air and/or nitrogen in the AHC Tank may dissipate slowly to atmosphere but air and/or nitrogen within the AHC/TEMS system remains “locked in” behind the closed Levelling Valves in the Control Valve Assembly (unless the ECU determines from the Height Control Sensor signals that height adjustment up or down is required). The contents of the Height Control Accumulator remain "locked in" behind the closed Solenoid Valve unless a height raise or a height drop is initiated at the switch on the centre console.

View attachment 3913678

NOTE: Any “locked in” air is under high AHC system pressure and is diffused in the fluid, not as visible bubbles. This is a ‘change of state’. The bubbles only emerge when the fluid is released to atmospheric pressure. This is much the same, but more intense, than gas in a sealed bottle of soda water. The bubbles are not visible until the cap is removed and the fluid is exposed to atmospheric pressure.

Interesting! I did know that.

Initially in the case of this vehicle, foamy AHC Fluid was observed on exit when bleeding at the Front and meant either air or nitrogen was present. This was eliminated by adding new AHC Fluid and bleeding.

Nitrogen in fluid, I expect to see grad count low or lower than was. Which may be yet to happen. I'll be watching count.

It was the bouncy rear, that made me first think. I'd blown a rear globe. Finding them and rear fluid good. Change my attention to rear hieght sensors. Again, currently no issues. True cause of temporary failure, not yet determined.

As for front shock leak (minoir weep), possible resulting in air getting into system. I never even considered weeping shock related to air in system, before now. But is something I'll be watching for on every flush I do. Look for correlation.

Can air really be drawn in past worn internal ‘shock absorber’ seals as suggested??

Here's my observation:

When I install AHC shocks. I flush them on the bench and pre fill. The shock works just like a syringe. I compress to expel old fluid. I pull (extend) shock apart to draw in fluid filling. So if a seal within shock leaking fluid out. It may grab a little air on extension, past weepy seal. Or so is what I'm now thinking, and will be watching for.

View attachment 3914267

This would mean that at some point between full compression and full rebound of the ‘shock absorber’, the fluid pressure inside the ‘shock absorber’ becomes less than atmospheric pressure.

Can this really happen??

I'm thinking yes. And as stated will I'll watch for.

If it does, then should this situation be expected and reported as a common occurrence on 18 to 27 years old 100 series vehicles with AHC??

I'd think, only reported, if results issue they're asking for help on.

I actually find ~1 out of 3 with air in either front or rear sometimes both. Most I see, have never been serviced (i.e no record, fluid very dark, DS rear sitting low and pressures high). Yet I'll see heavy foaming (air) sometimes. I really never even note foamy fluid, unfortunately. I do note weeping shocks, as to be watched.

Air could be drawn in past a defective seal on the small driveshaft between the AHC Pump Motor and the AHC Pump, or, at a defective seal at the outer case of the overall AHC Pump Assembly Part Number 48910-60012, or, at a defective seal where the AHC Tank joins the overall AHC Pump Assembly Part Number 48910-60012. All of these situations are on the low pressure (intake side) of the AHC Pump sub-assembly Part Number 48901-60010.

Particulates in AHC Fluid accumulating from debris in the AHC/TEMS systems along with gel formation and long term oxidation and degradation of AHC Fluid is likely to be MUCH more common in AHC/TEMS vehicles than is widely realised -- especially when good ‘hydraulic hygiene’ and regular fluid changes (using only new correct AHC Fluid Part Number 08886-01805) have been neglected over the years. This has been demonstrated recently with increasing recognition of problems caused by fouling of the Return Valve by particulates carried in old AHC Fluid. The returning AHC Fluid DOES NOT pass through a strainer and DOES NOT pass through the AHC Pump gears on its way back to the AHC Tank. Instead, the sludge accumulates in low pressure or low flow parts of the hydraulic circuit. The picture of the surrounds of the AHC Pump sub-assembly behind the outer cover is a good example. Other examples are the ‘unswept’ regions of the ‘shock absorbers’, long AHC pipes, rarely used Height Control Accumulator, etc.

