AHC C1762 (1 Viewer)

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I just replaced my center accumulator with a newer one. In the process the tube from the pump to the center accumulator emptied it self. So now the accumulator is change and the canister filled with oil and i keep getting this code

C1762 - Abnormal oil pressure for pump.

Is this message related to the pressure switch on the pump housing? IT must, because each time i remove the connector and unscrew the sensor and put it back in. The truck AHC start operating normaly, something onyl one cycle (N to L and back) and sometimes 3-4 cycles. Until the off start blinking and it throw this same error again C1762.

My guess is that when i change the accumulator that pipe emptied itself and now it as alot of air and when i cycle the AHC air eventualy go back in pump and into the pressure sensor nozzle and keep getting this code.?

Right now the only good chance i have to restart the AHC is by removing this pressure sensor and putting it back. Then i might be able to raise it normally
 
Well Yesterday i swapped a other pump assembly i had. That did not help, in fact it would work any more and it would always throw the error code 1762.

So this morning i decided to test the pump. I removed the AHC relay and installed 2 wires with a toggle switch. I unscrewed to outlet pipe on the pump. And even while operating the motor manually with toggle, no oil would come out. If i'd remove the pressure switch then yes a steady stream of oil would come out of pressure switch opening. So i decided to put the pump i had out of the truck appart and reassemble it while pooring oil so that it bleeded and no air is present in it. I removed the pump on the truck and reinstall the one i bleeded. This time with pressure sensor installed i had a steady flow from the outlet. So i was expecting that i had solved my problem that was probably due to air being present in the pump housing.

I was wrong, once everything reconnected it did the same thing. the pump operate for a second and i get the same error code C1762. So my guess at this point is that the pressure sensor is just not fonctionning. So i removed the sensor and made myself a 4.5v battery using 3x 1.5v. and hooked up the pressure sensor (but on the work bench) and checked voltage per this (last page of pdf);
https://forum.ih8mud.com/attachment.php?attachmentid=340409&d=1247845785
and got 0.5V, since i had a other pump but the pressure sensor on that one is from a UZJ and different i also tested this one and it gave 0.6V. According to FSM voltage should be 4.5V, so i must be very unlucky to have 2 defective sensor, OR the test is not meant to be done with sensor out of the truck.

1- my pump is bleeded and have a good oil output but just cannot built pressure
2- My sensor as failed

Both coincidentally when we change the center accumulator (that as nothing to do with pump). UNLESS the error code C1762 abnormal pressure at pump could be a other pressure sensor. When i look up live data with techstream there are 3 pressure sensor. 1- front 2- rear and 3- accumulator. That 3rd one is a) the device on the center accumulator or B) the pressure sensor on the pump. The device on the center accumulator does looks like a pressure sensor, so since i've swap that accumulator assembly, maybe the sensor on it was defective, that will be the next thing to swap. But since the error code 1762 says abnormal pressure at pump, i am unsure that is it.
 
Well i finally, fix it. It was air. There is a part called a pump attenuator mounted on the frame just under the ACH pump. I beleive this act as a releif valve to maintain high pressure from this attenuator to first accumulator. With air between the pump and this attenuator, it simply cannot build pressure. So you need to crack open the tube from the pump on the attenuator to let air out. Then bleed the center accumulator.

All is good now and gain a good inch of travel with this newer center accumulator!
 
Thanks for the update. I'm sure it's gonna help someone down the line!
 
I got problems with my AHC. Few times got a blinking "OFF" while driving. It's going worse and worse. Finished with a dropped to very low car. The error code shows 62 (pump/pressure). The pump was working (engine runs). I diassembled the pump. Replaced the two O-Rings near the pump gears with bigger ones so i can feel some resistance at turning by hand. Assembled. Nothing. Then i got a pressure gauge. The pump i giving 16MPa so i think this is ok. Bleeding at the accumulator gives not one single drop of oil. So i did bleeding at the pump, the pump was not giving any pressure again. I connected the pump from battery, no pressure, then connected in wrong direction and again in the correct drirection from battery, oil was comming out with pressure so i bleed at the pump attenuator and later at the accumulator. It's woriking now again.

I think the real cause was low pressure due to worn pump. Changing the O-rings gives the pump some extension of life....
After this the problem was air in the lines and pump. I dont know if there is a filter at the lower part of the tank, if yes this can be the cause too.

If You remove the cap of the pump relay You can run the pump without the engine running, it will help with the bleeding.
 
I am in the same boat, hopefully this is the fix. I am getting the same code, the ahc wont go into N or H and defaults to L and has the off light blinking. The ride is really bouncy as well.
 
UNLESS the error code C1762 abnormal pressure at pump could be a other pressure sensor. When i look up live data with techstream there are 3 pressure sensor. 1- front 2- rear and 3- accumulator. That 3rd one is a) the device on the center accumulator or B) the pressure sensor on the pump. The device on the center accumulator does looks like a pressure sensor
Thanks for the post @eleblanc -- it shows what can happen when air is trapped. Seems like you have fixed the problem. By the way, there is only one pressure sensor on the AHC system on LC100/LX470 vehicles. This is located at the AHC Pump. It measures maximum Front Pressure, maximum Rear Pressure and maximum Accumulator pressure, separately one-by-one, when the vehicle is being raised. These readings are shown on Techstream. There is a solenoid valve but no pressure sensor at the front of the Accumulator cylinder and there is no pressure sensor at the Actuators at each wheel.
 
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I dont know if there is a filter at the lower part of the tank, if yes this can be the cause too.
Post #67 by @BullElk at AHC pump out? - https://forum.ih8mud.com/threads/ahc-pump-out.1226629/page-4#post-13401098 for pictures of a clogged pump.

Posts #69 and #70 by @IndroCruise at AHC pump out? - https://forum.ih8mud.com/threads/ahc-pump-out.1226629/page-4#post-13407297 and onwards shows pictures of AHC Pump disassembly and also show the tiny filters (strainers) within the Pump. Pump disassembly, cleanup of the pump and backwashing the filters (strainers) are all straightforward and is made easy by removing the motor/pump/tank assembly from the vehicle. Even if the decision is to replace the pump, disassembly of the existing pump is still recommended. It is better to know how much 'gloop' is in the system before installing a new pump and risking another blockage.
 
Ran techstream on my lx, can anyone lmk if something is standing out from this reading? Getting same error code. C1762

3E715440-44C6-4A9D-8271-7F5B925528BB.jpeg
 
Well i finally, fix it. It was air. There is a part called a pump attenuator mounted on the frame just under the ACH pump. I beleive this act as a releif valve to maintain high pressure from this attenuator to first accumulator. With air between the pump and this attenuator, it simply cannot build pressure. So you need to crack open the tube from the pump on the attenuator to let air out. Then bleed the center accumulator.

All is good now and gain a good inch of travel with this newer center accumulator!
Where is the attenuator and in particular where do you Bleed it from?
 
Where is the attenuator and in particular where do you Bleed it from?

@Skruf -- if you go back to Post #6 in your thread Techstream Error - Techstream Error - https://forum.ih8mud.com/threads/techstream-error.1308912/#post-14901269 and look at the document at the link under the heading General Description of the AHC/TEMS systems with explanations and diagrams of components, you will find a description of what the Attenuator does, where it fits in the overall hydraulic circuit, and where the Attenuator will be found.

