99 LX, trying to resurrect AHC

[danlx]

I’ve been working on diagnosing an AHC failure this weekend. Techstream is showing C1732, C1763 & C1764. Blinking OFF light. Hard harsh ride. No raise or lower function.

I’ve read thru threads here. Downloaded TIS content for the truck, RM & EWD. Tested harnesses which seem to have good continuity where tested- (junction ID2 downstream to the accumulator and valve solenoids, both of which test as electrically functional).

I know a lot of folks just delete AHC. I do not want to. Has anyone else chased these things successfully? Could this be an ECU fail? (Does anyone have a functioning AHC ECU that you’ve removed (known functioning) that you’d part ways with?) Any other thoughts or tips? TIA.

 

[ramangain]

@suprarx7nut if you are close to him perhaps he can help

 

[ktmracer01]

DTC C1732: Front Damping Force Control Actuator LH Circuit Malfunction
DTC C1763: Abnormal Pump Motor Oil Pressure - The Suspension Control ECU monitors the Pump Motor Oil Pressure.
DTC C1764: Abnormal Leveling Valve Oil Pressure

Could be front left height control sensor.

1. Is the pump making any noise?
2. Has there been any service done to the AHC recently? E.g. AHC fluid flush, parts replacement, adjustment, etc.
3. Any other symptoms on the suspension other than the codes? E.g. bouncy, stiff ride, too soft, etc.

@suprarx7nut is one of the most knowledgeable and @IndroCruise has been very helpful in the past with diagnosis AHC suspension with a lot of technical input.

 

[suprarx7nut]

I’ve been working on diagnosing an AHC failure this weekend. Techstream is showing C1732, C1763 & C1764. Blinking OFF light. Hard harsh ride. No raise or lower function.

I’ve read thru threads here. Downloaded TIS content for the truck, RM & EWD. Tested harnesses which seem to have good continuity where tested- (junction ID2 downstream to the accumulator and valve solenoids, both of which test as electrically functional).

I know a lot of folks just delete AHC. I do not want to. Has anyone else chased these things successfully? Could this be an ECU fail? (Does anyone have a functioning AHC ECU that you’ve removed (known functioning) that you’d part ways with?) Any other thoughts or tips? TIA.

I can take a look at my manual this evening if I remember. I dealt with a similar forum member problem where one fault caused another (1763 was caused by an apparent wiring harness fault on the height accumulator circuit). I would measure resistance at the front left actuator and compare to factory spec. I can pull spec later today if I'm reminded.

If the actuator looks good at the actuator itself, I'd then pull the AHC ECU (involves removing the lower steering wheel trim and HVAC pipe to get access to it) and measure from the ECU connector.

Sounds like you've got wiring or actuator problem. Some targeted work with a multilmeter might reveal the answer quickly.

Happy to help here and offer suggestions (free) or if you'd like to come by the ADGU shop in Frederick and have us put our hands on it, we can take a look there as well ($$).

You're right to not want to remove it, IMO. Should be an easy fix.

 

[IndroCruise]

I’ve been working on diagnosing an AHC failure this weekend. Techstream is showing C1732, C1763 & C1764. Blinking OFF light. Hard harsh ride. No raise or lower function.

I’ve read thru threads here. Downloaded TIS content for the truck, RM & EWD. Tested harnesses which seem to have good continuity where tested- (junction ID2 downstream to the accumulator and valve solenoids, both of which test as electrically functional).

I know a lot of folks just delete AHC. I do not want to. Has anyone else chased these things successfully? Could this be an ECU fail? (Does anyone have a functioning AHC ECU that you’ve removed (known functioning) that you’d part ways with?) Any other thoughts or tips? TIA.

