Hi @old470,
You have invested heavily in renovating the AHC system on your 1999 LX470 and hopefully you have overcome the previous frustrations in getting the system to work?
If not, here are some further notes in case they are of assistance to you or to some other IH8MUD Member with similar problems.
Can you please give an update about your experiences? There is always something that can be learned from IH8MUD Members’ stories.
Further to Post #5 in this thread, here are my recommendations on what to do next. Some notes on “Background Issues and Symptoms” have been added at the end of these recommendations for further information.
My suggestion is to proceed in a sequence in a methodical way based on observations and measurements, rather than jump from one guess to the next.
In the notes below, please be very aware that the absence of a Diagnostic Trouble Code (DTC)
does not mean that there is no fault. The DTC’s only provide a guide and may or may not be revealed by Techstream or other scanner, especially when there is more than one related fault. As far as possible, it is best to check and test in a basic way that gives positive confirmation that a component or circuit is healthy.
A repeated DTC which cannot be cleared
does indicate a fault.
Step 1: Cross levelling and Sensor Check
First, check ‘cross level’ of the front of the vehicle by measuring with a tape-measure the distance from the centre of the hub to the lip of the fender vertically above, at each wheel.
Theoretically and as stated in the Factory Service Manual (FSM), ’cross levelling’ should be done when the vehicle is at “N” height.
In your case, you cannot raise to “N” height, so it is worthwhile to look at this even in the “LO” height position.
Just remember that this is a non-standard approach and it is important to ‘cross level’ properly at “N” height when the AHC system is working again. (There can be slightly different side-to-side effects at the extremes of “LO” and “HI” compared to “N” height due wear of springs and torsion bars or worn or out-of-adjustment mechanical components, especially on older vehicles).
When ‘cross-levelling’ the actual tape-measurement numbers at Front Right and Front Left hub-to-centre are not important but they should be
equal within 10mm (0.39 inches).
The purpose of front ‘cross levelling’ has two parts:
- to ensure that both torsion bars are carrying equal loads, and,
- to avoid a built-in ‘lean’. This in turn causes confused readings ‘to be sent to the ECU by the Height Control Sensors (even with new Sensors as in this case), and then in turn this may cause the ECU to switch to ‘fail safe mode’ which may prevent AHC operation.
If the static (engine “OFF”) front hub-to-fender heights not close to equal, adjust using the torsion bar adjusters
and NOT the Height Control Sensor adjusters to equalise the front side-to-side heights (at least within 10 mm difference, less difference is better) with the engine “OFF”.
Then read the Height Control Sensor adjusters again on your scanner. The readings won’t approach zero until the vehicle can be raised to “N”, so don’t worry about this yet, but reducing the
differences in the readings also is very important – see “Background Issues and Symptoms” notes far below.
If the vehicle has been ‘cross levelled’ as well as can be done, but the Height Control Sensors are still showing such widely different readings, then it is possible that the Electronic Control Unit (ECU) has put the AHC/TEMS system into ‘fail safe mode’ -- as explained this post and in the FSM reference given in my previous post. In ‘fail safe mode’, the AHC/TEMS system will not work properly and may not work at all. There are other possibilities and these also are mentioned in the “Background Issues and Symptoms” notes below.
As a starting point, persistent widely different Height Control Sensor readings lead to suspicion about the condition of the Height Control Sensors or the various connectors and/or reliable continuity of the harness wiring to the ECU.
For this vehicle, even though the Height Control Sensors are new, it is still worthwhile to remove them and check them physically and electrically. This is set out in the FSM and is summarised above in Post #5 and later in this Post.
At the same time, suggest check electrical continuity of all connectors and relevant harnesses. (Note: Harness damage is unusual but this is a 22 year old vehicle, so physically inspect the underbody carefully for harness damage, especially at places where any harness is visible and may have been struck by stones or road debris — although both are uncommon in Singapore. Follow up any suspicions by testing electrical continuity).
At this time, also suggest check the voltage at the connectors and at the relevant terminals (SHB and SHG) at the ECU as described in the same section of the FSM as the other Height Control Sensor checks (C1711 at Page DI-226).
Again, absence of a DTC does not tell you that the system is healthy.
Special Note 1: There is no point in trying to correct a cross-levelling problem by adjusting the Height Control Sensors. If the vehicle steering is pointed straight ahead, or if the vehicle is stationary, then the Gate Valves in the Control Valve Assembly are open and the front AHC pressures are the same on both sides. The Front AHC pressures are not changed by adjusting the position of the Right and Left Sensors and so the side-to-side heights cannot be changed by adjusting the position of the Sensors. Attempts at ‘cross levelling’ with Height Control Sensors will result only in confusion and frustration.
Step 2: Increase load on front torsion bars
Turn both Torsion Bar adjusters clockwise (when looking up from underneath the vehicle) by say 5 to 8 turns or even more.