The pipe from the AHC Pump runs to the Attenuator which is attached to a bracket on the chassis directly below the AHC Pump assembly -- meaning the whole Tank, Pump, Motor assembly located at the LHS (passenger) side -- of the firewall on your RHD vehicle. It can be reached from above or from the LHS wheel arch (on an RHD vehicle) after removing the covers.

The Attenuator is a small nitrogen-loaded device which smooths the flow from the AHC Pump -- acts in a similar way to the shock absorber valve sometimes fitted to buildings and which is meant to reduced water hammer.

The pics below may help you find the parts and connections.

With engine (and therefore AHC) "OFF" for safety, you could attempt to bleed the Attenuator by disconnecting the pipe on either side of the Attenuator -- not sure what bleeding here really will do for you, and also, bleeding of the whole AHC circuit would be necessary afterwards -- but the suggestion from @eleblanc is acknowledged.


AHC - Attenuator.jpg


Attenuator Location below AHC Pump Assembly on RHD vehicle:
(Smaller pipe comes from AHC Pump above, larger pipe goes to Height Control Accumulator mid-way long LHS chassis rail on the outboard side)

AHC - Attenuator Location.jpg



Attenuator connection to Height Control Accumulator -- and the connection to the Control Valve Assembly:
AHC Height Control Attenuator - corrected.jpg


Location of the Control Valve Assembly mid-way long LHS chassis rail on the inboard side -- seen here above Front LHS torsion bar):

AHC Control Panel Assembly on inner side of LHS chassis rail.jpg


AHC - Control Valve Assembly.jpg
 
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Absolute Champion

Thanks I'll give it a shot and come back to update

My Post #12 in this thread was a bit rushed and unclear -- I have made a few late edits to tidy it up and also added a few more pics in that Post.

Going further ....

Suggest just bear in mind that when bleeding the AHC system in the garage with vehicle at rest and with engine (and therefore AHC "OFF") for safety, nothing much will be seen when attempting to bleed the Attenuator by opening the pipework on either side of it.

In this condition with vehicle at rest and with engine (and therefore AHC "OFF"), the hydraulic circuit then is in the condition shown below.

In this situation, the Levelling Valves in the Control Valve Assembly are closed, the Gate Valves are open -- meaning that AHC Fluid is "locked" behind the Levelling Valves and is not going forward of that point (except in the unusual event of faulty, leaky valves). In other words, this how the vehicle holds its height. Not particularly relevant here but AHC Fluid can move between Front Left and Front Right (and Rear Left and Rear Right) because the Gate Valves are open -- but AHC Fluid cannot move between Front and Rear. Also in this condition, the solenoid valve at the Height Control Accumulator remains closed. AHC Fluid will neither enter nor leave the Height Control Accumulator in this condition -- a signal from the Suspension ECU is required to open the solenoid. If the Height Control Accumulator bleeder valve is opened, then the AHC Fluid under high pressure in the Height Control Accumulator will be released through the bleeder valve to an external container or onto the ground.

AHC - Vehicle at Rest, Engine and AHC OFF.jpg


Height Control Accumulator Description.jpg
 
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My Post #12 in this thread was a bit rushed and unclear -- I have made a few late edits to tidy it up and also added a few more pics in that Post.

Going further ....

Suggest just bear in mind that when bleeding the AHC system in the garage with vehicle at rest and with engine (and therefore AHC "OFF") for safety, nothing much will be seen when attempting to bleed the Attenuator by opening the pipework on either side.

In this condition with vehicle at rest and with engine (and therefore AHC "OFF"), the hydraulic circuit then is in the condition shown below.

In this situation, the Levelling Valves in the Control Valve Assembly are closed, the Gate Valves are open -- meaning that AHC Fluid is "locked" behind the Levelling Valves and is not going forward of that point (except in the unusual event of faulty, leaky valves). In other words, this how the vehicle holds its height. Not particularly relevant here but AHC Fluid can move between Front Left and Front Right (and Rear Left and Rear Right) because the Gate Valves are open -- but AHC Fluid cannot move between Front and Rear. Also in this condition, the solenoid valve at the Height Control Accumulator remains closed. AHC Fluid will neither enter nor leave the Height Control Accumulator in this condition -- a signal from the Suspension ECU is required to open the solenoid. If the Height Control Accumulator bleeder valve is opened, then the AHC Fluid under high pressure in the Height Control Accumulator will be released through the bleeder valve to an external container or onto the ground.

View attachment 3288683

View attachment 3288685
Thanks for the extra detail.

So I bled the line on the pump side of the attenuator (engine and pump running oops). Didn't seem to be any air in there and the was making what seemed like no shortage of pressure.

I also bled the accumulator, the fluid in that looked very aerated. It's now seemingly empty...there was very little in it and it won't refill.

Anyway, it is still stuck in low and the pump is still labouring and then dropping a code.


c1762 Abnormal oil pressure for pump

C1751 Continious current to compressor motor.

It seems like fluid is not going past the attenuator?
I've also noticed the left and right suspension are at different heights.

I know I really need to sit down and spend a couple of days reading and learning about the system but unfortunately
I've been rushing to try and fix because I have to drive the vehicle a couple of hundred km tomorrow and I'm desperate to get it sorted.

At least I have techstream going now

If you have any last minute things I could try please let me know.

Thanks again for the help
 
Thanks for the extra detail.

So I bled the line on the pump side of the attenuator (engine and pump running oops). Didn't seem to be any air in there and the was making what seemed like no shortage of pressure.

I also bled the accumulator, the fluid in that looked very aerated. It's now seemingly empty...there was very little in it and it won't refill.

Anyway, it is still stuck in low and the pump is still labouring and then dropping a code.


c1762 Abnormal oil pressure for pump

C1751 Continious current to compressor motor.

It seems like fluid is not going past the attenuator?
I've also noticed the left and right suspension are at different heights.

I know I really need to sit down and spend a couple of days reading and learning about the system but unfortunately
I've been rushing to try and fix because I have to drive the vehicle a couple of hundred km tomorrow and I'm desperate to get it sorted.

At least I have techstream going now

If you have any last minute things I could try please let me know.

Thanks again for the help

Once C1751 and C1762 appear separately or together, the Suspension ECU prohibits the operation of both the AHC system and the TEMS system, and, Suspension ECU initiates what the FSM calls ‘fail safe function’ – see FSM extracts far below.

If the vehicle has dropped to LO height, it will stay there until the fault(s) are cleared. The AHC Pump will not raise the vehicle in this condition and therefore will not re-charge the Height Control Accumulator -- which only happens at the end of a raise.

You may be able to raise the vehicle with the “Height Control Operation Test (Active Test)” -- see Page 6 of the Pre-check attachment, use unbent paper-clip in place of special service tool SST 09843-18020) but this may not work when the vehicle is in ‘fail safe function’ ....

So before attempting anything else, clear the DTC’s. How to do this?

I wrote that up today for someone else – and you can read it here.

The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-22#post-14910236

It cannot be said that it is impossible that there is a blockage at the Attenuator -- but it can be said that historically on AHC forums that this is one of the least likely causes, a rarely if ever reported experience.