*** Late edit added referring to AHC-B fuse, AHC-IG fuse and AHC Main Relay ***

I was writing a few thoughts while others were doing similar. Given that @suprarx7nut is much closer to you and is very knowledgable, suggest follow his lead in his Post #4 above in this thread, rather than have too many diverse views. Meanwhile, I may as well share what I had already written in case any of these ideas are of assistance:

It seems like you have access to the appropriate Factory Workshop Manual (FSM) but just in case, here is a good on-line reference:
https://lc100e.github.io/manual/
then work through tabs at left hand side of the opening page to find:
Repair Manual > DIAGNOSTICS > ACTIVE HEIGHT CONTROL & SKYHOOK TEMS
(This is an LC100 version but it applies equally to LX470 which is basically the same under the skin).

The exact conditions causing each of these DTC’s are described at this FSM reference.

Note that Techstream is not indicating DTC C1761 – Malfunction in ECU.

However,

• AHC light is blinking, and,
• No raise or lower function
• Harsh ride

Actions taken so far:

C1732 – Control Valve Solenoid and Accumulator Solenoid Circuits
--- tested solenoids on both devices per FSM and found satisfactory.

C1763 & C1764 – Fluid Pressure Abnormality
--- no actions reported.

Note that C1732 is consistent with C1763 and C1764. These DTC’s are likely to occur together because, as seen in the extract far below and regardless of the electrical health of these circuits, C1764 means that the ECU has prohibited the operation of the Height Control Accumulator solenoid and also prohibits height control – meaning the raise/lower function is prohibited. Not stated but also it is likely to include prohibition of active damping which means rough ride. This situation does not indicate an ECU failure.

Suggest pursue each of the causes listed under “Trouble Area” in the relevant section of the FSM (extract below), including the pressure sensor even though a pressure sensor DTC (C1762) has not appeared.

Note the particular 'fail safe' mode which occurs in the case of C1764. This would appear to correspond with your described symptoms.

As a starting point, suggest first use the Height Control Operation Test (also called the “Active Test” ) to determine whether the height control functions are capable of working if the fault conditions are resolved. See attachment at Page 6 – Item 5. Special Service Tool (SST) 09843-18020 is mentioned which is just a short jumper lead. Instead use an unbent clip to bridge the terminals shown. With ignition OFF, make the connections at DLC1, (usually in the engine bay above the RHS front wheel with the word "DIAGNOSTIC" marked on the cover), then with engine operating, perform the test exactly as described in the attachment. There will be an array of warning lights in the instrument panel during this test because parts of the circuits are being bypassed. These will disappear when the test is complete and the paper clip bridge is removed and the ON/OFF switch is pressed and held for a few seconds. (This procedure is similar to the FSM but for better convenience uses DLC1 rather than DLC3).

If there are no height control functions when the "Active Test" is used, then it is time to suspect the AHC Pump – either a lack of power, or, a possible blockage.

For power supply, check AHC-IG fuse as shown in “Power Source Circuit” in the FSM reference. Suggest also check the AHC-B fuse and the AHC Main Relay – see test details in the above referenced FSM section on AHC Diagnostics for C1743. Also see this link for background information and details which may help.
Quick question about AHC ride with extra weight - https://forum.ih8mud.com/threads/quick-question-about-ahc-ride-with-extra-weight.1247828/page-4#post-13959941
The point is that if power cannot get through due to fuse or relay problems, not much is going to happen in the AHC/TEMS systems.

Then suggest cut to the chase and test the AHC Pump by connecting the Pump Motor directly to the battery, making the connections exactly as shown for for DTC C1762 (even though this DTC is not showing). If the Pump can operate but there is insufficient pressure and flow, then a Pump blockage might be suspected.

Perhaps that is enough for a start.

 

[danlx]

DTC C1732: Front Damping Force Control Actuator LH Circuit Malfunction
DTC C1763: Abnormal Pump Motor Oil Pressure - The Suspension Control ECU monitors the Pump Motor Oil Pressure.
DTC C1764: Abnormal Leveling Valve Oil Pressure

Could be front left height control sensor.