View attachment 3912988

If any doubts remain about air in the system, then it may be worth devising a method and pursuing a ‘deep bleed’, such as variations on the idea discussed with @medtro in Post #184 in this thread -- or any other idea which allows the Levelling Valves to remain open (maybe by applying the test voltage to the Levelling Valve solenoids) AND while the bleeder valves also remain open while operating the AHC Pump (directly from the battery if necessary) to give continuous flow of air-free fluid. The aim would be to flush and clean out as much of the system as possible.

Control Valve Assembly and Levelling Valves and Gate Valves:

Personally, I have never had the courage to open and disassemble a Control Valve Assembly to see what can be understood about the ECU-controlled Levelling Valves beyond the FSM diagrams below:

I've never even pull one off, much less apart. I was just spitballing possibilities, of a pressure difference (low front high in rear) due to air in front and weep in front shock, result in fluke in a control valve.

View attachment 3912984

View attachment 3912985

Even if none of the relevant DTC’s 1731, 1732, 1733, 1734, 1736 have been received, it may be worthwhile to conduct the relevant tests and especially test the continuity of the circuits shown below and in the attachment at Page 46:

View attachment 3912986
As you have suggested, it is conceivable that particulates could cause obstructions or mechanical interference within the Levelling Valves in much the same manner as is known to happen elsewhere, such as in the Return Valve mechanism. In such a case, these electrical tests will not reveal DTC's about these mechanical issues unless an obstruction has caused a Levelling Valve solenoid coil to burn out!!

Similar problems within the Gate Valves also could be possible. These "normally open" valves control side-to-side fluid flow (and therefore pressures) until certain speeds are reached when the Gate Valves are closed by the ECU to better resist body roll. This does not appear to be relevant nor a cause of the symptoms described.

If all other possibilities have been eliminated and it is considered that obstructed Levelling Valve(s) are the most likely problem, then devising a method to hold the Levelling Valve(s) in the open position with an applied test voltage, then flushing fluid or compressed air through the valves, may displace the obstruction. This might best be done on a work bench after removing the Control Valve Assembly.

A broken or weakened spring(s) in the Levelling Valve(s) will not be repaired by attempts to flush and clean.

Or it may be simpler to replace the entire Control Valve Assembly with a new unit or a Second Hand unit obtained from someone who has abandoned the AHC system. As the provenance of a Second Hand unit would be unknown, pre-testing before installation would be wise. IMPEX offers a new OEM Control Valve Assembly (Toyota/Lexus Part Number 48940-60010) for USD300.89 plus delivery plus taxes.

View attachment 3912993

View attachment 3912994

 

[IndroCruise]

I'm learning more, each day. When it comes to AHC, I think you, have the best handle on the system.

I forgot to mention. I did raise to H, than disconnect the rear hieght sensor wire housing block, from sensor. Which raising with AHC to H, moving sensor arm by doing so and with floor jack and disconnecting wire housing block from sensor. May have jostled the contacts, both of wires and internal contacts. Which that may indeed be, where the fault was/is. If fails/falls again, i will inspect height sensor closely.

I regret pulling globes and flushing, before just seeing if raising and lower corrected.

If issue happens again. I will be pulling and inspect rear hieght sensor, first thing.


I've no idea, why C1340 was present. I do not think, it was affecting the AHC. As always, I cleared the codes, and watch to see what returns. I've rechecked for DTC, non present.

It may be FSM typo, or maybe it's concerned with condition of very high fluid temp. Or could it be, the AHC CPU uses the info to adjust pressure info it receives, to account for temperatures. I've not seen any info, why the system even has a temperature sensor! Have you?

I do know, in the VGRS systems. It monitors steering VGRS actuator temperature. That if its temperature gets to high, VGRS goes into fail safe. Which it locks at same ratio as non VGRS steering. The systems does this, without any notice to drive or setting off any DTC. One would have to be watching data stream in tech stream, to even know this happens. Or check if system working, by counting steering wheel turns (~2 1/3) from lock to lock.

It may be that if:

AHC CPU sees, temperature of AHC fluid to high. It locks out pump, maybe even control valves. Until temperature back (cooler) within operating range. Again, may be a case, where we don't know it is even happening. Where it a behind the sine function. Possible, FSM wants sensor disconnect. To make sure system doesn't go into fail safe as we check and adjust pressure.