If you consider that the Attenuator is stopping AHC Fluid flow from the AHC Pump, then suggest test that idea by disconnecting the larger pipe on the downstream side and try again to run the AHC Pump so that it sends AHC Fluid through the Attenuator. Then you will know one way or the other.

My sense, sight unseen, from far away, is as follows:

‘Best Case’ scenario is that these DTC’s are the expected consequence of switching ON the AHC Pump against an open circuit -- when the Attenuator pipe was disconnected. The Pressure Sensor (there is only one in the system and it is located next to the AHC Pump) then detects low pressure -- seemingly lots of flow but low pressure and no change for significant period of time, so you get C1751, and, the ECU also responds to low pressure with C1762 -- and then the ECU initiates ‘fail safe function’ as highlighted far below.

‘Maybe Easy Case’ scenario -- the vehicle has a ‘lean’ but neither hub-to-fender tape measurements nor Techstream Height Control Sensor readings nor a screenshot from Techstream are provided -- best always to show these details. The points here are that:
  • Easy -- a physical ‘lean’ on level ground (not in the street) is mechanical -- nothing to do with the AHC system. The cause needs to be understood but will have to do with mechanical wear and tear somewhere. It means that the vehicle should be Front ‘cross-levelled’ (Front physical heights equalised as measured by tape-measure using the torsion bar adjusters) -- very simple job, as described on the attached FSM extract,
  • Not so easy -- if a mechanically-caused physical ‘lean’ by itself, or, in combination with Height Control Sensors in poor condition, results in the Height Control Sensors sending incorrect signals to the Suspension ECU, then if the differences in signals is wide enough, then the Suspension ECU will not be able to resolve the correct height at which the vehicle should self-level at N (or any other height selected on the centre console). In that case, the Suspension ECU may initiate the ‘fail safe function’ and prohibit the operation of the AHC system and the TEMS system – as described at Post #3 at Techstream Error - https://forum.ih8mud.com/threads/techstream-error.1308912/#post-14898658
So the tape-measured Front physical heights should be equal within 10 millimetres and all three Height Control Sensors should read zero +/- 5 millimetres. If so, all good! (Note that absence of a Height Control Sensor DTC does not provide assurance of healthy Sensors).

‘Sad Case’ scenario is that somehow there is air in the system -- also resulting in these DTC’s, not uncommon. Fixing this requires determined, multiple bleeding efforts. You could start by clearing the DTC’s, then opening the RHS Rear bleeder (furthest from the AHC Pump) a little while using a plastic hose on the bleeder screw, seeing whether the emerging AHC Fluid is bubbly or not, then try to operate the AHC Pump to send a little AHC Fluid through the system with the bleeder open until the bubbles disappear. Repeat at the other bleeder screws, leaving the Height Control Accumulator until last. It may be necessary to clear repeated DTC’s (see above), then re-start the effort. If this works, attempt to raise the vehicle by repetitive starts of the AHC Pump (even though protection may cause it to stop, require DTC’s to be cleared, then re-start), until the vehicle starts to raise. If the bubbles do not reduce at the bleeders near each wheel, then it is time to suspect nitrogen from failing membranes from one or more ‘globes’. If the ‘globes’ are original on a 1999 LX470 (now 24 years old), then ‘globe’ failure would not be unexpected, so at this age this is a matter of luck.

‘Worst Case #1’ scenario is that one or more ‘globes’ HAVE failing membranes, allowing nitrogen into the AHC system -- also resulting in these DTC’s. This is very hard to fix without changing out the ‘globes’. Bleeding as above may help – but may not be effective until all nitrogen has been purged or all four of the ‘globes’ are replaced -- Front Part Number 49141-60010 x 2 required; Rear Part Number 49151-60010 x 2 required.

‘Worst Case #2’ scenario is that there is a partial blockage inside the AHC Pump sub-assembly – Part Number 49130-60010. This will result in these same DTC’s if the AHC Pump is starved of AHC Fluid due to partially blocked inlet strainer within the actual pump -- but also will be accompanied by an unusually rough screaming noise from cavitation within this tiny gear pump. The AHC Pump can be disassembled and cleaned -- see Post #9 in this thread -- but at 24 years of age, replacement of the AHC Pump sub-assembly Part Number 49130-60010 is the better idea.

There are other possibilities but that is the best I can manage at short notice late at night.

I wish you the very best of luck -- just a bit concerned that there may be no quick fixes here in the time you have available.

AHC - C1751 highlighted.jpg


AHC - C1762 Highlighted.jpg
 

Attachments

  • AHC suspension precheck and damper check.pdf
    406.1 KB · Views: 46
  • AHC - Cross level per FSM.pdf
    364.6 KB · Views: 44
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Once C1751 and C1762 appear separately or together, the Suspension ECU prohibits the operation of both the AHC system and the TEMS system, and, Suspension ECU initiates what the FSM calls ‘fail safe function’ – see FSM extracts far below.

If the vehicle has dropped to LO height, it will stay there until the fault(s) are cleared. The AHC Pump will not raise the vehicle in this condition and therefore will not re-charge the Height Control Accumulator -- which only happens at the end of a raise.

You may be able to raise the vehicle with the “Height Control Operation Test (Active Test)” -- see Page 6 of the Pre-check attachment, use unbent paper-clip in place of special service tool SST 09843-18020) but this may not work when the vehicle is in ‘fail safe function’ ....

So before attempting anything else, clear the DTC’s. How to do this?

I wrote that up today for someone else – and you can read it here.

The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-22#post-14910236

It cannot be said that it is impossible that there is a blockage at the Attenuator -- but it can be said that historically on AHC forums that this is one of the least likely causes, a rarely if ever reported experience.

If you consider that the Attenuator is stopping AHC Fluid flow from the AHC Pump, then suggest test that idea by disconnecting the larger pipe on the downstream side and try again to run the AHC Pump so that it sends AHC Fluid through the Attenuator. Then you will know one way or the other.

My sense, sight unseen, from far away, is as follows:

‘Best Case’ scenario is that these DTC’s are the expected consequence of switching ON the AHC Pump against an open circuit -- when the Attenuator pipe was disconnected. The Pressure Sensor (there is only one in the system and it is located next to the AHC Pump) then detects low pressure -- seemingly lots of flow but low pressure and no change for significant period of time, so you get C1751, and, the ECU also responds to low pressure with C1762 -- and then the ECU initiates ‘fail safe function’ as highlighted far below.

‘Maybe Easy Case’ scenario -- the vehicle has a ‘lean’ but neither hub-to-fender tape measurements nor Techstream Height Control Sensor readings nor a screenshot from Techstream are provided -- best always to show these details. The points here are that:
  • Easy -- a physical ‘lean’ on level ground (not in the street) is mechanical -- nothing to do with the AHC system. The cause needs to be understood but will have to do with mechanical wear and tear somewhere. It means that the vehicle should be Front ‘cross-levelled’ (Front physical heights equalised as measured by tape-measure using the torsion bar adjusters) -- very simple job, as described on the attached FSM extract,
  • Not so easy -- if a mechanically-caused physical ‘lean’ by itself, or, in combination with Height Control Sensors in poor condition, results in the Height Control Sensors sending incorrect signals to the Suspension ECU, then if the differences in signals is wide enough, then the Suspension ECU will not be able to resolve the correct height at which the vehicle should self-level at N (or any other height selected on the centre console). In that case, the Suspension ECU may initiate the ‘fail safe function’ and prohibit the operation of the AHC system and the TEMS system – as described at Post #3 at Techstream Error - https://forum.ih8mud.com/threads/techstream-error.1308912/#post-14898658
So the tape-measured Front physical heights should be equal within 10 millimetres and all three Height Control Sensors should read zero +/- 5 millimetres. If so, all good! (Note that absence of a Height Control Sensor DTC does not provide assurance of healthy Sensors).