1. Is the pump making any noise?
2. Has there been any service done to the AHC recently? E.g. AHC fluid flush, parts replacement, adjustment, etc.
3. Any other symptoms on the suspension other than the codes? E.g. bouncy, stiff ride, too soft, etc.

@suprarx7nut is one of the most knowledgeable and @IndroCruise has been very helpful in the past with diagnosis AHC suspension with a lot of technical input.

Sorry for

DTC C1732: Front Damping Force Control Actuator LH Circuit Malfunction
DTC C1763: Abnormal Pump Motor Oil Pressure - The Suspension Control ECU monitors the Pump Motor Oil Pressure.
DTC C1764: Abnormal Leveling Valve Oil Pressure

Could be front left height control sensor.

1. Is the pump making any noise?
2. Has there been any service done to the AHC recently? E.g. AHC fluid flush, parts replacement, adjustment, etc.
3. Any other symptoms on the suspension other than the codes? E.g. bouncy, stiff ride, too soft, etc.

@suprarx7nut is one of the most knowledgeable and @IndroCruise has been very helpful in the past with diagnosis AHC suspension with a lot of technical input.

Sorry for the slow reply. Work outside my garage has kept me wrapped up til today. The pump no longer makes attempts to actuate at startup. I’ve tested power to the pump and observed operating an sound by de-capping the pump relay in the engine room fuse block and manually switching the relay momentarily.
I did conduct a bleed of the accumulator and front globes to try to clear out the lines. This had no affect on resolving the system’s original problem.

 

[danlx]

*** Late edit added referring to AHC-B fuse, AHC-IG fuse and AHC Main Relay ***

I was writing a few thoughts while others were doing similar. Given that @suprarx7nut is much closer to you and is very knowledgable, suggest follow his lead in his Post #4 above in this thread, rather than have too many diverse views. Meanwhile, I may as well share what I had already written in case any of these ideas are of assistance:

It seems like you have access to the appropriate Factory Workshop Manual (FSM) but just in case, here is a good on-line reference:
https://lc100e.github.io/manual/
then work through tabs at left hand side of the opening page to find:
Repair Manual > DIAGNOSTICS > ACTIVE HEIGHT CONTROL & SKYHOOK TEMS
(This is an LC100 version but it applies equally to LX470 which is basically the same under the skin).

The exact conditions causing each of these DTC’s are described at this FSM reference.

Note that Techstream is not indicating DTC C1761 – Malfunction in ECU.

However,

• AHC light is blinking, and,
• No raise or lower function
• Harsh ride

Actions taken so far:

C1732 – Control Valve Solenoid and Accumulator Solenoid Circuits
--- tested solenoids on both devices per FSM and found satisfactory.

C1763 & C1764 – Fluid Pressure Abnormality
--- no actions reported.

Note that C1732 is consistent with C1763 and C1764. These DTC’s are likely to occur together because, as seen in the extract far below and regardless of the electrical health of these circuits, C1764 means that the ECU has prohibited the operation of the Height Control Accumulator solenoid and also prohibits height control – meaning the raise/lower function is prohibited. Not stated but also it is likely to include prohibition of active damping which means rough ride. This situation does not indicate an ECU failure.

Suggest pursue each of the causes listed under “Trouble Area” in the relevant section of the FSM (extract below), including the pressure sensor even though a pressure sensor DTC (C1762) has not appeared.

Note the particular 'fail safe' mode which occurs in the case of C1764. This would appear to correspond with your described symptoms.

As a starting point, suggest first use the Height Control Operation Test (also called the “Active Test” ) to determine whether the height control functions are capable of working if the fault conditions are resolved. See attachment at Page 6 – Item 5. Special Service Tool (SST) 09843-18020 is mentioned which is just a short jumper lead. Instead use an unbent clip to bridge the terminals shown. With ignition OFF, make the connections at DLC1, (usually in the engine bay above the RHS front wheel with the word "DIAGNOSTIC" marked on the cover), then with engine operating, perform the test exactly as described in the attachment. There will be an array of warning lights in the instrument panel during this test because parts of the circuits are being bypassed. These will disappear when the test is complete and the paper clip bridge is removed and the ON/OFF switch is pressed and held for a few seconds. (This procedure is similar to the FSM but for better convenience uses DLC1 rather than DLC3).

