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 491
41-60010 x 2 required; Rear Part Number 491
51-60010 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.
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