‘Sad Case’ scenario is that somehow there is air in the system -- also resulting in these DTC’s, not uncommon. Fixing this requires determined, multiple bleeding efforts. You could start by clearing the DTC’s, then opening the RHS Rear bleeder (furthest from the AHC Pump) a little while using a plastic hose on the bleeder screw, seeing whether the emerging AHC Fluid is bubbly or not, then try to operate the AHC Pump to send a little AHC Fluid through the system with the bleeder open until the bubbles disappear. Repeat at the other bleeder screws, leaving the Height Control Accumulator until last. It may be necessary to clear repeated DTC’s (see above), then re-start the effort. If this works, attempt to raise the vehicle by repetitive starts of the AHC Pump (even though protection may cause it to stop, require DTC’s to be cleared, then re-start), until the vehicle starts to raise. If the bubbles do not reduce at the bleeders near each wheel, then it is time to suspect nitrogen from failing membranes from one or more ‘globes’. If the ‘globes’ are original on a 1999 LX470 (now 24 years old), then ‘globe’ failure would not be unexpected, so at this age this is a matter of luck.

‘Worst Case #1’ scenario is that one or more ‘globes’ HAVE failing membranes, allowing nitrogen into the AHC system -- also resulting in these DTC’s. This is very hard to fix without changing out the ‘globes’. Bleeding as above may help – but may not be effective until all nitrogen has been purged or all four of the ‘globes’ are replaced -- Front Part Number 49141-60010 x 2 required; Rear Part Number 49151-60010 x 2 required.

‘Worst Case #2’ scenario is that there is a partial blockage inside the AHC Pump sub-assembly – Part Number 49130-60010. This will result in these same DTC’s if the AHC Pump is starved of AHC Fluid due to partially blocked inlet strainer within the actual pump -- but also will be accompanied by an unusually rough screaming noise from cavitation within this tiny gear pump. The AHC Pump can be disassembled and cleaned -- see Post #9 in this thread -- but at 24 years of age, replacement of the AHC Pump sub-assembly Part Number 49130-60010 is the better idea.

There are other possibilities but that is the best I can manage at short notice late at night.

I wish you the very best of luck -- just a bit concerned that there may be no quick fixes here in the time you have available.

View attachment 3288754

View attachment 3288756
Thanks mate, bumpy trip to go camping completed. I'll see if I can fix it for the return trip in a week
 
Once C1751 and C1762 appear separately or together, the Suspension ECU prohibits the operation of both the AHC system and the TEMS system, and, Suspension ECU initiates what the FSM calls ‘fail safe function’ – see FSM extracts far below.

If the vehicle has dropped to LO height, it will stay there until the fault(s) are cleared. The AHC Pump will not raise the vehicle in this condition and therefore will not re-charge the Height Control Accumulator -- which only happens at the end of a raise.

You may be able to raise the vehicle with the “Height Control Operation Test (Active Test)” -- see Page 6 of the Pre-check attachment, use unbent paper-clip in place of special service tool SST 09843-18020) but this may not work when the vehicle is in ‘fail safe function’ ....

So before attempting anything else, clear the DTC’s. How to do this?

I wrote that up today for someone else – and you can read it here.

The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-22#post-14910236

It cannot be said that it is impossible that there is a blockage at the Attenuator -- but it can be said that historically on AHC forums that this is one of the least likely causes, a rarely if ever reported experience.

If you consider that the Attenuator is stopping AHC Fluid flow from the AHC Pump, then suggest test that idea by disconnecting the larger pipe on the downstream side and try again to run the AHC Pump so that it sends AHC Fluid through the Attenuator. Then you will know one way or the other.

My sense, sight unseen, from far away, is as follows:

‘Best Case’ scenario is that these DTC’s are the expected consequence of switching ON the AHC Pump against an open circuit -- when the Attenuator pipe was disconnected. The Pressure Sensor (there is only one in the system and it is located next to the AHC Pump) then detects low pressure -- seemingly lots of flow but low pressure and no change for significant period of time, so you get C1751, and, the ECU also responds to low pressure with C1762 -- and then the ECU initiates ‘fail safe function’ as highlighted far below.

‘Maybe Easy Case’ scenario -- the vehicle has a ‘lean’ but neither hub-to-fender tape measurements nor Techstream Height Control Sensor readings nor a screenshot from Techstream are provided -- best always to show these details. The points here are that:
  • Easy -- a physical ‘lean’ on level ground (not in the street) is mechanical -- nothing to do with the AHC system. The cause needs to be understood but will have to do with mechanical wear and tear somewhere. It means that the vehicle should be Front ‘cross-levelled’ (Front physical heights equalised as measured by tape-measure using the torsion bar adjusters) -- very simple job, as described on the attached FSM extract,
  • Not so easy -- if a mechanically-caused physical ‘lean’ by itself, or, in combination with Height Control Sensors in poor condition, results in the Height Control Sensors sending incorrect signals to the Suspension ECU, then if the differences in signals is wide enough, then the Suspension ECU will not be able to resolve the correct height at which the vehicle should self-level at N (or any other height selected on the centre console). In that case, the Suspension ECU may initiate the ‘fail safe function’ and prohibit the operation of the AHC system and the TEMS system – as described at Post #3 at Techstream Error - https://forum.ih8mud.com/threads/techstream-error.1308912/#post-14898658
So the tape-measured Front physical heights should be equal within 10 millimetres and all three Height Control Sensors should read zero +/- 5 millimetres. If so, all good! (Note that absence of a Height Control Sensor DTC does not provide assurance of healthy Sensors).

‘Sad Case’ scenario is that somehow there is air in the system -- also resulting in these DTC’s, not uncommon. Fixing this requires determined, multiple bleeding efforts. You could start by clearing the DTC’s, then opening the RHS Rear bleeder (furthest from the AHC Pump) a little while using a plastic hose on the bleeder screw, seeing whether the emerging AHC Fluid is bubbly or not, then try to operate the AHC Pump to send a little AHC Fluid through the system with the bleeder open until the bubbles disappear. Repeat at the other bleeder screws, leaving the Height Control Accumulator until last. It may be necessary to clear repeated DTC’s (see above), then re-start the effort. If this works, attempt to raise the vehicle by repetitive starts of the AHC Pump (even though protection may cause it to stop, require DTC’s to be cleared, then re-start), until the vehicle starts to raise. If the bubbles do not reduce at the bleeders near each wheel, then it is time to suspect nitrogen from failing membranes from one or more ‘globes’. If the ‘globes’ are original on a 1999 LX470 (now 24 years old), then ‘globe’ failure would not be unexpected, so at this age this is a matter of luck.