If there are no height control functions when the "Active Test" is used, then it is time to suspect the AHC Pump – either a lack of power, or, a possible blockage.

For power supply, check AHC-IG fuse as shown in “Power Source Circuit” in the FSM reference. Suggest also check the AHC-B fuse and the AHC Main Relay – see test details in the above referenced FSM section on AHC Diagnostics for C1743. Also see this link for background information and details which may help.
Quick question about AHC ride with extra weight - https://forum.ih8mud.com/threads/quick-question-about-ahc-ride-with-extra-weight.1247828/page-4#post-13959941
The point is that if power cannot get through due to fuse or relay problems, not much is going to happen in the AHC/TEMS systems.

Then suggest cut to the chase and test the AHC Pump by connecting the Pump Motor directly to the battery, making the connections exactly as shown for for DTC C1762 (even though this DTC is not showing). If the Pump can operate but there is insufficient pressure and flow, then a Pump blockage might be suspected.

Perhaps that is enough for a start.

View attachment 2754360

Thank you for your detailed step by step and supporting docs. My slow reply is due to me being crazy busy launching my business. Now with some time to twist wrenches this afternoon I’m walking thru this diagnostic.

According to what you’re saying here, since no raise or lower function occurs in diagnostic mode, jumping Ts to E1, the pump seems suspect.

 

[danlx]

I can take a look at my manual this evening if I remember. I dealt with a similar forum member problem where one fault caused another (1763 was caused by an apparent wiring harness fault on the height accumulator circuit). I would measure resistance at the front left actuator and compare to factory spec. I can pull spec later today if I'm reminded.

If the actuator looks good at the actuator itself, I'd then pull the AHC ECU (involves removing the lower steering wheel trim and HVAC pipe to get access to it) and measure from the ECU connector.

Sounds like you've got wiring or actuator problem. Some targeted work with a multilmeter might reveal the answer quickly.

Happy to help here and offer suggestions (free) or if you'd like to come by the ADGU shop in Frederick and have us put our hands on it, we can take a look there as well ($$).

You're right to not want to remove it, IMO. Should be an easy fix.

Hey @suprarx7nut, thank you for weighing in. I’m pretty stoked to learn about Air Down Gear Up. Being in Westminster, you’re pretty close!

With some free time this weekend, I’m testing further; looks like the pump is suspect. Going to attempt a dismount, clean out and rebuild.

 

[IndroCruise]

Thank you for your detailed step by step and supporting docs. My slow reply is due to me being crazy busy launching my business. Now with some time to twist wrenches this afternoon I’m walking thru this diagnostic.

According to what you’re saying here, since no raise or lower function occurs in diagnostic mode, jumping Ts to E1, the pump seems suspect.

As mentioned, vehicle may be in 'fail safe' mode and if so, there will be no AHC functionality -- some tests such as AHC pressures, overall 'globe' condition etc cannot be conducted until this condition is resolved.

Just a few double-checks concerning power supply to the AHC Pump unit before considering Pump disassembly:

• Check that “Active Test” is carried out exactly as described in attachment to previous post – paperclip bridge in place, engine running, centre console switch in “COMFORT” position, then test functions as described,

• Check that fuses AHC-IG and AHC-B are satisfactory and that AHC Main Relay is operating satisfactorily – any of these may interrupt power to the motor driving the AHC Pump, in which case there will be no AHC action,

• Check actual pump operation by direct connection of the motor to the battery per procedure found at FSM Diagnostics item C1762. This avoids any assumptions about any other circuit and establishes whether or not the pump motor is capable of operation. (It has to be said that failure of the pump motor is very rare):

If proceeding to pump dis-assembly, a description of step-by-step pump removal with pics can be found at this thread and may be helpful:

AHC pump removal with pics - https://forum.ih8mud.com/threads/ahc-pump-removal-with-pics.1227554/

Here are a few posts depicting what is sometimes found inside the AHC Pump:

Posts #58, #60, #62 at the thread linked below show all carry very good pics.