Yep. 11 grads.

January 2020 I saw 10 grads. But I'll note, that fluid Temperature was 64f, at time of count.
Now (May 2025) I'm seeing improvement, with it at 11 grads. But this time, I check when temp at 122F. Which I have notice grad counts, do slightly increase at higher temps. Also splitting hairs, when dealing with just one grad.

I've also found count changes with balance of system. In that, let say a system has never been flushed or adjusted, Counting grad's, I almost always see a low count. Flush and adjust, count almost always comes up.

It surprised me, I needed 1 3/4 CW turns, to get to 6.9Mpa this time (May 2025). Perhaps I did get air in front, back then. But I really felt, I did not. I actually double flush, back then ( 5 years, 12K miles ago). Perhaps I should have driven to settle before adjusting T-bars for pressure.

I should also note. Front Pressure change from 6.9 to 6.4Mpa, after driving to settle. It was on this drive, where I stopped and reconnect temperature sensors. I've not readjusted T-bar yet, to bring front up to 6.9. It could be, vehicle just settle. I may be giving back the 1 3/4 CW turns, to get my 6.9Mpa front. Which may also affect rear reading.

A little strange:

I found my RR fender to axle hieght low. RL at 20 1/8, RR at 19 7/8" (fronts: FL & FR 19 3/16) I'll note: in 2020 I measured by hand. In 2025, I used the tool sold on mud, that centers on wheel by slipping over 3 lug nuts. I've found the tool, is subject to variation in measurement, depending on orientation. That my hand measure is more accurate. I'll also note the fender to axle, which I'm very accurate and consistent in procedure.
As always, fender damage. One day I'll get myself a measure stick, and check at FSM points/method.

I found (in 2025) rear pressure at 5.7Mpa, with full tank of gas. 5 years ago (in 2020), I found rear at 6.4Mpa with ~ 2/3 tank of gas.

I'll be rechecking heights, pressures and adjust again soon. Currently it's riding fine, but does feel like pressure a bit low in front. That is more weight on T-bars, reducing dampening (stiff ride).


Perhaps when I double bleed 5 years ago. I didn't double bleed accultor.

I actually, have been using AHC height function often. I hate door dings. So when I park in a public lot. I raise to H. This is to get typically door ding hit, onto plastic side cladding.

I also periodical take to low, just to exercise (move fluid) system.


Interesting! I did know that.

Nitrogen in fluid, I expect to see grad count low or lower than was. Which may be yet to happen. I'll be watching count.

It was the bouncy rear, that made me first think. I'd blown a rear globe. Finding them and rear fluid good. Change my attention to rear hieght sensors. Again, currently no issues. True cause of temporary failure, not yet determined.

As for front shock leak (minoir weep), possible resulting in air getting into system. I never even considered weeping shock related to air in system, before now. But is something I'll be watching for on every flush I do. Look for correlation.



Here's my observation:

When I install AHC shocks. I flush them on the bench and pre fill. The shock works just like a syringe. I compress to expel old fluid. I pull (extend) shock apart to draw in fluid filling. So if a seal within shock leaking fluid out. It may grab a little air on extension, past weepy seal. Or so is what I'm now thinking, and will be watching for.



I'm thinking yes. And as stated will I'll watch for.

I'd think, only reported, if results issue they're asking for help on.

I actually find ~1 out of 3 with air in either front or rear sometimes both. Most I see, have never been serviced (i.e no record, fluid very dark, DS rear sitting low and pressures high). Yet I'll see heavy foaming (air) sometimes. I really never even note foamy fluid, unfortunately. I do note weeping shocks, as to be watched.


I've never even pull one off, much less apart. I was just spitballing possibilities, of a pressure difference (low front high in rear) due to air in front and weep in front shock, result in fluke in a control valve.

Your very thoughtful reply is greatly appreciated. It will be interesting to see what conclusions emerge from ongoing monitoring.