‘Worst Case #1’ scenario is that one or more ‘globes’ HAVE failing membranes, allowing nitrogen into the AHC system -- also resulting in these DTC’s. This is very hard to fix without changing out the ‘globes’. Bleeding as above may help – but may not be effective until all nitrogen has been purged or all four of the ‘globes’ are replaced -- Front Part Number 49141-60010 x 2 required; Rear Part Number 49151-60010 x 2 required.

‘Worst Case #2’ scenario is that there is a partial blockage inside the AHC Pump sub-assembly – Part Number 49130-60010. This will result in these same DTC’s if the AHC Pump is starved of AHC Fluid due to partially blocked inlet strainer within the actual pump -- but also will be accompanied by an unusually rough screaming noise from cavitation within this tiny gear pump. The AHC Pump can be disassembled and cleaned -- see Post #9 in this thread -- but at 24 years of age, replacement of the AHC Pump sub-assembly Part Number 49130-60010 is the better idea.

There are other possibilities but that is the best I can manage at short notice late at night.

I wish you the very best of luck -- just a bit concerned that there may be no quick fixes here in the time you have available.

View attachment 3288754

View attachment 3288756


Ok so.

1. I've cleared all the faults repeatedly. I can hear the pump working to raise it but it won't go up. Inevitably throws a code and reverts back to low
2. Re: best case scenario- it was throwing the codes prior to trying to bleed the Attenuator. Given that the fluid was spurting out under pressure I can't see how that could have introduced air into the system.
3. Tried the manual raise with paperclip. Got it into test mode but couldn't hear the pump starting when I tested it. In any case I already know the pump is working and nothing is moving.

So next is the height control Sensors.

They are out from each other more than the spec you listed above.

I'll try to get these fixed tomorrow.
Could it be that clearing codes and putting these back to spec might solve the problem?

Thanks again for your help
 
Ok so.

1. I've cleared all the faults repeatedly. I can hear the pump working to raise it but it won't go up. Inevitably throws a code and reverts back to low
2. Re: best case scenario- it was throwing the codes prior to trying to bleed the Attenuator. Given that the fluid was spurting out under pressure I can't see how that could have introduced air into the system.
3. Tried the manual raise with paperclip. Got it into test mode but couldn't hear the pump starting when I tested it. In any case I already know the pump is working and nothing is moving.

So next is the height control Sensors.

They are out from each other more than the spec you listed above.

I'll try to get these fixed tomorrow.
Could it be that clearing codes and putting these back to spec might solve the problem?

Thanks again for your help

This must be so @#$%^& frustrating when you are away camping!!

If you could post the full screenshot from Techstream, that would save a lot of guesswork.

The vehicle appears to have some problems which are not difficult to fix with a few tools and a multi-meter and Techstream. Sadly, some issues may require some supplies and replacement parts -- and these may make things difficult on a camping trip.

It may be that the best that can be achieved is a ‘limp home’ outcome as described in the response below to your question #3.

Going through your list of comments/questions ….


1. I've cleared all the faults repeatedly. I can hear the pump working to raise it but it won't go up. Inevitably throws a code and reverts back to low.

Noted. It is a basic step when problem-solving to clear DTC’s then restart the vehicle. This makes sure that only current issues are indicated by the ECU. The idea is for current issues to over-write previous events held in the ECU memory and which may no longer be relevant.

The actual codes (DTC’s) coming up on Techstream after clearing are not mentioned -- presumably one or both of C1751 and C1762 are repeating??

So which code or codes are appearing now? This detail is important -- helps direct attention to the likely problem, helps avoid chasing an unlikely cause.

The C1762 is almost instantaneous (~0.6 seconds -- see description highlighted in yellow in the FSM extract for C1762 shown at Post #16).

So unless you say otherwise, let’s assume that the symptoms described on your vehicle better match those for C1751?

This means that after clearing the DTC, the AHC Pump starts again. The ECU then detects the defined fault condition highlighted in yellow in the FSM extract for C1751, shown at Post #16.

At that point, the ECU initiates ‘fail safe function’ defined for C1751 -- meaning that

  • height control is prohibited, and,
  • the AHC Pump stops and will not raise the vehicle, and,
  • the vehicle adopts the height of the lowest wheel according to the signals sent by the Height Control Sensors to the ECU -- to do this the ECU opens the Front and Rear Levelling Valves in the Control Valve Assembly, allowing AHC Fluid to escape back to the AHC Tank, and so the vehicle settles.
That seems to describe the symptoms on your vehicle?

The question then is

“What is causing the slow rise or no rise of the vehicle, slow or no build up of AHC pressures, ‘fail safe function’ and cut-out of the AHC system?”

As distinct from C1762, for which the question would be

“What is causing immediate low delivery pressure, ‘fail safe function’ and cut-out of the AHC system, such as, AHC Fluid starvation on the inlet side of the pump due to blockage at the internal inlet strainer, or, build-up of air (or nitrogen from failing ‘globe’ membranes) which have the same effect?”

In either case, the box marked “Trouble Area” in the FSM extract points to possible causes.

Back to C1751 and the list of “Trouble Areas”:

  • AHC Pump & Motor or AHC Pump & Motor circuits ? No -- the Pump and Motor work fine,
  • AHC motor relay? No -- AHC Pump & Motor work fine, so no point in testing this relay (which is in the fuse/relay box in the engine bay), nor the power circuits to the pump motor, nor the power circuits to the AHC system (via AHC Main Relay),
  • Height Control Sensor linkage damage? Possible, worth inspecting all three Sensors under the vehicle. Faulty Height Control Sensors AND ALSO faults in the Height Control Sensor OR in the circuits between the Sensors and the ECU can be a cause of exactly the same symptoms WITH OR WITHOUT Height Sensor DTC’s C1711, C1712, C1713 -- see extract below and attached,
  • AHC Fluid leakage or low AHC Fluid levels? Unlikely -- assume that these would have been discovered by now,
  • Fluid clog in lines or in solenoid valves? Too early to eliminate this cause, especially partial blockage of the AHC Pump Inlet Strainer (not to be confused with the large strainer at the top of the AHC Tank) -- noting the 24 years of age and 400,000 kilometres on the vehicle and the possibility that change/flush of AHC Fluid may not always have occurred in line with the Owner’s Manual? If not, there almost certainly will be old sludgy AHC Fluid lurking in the system. If so, a blockage is highly likely,
Not directly mentioned in the FSM-specified “Trouble Areas” for C1751 are the following:
  • Vehicle weight – if the vehicle weight exceeds the FSM-specified capability of the AHC system, then AHC pressures will be excessive and the vehicle will not rise -- see extract below:
AHC Load Limits.jpg

  • Unadjusted AHC pressures -- if the suspension has not been reviewed and adjusted periodically, then gradual weakening of Front torsion bars and especially Rear coil springs will result in AHC pressures increasing over the 24 years, 400,000 kilometres of life of the vehicle. Presumably, the necessary checks and adjustments were made when KING KTRS-79 Rear springs were fitted? If AHC pressures have remained within the FSM-specified ranges, then AHC presures should not be a problem. I have KTRS-79 springs and they work just fine with the AHC and TEMS systems. However, you won't be able to check pressures with Techstream until you can move the vehicle heights, N > LO > N.