The pump unit itself is a small but robust simple gear pump – meaning that it is a positive displacement pump which works by pushing fluid between the gear teeth to the discharge side of the pump, there by developing pressure and flow. The gears themselves are not much bigger than a fingernail in diameter. The pump really is not likely to fail except in extreme circumstances in which somehow a lot of grit gets in there and chews out the gears and/or seals.

AHC Help PNW area - https://forum.ih8mud.com/threads/ahc-help-pnw-area.1255474/page-4#post-13976383

Older pumps can suffer from material blockages as described in the attached Technical Services Bulletin (TSB CP-3006).

Additionally, ancient AHC Fluid can form a soft jelly-like substance which can accumulate as shown in the various pics.

The pump is protected on both sides of the gears by tiny strainers as seen in the pics at the links within the link given above. There can be blockages here, either from particles picked up in the system or by jelly-like material. The strainers can be pulled out of the already dis-assembled pump unit with a pick (carefully as a source of replacement strainers is unknown), or, strainers may be cleaned by backwashing while in place in the pump block.

From memory, a replacement pump costs about USD170-USD200 ex Partsouq (just the pump, not the whole pump/motor/tank assembly which also comes with new pressure and temperature sensors and is much more expensive).

If not done already, it is worth taking the time to read the general description and have a look at the diagrams of the AHC/TEMS systems in the second attachment -- this comes from the FSM.

LATE EDIT: Just to complete the set, see Post #11 at the link below for pictures of the dis-assembled motor which drives the AHC Pump:

AHC help ... yeah another one.

Hey ... Yep another ahc post .... last year I bought new spheres etc fitted em and cleaned the pump assembly everything was fine. This year I decided to do my yearly ahc bleed / flush. Went well everything was OK then one day car was parked a few days and when I went to drive it suspension...

forum.ih8mud.com

As mentioned earlier, failure of this motor is extremely rare -- cannot recall or find any other reports on IH8MUD or elsewhere. The motor in these pics looks good -- brushes are OK, no burn marks on commutator, no evidence of flashover, no grit around bearings -- of course, if pulled down it should be inspected and tested properly both electrically and mechanically.

 

[suprarx7nut]

Hey @suprarx7nut, thank you for weighing in. I’m pretty stoked to learn about Air Down Gear Up. Being in Westminster, you’re pretty close!

With some free time this weekend, I’m testing further; looks like the pump is suspect. Going to attempt a dismount, clean out and rebuild.

Anxious to hear how it goes.

My wager is it's a wiring fault on that damper circuit.

The damper solenoid circuit can cause the other codes and prevent pump motor running. The pump motor can't cause the other faults. That pump/motor can die, but it's very unusual.

Let us know what you find! I like a good mystery.

 

[IndroCruise]

Anxious to hear how it goes.

My wager is it's a wiring fault on that damper circuit.

The damper solenoid circuit can cause the other codes and prevent pump motor running. The pump motor can't cause the other faults. That pump/motor can die, but it's very unusual.

Let us know what you find! I like a good mystery.

Agree with @suprarx7nut -- that is another worthwhile and easy check before pulling things to pieces.

 

[JOSHD]

Hey @suprarx7nut, thank you for weighing in. I’m pretty stoked to learn about Air Down Gear Up. Being in Westminster, you’re pretty close!

With some free time this weekend, I’m testing further; looks like the pump is suspect. Going to attempt a dismount, clean out and rebuild.

Hello Dan. I swapped out the AHC in my 99 LX470 with a traditional lift kit. The AHC was working fine before I removed it. If your pump turns out to be bad, I have mine already removed you can have for the cost of shipping.