 

[IndroCruise]

From @2000LC: “It may be FSM typo, or maybe it's concerned with condition of very high fluid temp. Or could it be, the AHC CPU uses the info to adjust pressure info it receives, to account for temperatures. I've not seen any info, why the system even has a temperature sensor! Have you?”

I don’t really know the answer to this question in above Post #188 -- nor do I know what the Toyota-Lexus Design Engineers had in mind.

Here are a few rambling thoughts, probably the result of ‘over-thinking’!!

As we all know, the theory is that a hydraulic fluid temperature sensor for an oil gear pump is helpful in monitoring fluid temperature – which is important to avoid the loss of viscosity, lubricity, fluid degradation, system efficiency, safety, component damage, etc:

Viscosity and Lubricity:

Hydraulic oil viscosity is temperature-dependent. If the oil is too hot, it thins out, reducing its lubricating ability and leading to increased wear on pump components. Conversely, if the oil is too cold, it thickens, making it harder for the pump to operate efficiently.

Fluid Degradation:

High temperatures accelerate the breakdown of hydraulic oil, leading to the formation of sludge and varnish that can contaminate the system and damage components. Maintaining proper temperature helps extend the lifespan of the hydraulic fluid.

Component Damage:

High temperatures can also damage seals, hoses, and other components in the hydraulic system, leading to leaks and increased maintenance costs.

System Efficiency:

Optimal fluid temperature ensures the hydraulic system operates at its peak efficiency, reducing energy consumption and increasing productivity.

Safety:

Monitoring temperature can help prevent potentially hazardous situations, such as the formation of vapor or catastrophic component failures.

This is all good well-known theoretical stuff -- following much the same principles as for the other three obvious hydraulic systems on LC100/LX470 – namely Auto Transmission, Brakes (especially with ABS) and Steering (especially with VGRS).

One of the differences may be that the AHC/TEMS system is a relatively low-temperature system and so a light mineral oil (with a few additives) is used as the Toyota/Lexus branded AHC Fluid.

Brakes and Auto Transmissions require different, higher temperature hydraulic fluids – these higher temperature fluids (sometimes ether/glycol-based for Brakes) definitely are incompatible with the AHC/TEMS system and are known to cause corrosion and damage of AHC seals, membranes and valves.

So maybe AHC/TEMS needs a Temperature Sensor to manage lower temperature capability of AHC Fluid – in a similar way to Steering VRGS as @2001LC has suggested?

As far as I can find, the information in the LC100/LX470 Factory Service Manuals (FSM) tells us only how the AHC Temperature Sensor is connected to the Suspension Electronic Control Unit (ECU) and how to test the AHC Temperature Sensor.

There are only the following vague indications (using voltage signals and timings) and nothing clear and practical to tell us what responses actually occur at what temperatures (or equivalent voltage signals from the AHC Temperature Sensor). The attached FSM explanation of the Detecting Condition for DTC C1719 AHC Temperature Sensor states as follows:

“When detecting the abnormal signal from the fluid temp. sensor (Fluid temp. sensor terminal voltage of ECU is 0.1 V or less or 4.8 V or more) for every 0.01 sec. and that condition continued for 0.5 sec. and had been consisted 10 times”.

The FSM-specified DTC C1719 test procedure is straightforward – it can be used to check whether the variation in signal voltages and timings correlate with the temperature indicated by Techstream (or other scanner) in a way that makes sense.

There is no “fail safe function" defined in the FSM for DTC C1719 for the AHC Temperature Sensor.

The connection of the Temperature Sensor to the ECU can be seen on the logic chart and more clearly at “TOIL” on the Electrical Wiring Diagram (EWD)

The point is that this “TOIL” connection at the ECU seems to be a “GO/NO GO” control -- in much the same way as the ECU connections for the Engine Running connection at “REG”, the STOP Light connection at “STP”, and the Door Switch connection at “DOOR”, and various others – many of which do not have an FSM-defined “fail safe function" and do not cause a DTC.

Some of these situations are not exclusive to AHC/TEMS and are defined in the Multiplex System DTC's:

LC100 Factory Service Manual

Landcruiser 100 Series Factory Workshop Manual

lc100e.github.io

However, we know from experience, for example, that the AHC/TEMS systems do not operate and cannot be tested with a door open or a foot on the brake, etc.