  • Aged or worn or damaged AHC Pump -- this is a simple positive displacement small gear pump. Wear and tear over time, or induced by persistent cavitation, results in the AHC Fluid by-passing the gears internally (fluid passes above and below the gears then passes through the gearcases and in effect recirculates). The pump then is no longer capable of delivering the required flow. It is noted that your Posts on both “100 Series” and “200 Series” forums describe a very noisy AHC Pump. This probably indicates an AHC Pump which is worn and/or cavitating (or aerated -- different to cavitation, similar effect, happens when a starved pump is drawing air past the seals).
The AHC Pump sub-assembly Part Number 48901-60010 can be disassembled and cleaned -- but replacement is the better idea.

AHC pump removal with pics - https://forum.ih8mud.com/threads/ahc-pump-removal-with-pics.1227554/#post-13401739

A few more pics by @BullElk at Post #67 and additional pics by me at Posts #69 and #70 in another thread (below) show what the internals of the actual AHC Pump sub-assembly 48901-60010 look like when it is extracted from the overall assembly 48910-60012 ….

AHC pump out? - https://forum.ih8mud.com/threads/ahc-pump-out.1226629/page-4#post-13401098

2. Re: best case scenario- it was throwing the codes prior to trying to bleed the Attenuator. Given that the fluid was spurting out under pressure I can't see how that could have introduced air into the system.

Any time the AHC system is opened -- such as when replacing parts or such as your Attenuator test -- there is an opportunity for air to enter, if not at the AHC Pump, then into the Attenuator and downstream pipelines, and bleeding of the system is essential. This could require (say) 2 x 2.5 litre cans of Toyota/Lexus AHC Fluid Part Number 08886-01805 -- might not need it all but it is very frustrating to be part way through a job and run out of fluid. Some folks are happy to re-use fluid -- makes as much or as little sense as re-using the relevant fluid in any other system on the vehicle!!

Previously, you have mentioned bubbly fluid emerging when bleeding the Height Control Accumulator. If air (or nitrogen) is found here, then however it got there, it is now throughout the system.

When a vehicle with a healthy AHC system is raised, the ECU opens the solenoid valve at the front of the Height Control Accumulator and the AHC Fluid contained under pressure of about 10.4 Mpa is sent to all four ‘shock absorbers’ (which actually are weight-bearing hydraulic struts in an AHC-equipped vehicle). This is how the Height Control Accumulator assists in raising the vehicle -- and this is its only purpose, it plays no role in damping performance.

When the vehicle is lowered, the Levelling Valves are opened, the vehicle drops until the ECU receives the relevant voltage signal from (healthy!) Height Control Sensors, and AHC Fluid returns to the AHC Tank. The AHC Pump then uses the fluid in the tank to re-charge the Height Control Accumulator and also when self-levelling the vehicle.

In this way, once air (or nitrogen) is in the hydraulic circuit in the AHC system, the whole system is polluted with air (or nitrogen) and remains so until the gas is removed by bleeding, or escapes very, very slowly to atmosphere at the AHC Tank. However, some gas will remain dissolved or entrained in the fluid, locked in behind the Levelling Valves, or in the unswept volume of the ‘shock absorbers’, nowhere near the AHC Tank. Suggest eyeball the hydraulic circuit in Post #14 to see this.

If the ‘globes’ on the vehicle are original, then at 24 years of age and 400,000 kilometres, the ‘globes’ are long past their best. It is highly likely that nitrogen has passed through deteriorated membranes within the ‘globes’ and into the AHC Fluid. Leakage of nitrogen may be continuing. If this is happening, this continuous ‘supply’ of new gas bubbles will not be removed until the ‘globes’ are completely purged of nitrogen or the ‘globes’ are replaced. Until then, fine bubbles or entrained gas will continue to pollute the whole AHC system. Bubbles will continue to be observed during bleeding as gas emerges or expands when arriving from the bleed tube at atmospheric pressure.


3. Tried the manual raise with paperclip. Got it into test mode but couldn't hear the pump starting when I tested it. In any case I already know the pump is working and nothing is moving.

Yes -- it was understood that the AHC Pump was and is working, so there is no need to look for problems in the power circuit to the pump motor and reasonable to assume that the power circuit through the AHC Main Relay to the ECU works, at least until the ECU finds a defined fault, such as C1751 or other faults.

The purpose of the “Active Test" was to raise the vehicle.

Suggest double-check that the test was carried out exactly in the sequence described in the attachment at Post #16:

(a) bridge terminals Ts and E1 at DLC1

(b) Initiate test function by pushing the DOWN button of the height selection switch 5 times or more within 5 seconds of starting the engine


(c) Select COMFORT mode

(d) Test Front by pushing UP and DOWN Buttons

(e) Test Rear by push and hold height control ON/OFF switch AND simultaneously pushing UP and DOWN buttons.

If this can be made to work and the vehicle raised to N height, then a temporary 'workaround' might be to de-power the AHC system and leaving the vehicle “locked” in N height. This would mean:

(i) de-power the AHC Pump motor by pulling the AHC 50 Amp fuse in the engine bay, and,

(ii) depower the AHC Main Relay and the AHC ECU and system functions by pulling the AHC-B fuse and the AHC-IG fuse behind the LHS kick-panel forward of the LHS Front Door.

This might at least allow you to travel home at N height, hopefully with damping locked at Step 8 (of 16 Steps), still not good, but better than long distance travel at LO height. (If there are other faults, the ‘fail safe function’ may cause damping to be locked at Step 16 of 16 Steps). If the ‘globes’ are blown, damping will be very poor anyway.

If the “Active Test” will not work when correctly executed, then that is confirmation of other problems in the AHC circuits.

It is possible to power the AHC Pump directly from the battery -- but to raise the vehicle, the Levelling Valves have to be opened by the ECU -- refer to the hydraulic circuit at Post #14.


So next is the height control Sensors.
They are out from each other more than the spec you listed above.
I'll try to get these fixed tomorrow.
Could it be that clearing codes and putting these back to spec might solve the problem?


This is possible -- I do think that you have more of the other problems mentioned above -- but let’s hope that luck is on your side ….

Again, it would be good to see a Techstream screenshot on level ground. If there are very wide differences between signals from the Height Control Sensors, then the ECU will be unable to resolve the height setting required. That may result in strange height behaviours at Front and/or Rear of the vehicle and/or the ECU initiating ‘fail safe function’ as defined for the Height Control Sensors and/or circuits. Such a situation can be due to any of

  • Damaged or worn linkages at the Sensors,
  • Wear and tear, moisture or corrosion within the Sensors,
  • Wiring damage in the connectors and/or harnesses which connect the Sensors and the ECU.
It also can easily happen that a faulty Sensor circuit sends a voltage to the ECU which is within the FSM-specified range but is the incorrect voltage for the actual height of the vehicle. In such cases there will be no DTC recorded by ECU and shown on Techstream. The Sensor and/or circuit fault must be diagnosed by recognising the vehicle behaviour, then testing the suspected Sensor(s) and/or circuit(s).

Checking the linkage and adjustment of the Height Control Sensors is worthwhile. Removal, disassembly and cleaning of the Sensors may help for a while, but OEM replacements of long-aged Sensors is the best idea.