 

[danlx]

Anxious to hear how it goes.

My wager is it's a wiring fault on that damper circuit.

The damper solenoid circuit can cause the other codes and prevent pump motor running. The pump motor can't cause the other faults. That pump/motor can die, but it's very unusual.

Let us know what you find! I like a good mystery.

@indocruise @suprarx7nut Thanks everyone who contributed time and knowledge on this thread. Short story long, I got AHC back online today. Temporary now, need to wire it permanent.

Last weekend I did tear the pump down to rule out gel blockage. There was no gunk, just clean AHC fluid, and a strong motor.

Today I pulled open the dash and got out the multimeter, ran down continuity checks from the AHC ECU to the accumulator and valve block.

The failure ended up being in the conductor for the front leveling valve somewhere downstream of connector ID2. Rather than tear up all those harnesses, some of which are sandwiched between the frame and body out of reach, I’m thinking about splicing in a wire to bypass the harness between the valve block plug and the ID2 connector in the LH footwell.

Any tips for routing the wire to be clean, permanent and long lasting?

 

[danlx]

@JOSHD Thank you for the offer of the hardware! I believe I have been able to solve the problem without new parts. Just need to put in some clean wiring to bypass the dead circuit. See #13 on the thread. Really appreciate your offer!

 

[danlx]

Agree with @suprarx7nut -- that is another worthwhile and easy check before pulling things to pieces.

@IndroCruise Thank you for all the time and expertise you've contributed on this thread! I'm hugely grateful.

 

[danlx]

@indocruise @suprarx7nut Thanks everyone who contributed time and knowledge on this thread. Short story long, I got AHC back online today. Temporary now, need to wire it permanent.

Last weekend I did tear the pump down to rule out gel blockage. There was no gunk, just clean AHC fluid, and a strong motor.

Today I pulled open the dash and got out the multimeter, ran down continuity checks from the AHC ECU to the accumulator and valve block.

The failure ended up being in the conductor for the front leveling valve somewhere downstream of connector ID2. Rather than tear up all those harnesses, some of which are sandwiched between the frame and body out of reach, I’m thinking about splicing in a wire to bypass the harness between the valve block plug and the ID2 connector in the LH footwell.

Any tips for routing the wire to be clean, permanent and long lasting?

@suprarx7nut This project is a wrap for now. I found a body grommet near the fuel tank wire pass-through. Used it to run the bypass wire. I’ve created a fairly vulnerable extra SPOF. But at least I know the work and where to find it. This 22 year old beast once again feels like a luxury vehicle to drive. Thanks again for all the help. I can’t wait to stop by your shop.

 

[suprarx7nut]

@suprarx7nut This project is a wrap for now. I found a body grommet near the fuel tank wire pass-through. Used it to run the bypass wire. I’ve created a fairly vulnerable extra SPOF. But at least I know the work and where to find it. This 22 year old beast once again feels like a luxury vehicle to drive. Thanks again for all the help. I can’t wait to stop by your shop.

I have another ID2 downstream failure point I'm working on for a customer. Interesting these seem to be similar, but with different failure conductors.

Thanks for the follow up!

 

[ramangain]

@indocruise @suprarx7nut Thanks everyone who contributed time and knowledge on this thread. Short story long, I got AHC back online today. Temporary now, need to wire it permanent.

Last weekend I did tear the pump down to rule out gel blockage. There was no gunk, just clean AHC fluid, and a strong motor.

Today I pulled open the dash and got out the multimeter, ran down continuity checks from the AHC ECU to the accumulator and valve block.

The failure ended up being in the conductor for the front leveling valve somewhere downstream of connector ID2. Rather than tear up all those harnesses, some of which are sandwiched between the frame and body out of reach, I’m thinking about splicing in a wire to bypass the harness between the valve block plug and the ID2 connector in the LH footwell.

Any tips for routing the wire to be clean, permanent and long lasting?

Nice work!