Anyway, the Temperature Sensor is connected and provides inputs to the ECU for some reason. My GUESS is that IF …

Fluid temp. sensor terminal voltage of ECU is 0.1 V or less or 4.8 V or more) for every 0.01 sec. and that condition continued for 0.5 sec. and had been consisted 10 times

THEN ….

All or some of the AHC/TEMS does not operate, maybe in the same way as the “fail safe function" for other fault conditions, even though this is not explained in the FSM??

In Post #185 this thread, the following difference in readings of AHC pressures are reported with and without the Temperature Sensor connected:

Frt 6.9Mpa, R 5.8Mpa, Acc 0 T-22F after short drive.
Plug in Temp sensor.
Frt 6.3Mpa, R 5.7Mpa, Acc 10.4 Mpa, T122F

NOTE: In the Toyota on-line version of the FSM for AHC/TEMS, the instruction to "Disconnect the fluid temperature sensor connector" before testing AHC pressures with the hand-held testor (alias Techstream) occurs ONLY for the Rear AHC test, and not for the Front AHC test:

LC100 Factory Service Manual

Landcruiser 100 Series Factory Workshop Manual

lc100e.github.io

The design of the ECU is ancient (commercialised in 1998 for LC100/LX470, evolved from TEMS in the Toyota Soarer, written up by Toyota in a paper for the US Society of Automotive Engineers in 1984). It seems unlikely that the ECU is performing some different kind of internal calculation of different pressures. It seems more likely that the ECU simply is responding to incoming voltage signals over time from the various sensors -- so why are these different pressures being experienced??

Different AHC pressures may relate to

• different tape-measured hub-to-fender vehicle ride heights (and consistent Height Control Sensor readings on Techstream) caused by out-of-range voltage and timing from the AHC Temperature Sensor, making the ECU cause default heights to be adopted by the vehicle -- as happens for some other fault conditions and as may happen with the AHC Temperature Sensor disconnected, or,
• some resistances to up/down movement due to wear or imperfections somewhere in the vehicle suspension and/or axle mechanisms and/or bushes, or,
• different loads on the vehicle.

So I am wondering if the tape-measured hub-to-fender vehicle ride heights are different between one set of measurements with the AHC Temperature Sensor connected and the other set of measurements with the AHC Temperature Sensor disconnected, in the above readings??

At the moment, ‘my’ LC100 is in the hands of my son-in-law, 5,216 kilometres (3,216 miles) away on the other side of Australia, so I cannot test this idea for myself.

[I am only looking at one vehicle whereas @2001LC has looked at many vehicles over many years and will have a much better handle and opinion on these kinds of variations].

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Aside: There are differences in Diagnostic Trouble Codes (DTC's) across the parts of the AHC/TEMS systems. For example, the Height Control Sensors (DTC 1711, C1712, C1713) have an FSM-defined “fail safe function". There are differences in the FSM-defined “fail safe function" for each of the Damping Force Control Actuator (DTC 1721), Control Valve Solenoid & Accumulator Solenoid Circuits (DTC 1731/1733, 1732/1734 & C1736), AHC Motor Relay Circuit (DTC 1741), AHC Main Relay Circuit (DTC 1743), AHC Pump Motor Circuit (DTC 1751), AHC Fluid Pressure Abnormality – Pump & Motor Does Not Supply Fluid (DTC1762).

Fluid Pressure Abnormality – Valve Does Not Open (DTC C1763, C1764) – are two are the more complicated FSM-defined “fail safe functions” – they may relate to clogged valves in the Control Valve Assembly or the Height Control Accumulator – but may simply relate to sensor or circuit faults.

The point is that the cause of each Diagnostic Trouble Code (DTC) is the result of the FSM-defined “decision rules” programmed into the ECU and which have been breached in terms of voltage signals and timings from the relevant Sensor. The DTC is an indirect indication of condition. By themselves, the DTC’s are helpful but insufficient – fault diagnosis also requires observation of the vehicle and system behaviour, including consistency or otherwise with the specific individual FSM-defined “fail safe function" for the relevant part of the AHC/TEMS systems.
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