It would be worthwhile to bring the Height Control Sensor adjusters as close as possible to zero on level ground, (engine OFF, ignition ON) in an effort to eliminate problems possibly caused by conflicting Sensor signals. If the Sensors are faulty, but the AHC Pump actually works and now responds to “Active Test”, this might result in the vehicle arriving at strange Front and Rear heights. This may be acceptable for ‘limp home’ purposes until faulty Sensors can be replaced.

See below on Height Control Sensors -- note in particular the defined causes of a DTC and also consider faults which can and do occur and which do not record a DTC -- basically when changed internal resistance causes a voltage signal below the minimum of 0.3 volts or above the maximum of 4.7 volts.

The FSM procedure for Sensor testing is shown in the attachment below. Or use the alternative at Post #2 in this thread:


Happy to try and help further -- but the above is about the best I can offer in the absence of more information.

All this is too much reading for a holiday! Hope you can enjoy your trip and arrive home safely despite these distractions!!

AHC - Height Control Sensors - DTC.jpg


AHC Fuse Locations for 2006 LC100 with AHC (may be slightly different for 1999 LX470):

AHC Fuse Locations.jpg
 

Attachments

  • AHC - Height Control Troubleshooting m_di_0224.pdf
    80.4 KB · Views: 45
Last edited:
This must be so @#$%^& frustrating when you are away camping!!

If you could post the full screenshot from Techstream, that would save a lot of guesswork.

The vehicle appears to have some problems which are not difficult to fix with a few tools and a multi-meter and Techstream. Sadly, some issues may require some supplies and replacement parts -- and these may make things difficult on a camping trip.

It may be that the best that can be achieved is a ‘limp home’ outcome as described in the response below to your question #3.

Going through your list of comments/questions ….


1. I've cleared all the faults repeatedly. I can hear the pump working to raise it but it won't go up. Inevitably throws a code and reverts back to low.

Noted. It is a basic step when problem-solving to clear DTC’s then restart the vehicle. This makes sure that only current issues are indicated by the ECU. The idea is for current issues to over-write previous events held in the ECU memory and which may no longer be relevant.

The actual codes (DTC’s) coming up on Techstream after clearing are not mentioned -- presumably one or both of C1751 and C1762 are repeating??

So which code or codes are appearing now? This detail is important -- helps direct attention to the likely problem, helps avoid chasing an unlikely cause.

The C1762 is almost instantaneous (~0.6 seconds -- see description highlighted in yellow in the FSM extract for C1762 shown at Post #16).

So unless you say otherwise, let’s assume that the symptoms described on your vehicle better match those for C1751?

This means that after clearing the DTC, the AHC Pump starts again. The ECU then detects the defined fault condition highlighted in yellow in the FSM extract for C1751, shown at Post #16.

At that point, the ECU initiates ‘fail safe function’ defined for C1751 -- meaning that

  • height control is prohibited, and,
  • the AHC Pump stops and will not raise the vehicle, and,
  • the vehicle adopts the height of the lowest wheel according to the signals sent by the Height Control Sensors to the ECU -- to do this the ECU opens the Front and Rear Levelling Valves in the Control Valve Assembly, allowing AHC Fluid to escape back to the AHC Tank, and so the vehicle settles.
That seems to describe the symptoms on your vehicle?

The question then is

“What is causing the slow rise or no rise of the vehicle, slow or no build up of AHC pressures, ‘fail safe function’ and cut-out of the AHC system?”

As distinct from C1762, for which the question would be

“What is causing immediate low delivery pressure, ‘fail safe function’ and cut-out of the AHC system, such as, AHC Fluid starvation on the inlet side of the pump due to blockage at the internal inlet strainer, or, build-up of air (or nitrogen from failing ‘globe’ membranes) which have the same effect?”

In either case, the box marked “Trouble Area” in the FSM extract points to possible causes.

Back to C1751 and the list of “Trouble Areas”:

  • AHC Pump & Motor or AHC Pump & Motor circuits ? No -- the Pump and Motor work fine,
  • AHC motor relay? No -- AHC Pump & Motor work fine, so no point in testing this relay (which is in the fuse/relay box in the engine bay), nor the power circuits to the pump motor, nor the power circuits to the AHC system (via AHC Main Relay),
  • Height Control Sensor linkage damage? Possible, worth inspecting all three Sensors under the vehicle. Faulty Height Control Sensors AND ALSO faults in the Height Control Sensor OR in the circuits between the Sensors and the ECU can be a cause of exactly the same symptoms WITH OR WITHOUT Height Sensor DTC’s C1711, C1712, C1713 -- see extract below and attached,
  • AHC Fluid leakage or low AHC Fluid levels? Unlikely -- assume that these would have been discovered by now,
  • Fluid clog in lines or in solenoid valves? Too early to eliminate this cause, especially partial blockage of the AHC Pump Inlet Strainer (not to be confused with the large strainer at the top of the AHC Tank) -- noting the 24 years of age and 400,000 kilometres on the vehicle and the possibility that change/flush of AHC Fluid may not always have occurred in line with the Owner’s Manual? If not, there almost certainly will be old sludgy AHC Fluid lurking in the system. If so, a blockage is highly likely,
Not directly mentioned in the FSM-specified “Trouble Areas” for C1751 are the following:
  • Vehicle weight – if the vehicle weight exceeds the FSM-specified capability of the AHC system, then the vehicle will not rise -- see extract below:
View attachment 3292155
  • Aged or worn or damaged AHC Pump -- this is a simple positive displacement small gear pump. Wear and tear over time, or induced by persistent cavitation, results in the AHC Fluid by-passing the gears internally (fluid passes above and below the gears then passes through the gearcases and in effect recirculates). The pump then is no longer capable of delivering the required flow. It is noted that your Posts on both “100 Series” and “200 Series” forums describe a very noisy AHC Pump. This probably indicates an AHC Pump which is worn and/or cavitating (or aerated -- different to cavitation, similar effect, happens when a starved pump is drawing air past the seals).
The AHC Pump sub-assembly Part Number 48901-60010 can be disassembled and cleaned -- but replacement is the better idea.

AHC pump removal with pics - https://forum.ih8mud.com/threads/ahc-pump-removal-with-pics.1227554/#post-13401739

A few more pics by @BullElk at Post #67 and additional pics by me at Posts #69 and #70 in another thread (below) show what the internals of the actual AHC Pump sub-assembly 48901-60010 look like when it is extracted from the overall assembly 48910-60012 ….

AHC pump out? - https://forum.ih8mud.com/threads/ahc-pump-out.1226629/page-4#post-13401098

2. Re: best case scenario- it was throwing the codes prior to trying to bleed the Attenuator. Given that the fluid was spurting out under pressure I can't see how that could have introduced air into the system.

Any time the AHC system is opened -- such as when replacing parts or such as your Attenuator test -- there is an opportunity for air to enter, if not at the AHC Pump, then into the Attenuator and downstream pipelines, and bleeding of the system is essential. This could require (say) 2 x 2.5 litre cans of Toyota/Lexus AHC Fluid Part Number 08886-01805 -- might not need it all but it is very frustrating to be part way through a job and run out of fluid. Some folks are happy to re-use fluid -- makes as much or as little sense as re-using the relevant fluid in any other system on the vehicle!!

Previously, you have mentioned bubbly fluid emerging when bleeding the Height Control Accumulator. If air (or nitrogen) is found here, then however it got there, it is now throughout the system.