Purpose: The vehicle is stuck in the “LO” height position, so AHC pressures cannot be measured by raising from “LO” to “N”. Therefore it is necessary to make a guess to increase Torsion Bar load to make sure that the AHC system is
not overloaded. When the AHC system is working again, AHC pressures can be measured correctly and then at least front pressures easily can be adjusted correctly using the Torsion Bar adjusters.
Step 3: Test AHC Pump operation direct from 12 volt battery (not via ‘Active Test’) and related tests:
To avoid doubts about the condition of the ECU or the effect of some conditions in ‘fail safe mode’, remove the power connector from the electric motor which drives the AHC Pump, then use leads to connect the motor terminals correctly to the vehicle battery (or some other 12 volt battery). Be very careful to avoid arcing by not to touching the leads together and
wear eye protection in case of accidents. For correct polarity, refer carefully to the diagrams in the FSM at this reference:
https://lc100e.github.io/manual/
Then follow these tabs:
Repair Manual > DIAGNOSTICS > ACTIVE HEIGHT CONTROL SUSPENSION & SKYHOOK TEMS > C1762 at Page DI-257 – Fluid Pressure Abnormality
Purpose: To check in the above test that the AHC Motor and Pump
actually are capable of running when voltage is supplied.
Then, even if no DTC’s are visible, also suggest doing the following tests using the procedure described “IN CASE OF NOT USING HAND HELD TESTER”. In your case, it is important test in this way to avoid any confusions with problems in other circuits, or with the ECU problems, or with the Techstream or other scanner reading a faulty ECU and which then gives faulty readings.
As part of the diagnosis, note in each case the specific, different ‘fail safe mode’ described in these sections of the FSM and compare these with symptoms observed on the vehicle. There is more than one ‘fail safe mode’.
Purpose: To check the health of the following circuits and components, as listed below:
C1718 at Page DI-229 – Fluid Pressure Sensor Circuit
C1719 at Page DI-233 – Fluid Temperature Sensor Circuit
C1731 at Page DI-239 – Control Valve Solenoid Circuit and Accumulator Solenoid Circuit
(Actually, it should be possible to hear and feel the operation of these valves – so these tests may not be necessary)
C1741 at Page DI-243 – AHC Motor Relay Circuit
C1743 at Page DI-248 – AHC Main Relay Circuit
C1751 at Page DI-252 – AHC Pump & Motor Circuit
Power Source Circuit at Page DI-294
Step 4: Check health of AHC Electronic Control Unit as described in FSM
C1761 at Page DI-256 – Malfunction in ECU (Memory Error)
This DTC was reported on this vehicle and refused to clear – see Post #1.
If the above Steps 1 to 3 have been completed with positive results in the checks and tests, then that indicates that those components and circuits in the AHC system are in good health.
Then, regardless of its appearance in the pictures in Post #1, an ongoing DTC C1761 certainly forces suspicion on the ECU – Part No. 89290-60010.
In such a case, after eliminating all other possibilities as described in the previous Steps 1 to 3, the ECU should be replaced.
Step 5: In case of previous incorrect Sensor adjustments, force different Sensor positions
The following method only is worthwhile after the following conditions have been achieved:
- cross levelling has been corrected as best possible,
- condition of Sensors, connectors and harnesses are healthy and reliable,
- AHC pressures are likely to be at the low end of the range,
- ECU is healthy
If these conditions have been satisfied and Sensor readings remain widely different and if AHC still is not working, then consider whether there is a possibility that mistakes by some person have been made in the past concerning Height Control Sensor adjustment.
It is possible that in the absence of a good understanding, incorrect adjustment of the Sensors have been left them in a position in which they are transmitting different and incorrect voltages to the ECU. If so, the ECU may have forced ‘fail safe mode’ which stops AHC operation.
If this is suspected, the following method may help to rectify differences in Sensor settings and remove the ‘failure mode’ effect, even though the vehicle is stuck at “LO” height:
- Firstly, for personal safety, secure the vehicle with stands under chassis rails so that it is impossible for downward movement of the car body onto a human body while working under the vehicle. This VERY important to prevent accidents and injury,
- With Techstream or scanner connected, ignition key “ON” BUT engine “OFF” (important – do not want AHC to ‘wake up’ and raise or lower vehicle body during adjustments), eliminate differences in the Sensor readings at “LO” by moving all Sensors until all scanner readings are approximately the same as the lowest Sensor, shown as -62.4mm in Post #1.
- Start engine, hoping that AHC will be ready to operate, press “UP” switch to see whether vehicle now raises from “LO” to “N”.
Best wishes for success!!
Can you please reply indicating whether there has been any progress with your vehicle?
, but I feel like he would have said something here at least.