When a vehicle with a healthy AHC system is raised, the ECU opens the solenoid valve at the front of the Height Control Accumulator and the AHC Fluid contained under pressure of about 10.4 Mpa is sent to all four ‘shock absorbers’ (which actually are weight-bearing hydraulic struts in an AHC-equipped vehicle). This is how the Height Control Accumulator assists in raising the vehicle -- and this is its only purpose, it plays no role in damping performance.

When the vehicle is lowered, the Levelling Valves are opened, the vehicle drops until the ECU receives the relevant voltage signal from (healthy!) Height Control Sensors, and AHC Fluid returns to the AHC Tank. The AHC Pump then uses the fluid in the tank to re-charge the Height Control Accumulator and also when self-levelling the vehicle.

In this way, once air (or nitrogen) is in the hydraulic circuit in the AHC system, the whole system is polluted with air (or nitrogen) and remains so until the gas is removed by bleeding, or escapes very, very slowly to atmosphere at the AHC Tank. However, some gas will remain dissolved or entrained in the fluid, locked in behind the Levelling Valves, or in the unswept volume of the ‘shock absorbers’, nowhere near the AHC Tank. Suggest eyeball the hydraulic circuit in Post #14 to see this.

If the ‘globes’ on the vehicle are original, then at 24 years of age and 400,000 kilometres, the ‘globes’ are long past their best. It is highly likely that nitrogen has passed through deteriorated membranes within the ‘globes’ and into the AHC Fluid. Leakage of nitrogen may be continuing. If this is happening, this continuous ‘supply’ of new gas bubbles will not be removed until the ‘globes’ are completely purged of nitrogen or the ‘globes’ are replaced. Until then, fine bubbles or entrained gas will continue to pollute the whole AHC system. Bubbles will continue to be observed during bleeding as gas emerges or expands when arriving from the bleed tube at atmospheric pressure.


3. Tried the manual raise with paperclip. Got it into test mode but couldn't hear the pump starting when I tested it. In any case I already know the pump is working and nothing is moving.

Yes -- it was understood that the AHC Pump was and is working, so there is no need to look for problems in the power circuit to the pump motor and reasonable to assume that the power circuit through the AHC Main Relay to the ECU works, at least until the ECU finds a defined fault, such as C1751 or other faults.

The purpose of the “Active Test was to raise the vehicle.

Suggest double-check that the test was carried out exactly in the sequence described in the attachment at Post #16:

(a) bridge terminals Ts and E1 at DLC1

(b) Initiate test function by pushing the DOWN button of the height selection switch 5 times or more within 5 seconds of starting the engine


(c) Select COMFORT mode

(d) Test Front by pushing UP and DOWN Buttons

(e) Test Rear by push and hold height control ON/OFF switch AND simultaneously pushing UP and DOWN buttons.

If this can be made to work and the vehicle raised to N height, then an idea might be to de-power the AHC system and leaving the vehicle “locked” in N height. This means:

(i) de-power the AHC Pump motor by pulling the AHC 50 Amp fuse in the engine bay, and,

(ii) depower the AHC Main Relay and the AHC ECU and system functions by pulling the AHC-B fuse and the AHC-IG fuse behind the LHS kick-panel forward of the LHS Front Door.

This might at least allow you to travel home at N height, hopefully with damping locked at Step 8 (of 16 Steps), still not good, but better than long distance travel at LO height. (If there are other faults, the ‘fail safe function’ may cause damping to be locked at Step 16 of 16 Steps). If the ‘globes’ are blown, damping will be very poor anyway.

If the “Active Test” will not work when correctly executed, then that is confirmation of other problems in the AHC circuits.

It is possible to power the AHC Pump directly from the battery -- but to raise the vehicle, the Levelling Valves have to be opened by the ECU -- refer to the hydraulic circuit at Post #14.


So next is the height control Sensors.
They are out from each other more than the spec you listed above.
I'll try to get these fixed tomorrow.
Could it be that clearing codes and putting these back to spec might solve the problem?


This is possible -- I do think that you have more of the other problems mentioned above -- but let’s hope that luck is on your side ….

Again, it would be good to see a Techstream screenshot on level ground. If there are very wide differences between signals from the Height Control Sensors, then the ECU will be unable to resolve the height setting required. That may result in strange height behaviours at Front and/or Rear of the vehicle and/or the ECU initiating ‘fail safe function’ as defined for the Height Control Sensors and/or circuits. Such a situation can be due to any of

  • Damaged or worn linkages at the Sensors,
  • Wear and tear, moisture or corrosion within the Sensors,
  • Wiring damage in the connectors and/or harnesses which connect the Sensors and the ECU.
It also can easily happen that a faulty Sensor circuit sends a voltage to the ECU which is within the FSM-specified range but is the incorrect voltage for the actual height of the vehicle. In such cases there will be no DTC recorded by ECU and shown on Techstream. The Sensor and/or circuit fault must be diagnosed by recognising the vehicle behaviour, then testing the suspected Sensor(s) and/or circuit(s).

Checking the linkage and adjustment of the Height Control Sensors is worthwhile. Removal, disassembly and cleaning of the Sensors may help for a while, but OEM replacements of long-aged Sensors is the best idea.

It would be worthwhile to bring the Height Control Sensor adjusters as close as possible to zero on level ground, (engine OFF, ignition ON) in an effort to eliminate problems possibly caused by conflicting Sensor signals. If the Sensors are faulty, but the AHC Pump actually works and now responds to “Active Test”, this might result in the vehicle arriving at strange Front and Rear heights. This may be acceptable for ‘limp home’ purposes until faulty Sensors can be replaced.

See below on Height Control Sensors -- note in particular the defined causes of a DTC and also consider faults which can and do occur and which do not record a DTC -- basically when changed internal resistance causes a voltage signal below the minimum of 0.3 volts or above the maximum of 4.7 volts.

The FSM procedure for Sensor testing is shown in the attachment below. Or use the alternative at Post #2 in this thread:


Happy to try and help further -- but the above is about the best I can offer in the absence of more information.

All this is too much reading for holiday! Hope you can enjoy your trip and arrive home safely despite these distractions!!

View attachment 3292161


AHC Fuse Locations for 2006 LC100 with AHC (may be slightly different for 1999 LX470):

View attachment 3292169
Wow, that's so much info. Thanks again for taking this effort to help me out.

I've attached the techstream results. They were on a flat as I could find surface so clearly they are miles out.


1. Error codes remain unchanged.
2. Note that I have previously flushed ahc with new fluid
3.
At that point, the ECU initiates ‘fail safe function’ defined for C1751 -- meaning that
  • height control is prohibited, and,
  • the AHC Pump stops and will not raise the vehicle, and,
  • the vehicle adopts the height of the lowest wheel according to the signals sent by the Height Control Sensors to the ECU -- to do this the ECU opens the Front and Rear Levelling Valves in the Control Valve Assembly, allowing AHC Fluid to escape back to the AHC Tank, and so the vehicle settles.
That seems to describe the symptoms on your vehicle?


The scenario you described DOES sound like what's happening.

I'm juggling 2 little kids and having to have a go at this bit by bit so excuse me if I can't get all the info for you immediately.

Gotta run but I'll have another read later

Cheers

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