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Rear Ride Height Problem

Mike Floutier

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My 2002 LS430 has always behaved well and has reached 225,000 miles.

However a few days ago without warning the rear of the car rose up leaving a good 6 - 8 inch gap between the tire and wheel arch - same on both sides; the front stayed where it was. The dashboard message was "Height Hi" and the red warning triangle came on. The ride became bouncy and bumpy.

I read all the forum reports on this. It seems fairly common and the usual outcome is that one of the height sensors has to be changed (a $400 potentiometer - often 2 or more!!)

I removed both rear sensors and tested them. They were both fine - indeed they were spotlessly clean inside. I also tried the connecting links in high and low positions (on the car) but there was no response.

The compressor still works but I have now disabled it by removing the 20A fuse as it would still try to fill the system occaisonally causing the ride to become even worse and eventually some air being blown off via what I assume is a safety valve (not sure if it's in the struts, guess it must be)

I'm not sure how to proceed. Can anyone help me with this?




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I am no expert on this system but maybe have a few ideas to consider.

Since front seems OK that would seem to imply the air compressor, front shocks, front sensors, front air valve servo, and ECU are all in tact. Since ECU and compressor are common then it might be safe to give them a thumbs up for the rear as well (not absolutely true but seems reasonable). That then leads you to the rear sensors, rear shocks, and rear air valve servo.

Since the car rises it seems reasonable to assume the shocks are good. A leaky shock wouldn’t rise like you are seeing. This then suggests the sensor or the air valve servo. You tried the sensor. Maybe it’s the air valve stuck open sending full reservoir air pressure to shocks all the time.

Also your test of the sensor is fine but there could be electronics in the ECU that send a particular current across the pot (i.e. if it is a 1k ohm pot, 5mA current would establish 5VDC across the pot). If the wiring is dirty or broken or the pot resistive material is cracked that current might not flow rendering the sensor useless. You could test this by putting a DVM in series with the wire to either end of the POT. Then compare that current to what you see in the front sensor. If they are around the same sounds like outer ring of POT is OK. Then you have to check wiper of POT and see that it doesn’t have a worn spot. That is a valid voltage output across all the entire range. Since the wiper spends most of its time in the same position the wear is very concentrated on a small spot. If that spot it worn out the wiper could put out an invalid reading (basically open circuit). You’d only notice this if you measure with the sensor in that position.

One last thought is if the ecu detects an error in sensor or wiring does it default to full pressure in shocks as a fail safe feature. Maybe the high height is telling you its entered into a fail safe mode until the problem corrected and ecu reset. Just a guess on this one.

Anyway I’m no expert on this but maybe a few ideas to run with.

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Many thanks CuriousB, I can't tell you what a relief it is to talk to a thinking person about this.

1. I agree about concentrating on the rear but will test the front sensors in high and low positions (on car) to establish that they (and the other bits involved) work ok - ie. the front rises and falls.

2. You mentioned the air valve servos being stuck open. Good point but since there are 2, one for each side, they wouldn't have both failed simultaneously.

3. I see what you are saying about the POT. what I've done is to set the POT link rods in extreme HIGH and then LOW positions and restarted the car each time. There was no response so it seems the POTS are ok. HOWEVER, I want to check the voltage at the suspension ECU and the continuity of the relevant wiring as the sensor CIRCUITS seem to be implicated (as you agree). My problem is that I can't locate the ECU even with the help of the manual - do you know how to find it?

Also I would mention that the only problems I have had with this car have been caused by electrical connectors needing cleaning (throttle position sensor AND igntion coil) so I'm cleaning every connector I come across.

4. I agree that the high rear position is most likely a "failsafe" response by the ECU to a lack of sensor input - it would be the only rational response.

I plan to test the exhaust valve as soon as I can get a connector for it (can't access the contacts without one, it's to remote).

I also interested to know if there are seperate air valves for filling and venting the shocks or whether the same one performs both functions - if there is only one for both then we know it works as the shocks are being filled but not vented. The lack of venting could be the exhaust valve (as mentioned).

I do need to get access to the ECU though and this is my main help request at this stage.

Many thanks for reading all this and for all your thoughts.

Kind regards,


PS I should also mention that I've now disabled the system by pulling out the 20A fuse in the driver's side junction box. This stops the compressor from continuing to overload the rear shocks BUT it also shows that the whole system has no leaks as it's been dieconnected for a couple of days now.

Later on... I checked the wiring diagram for the height control valves and there is only one valve for each strut that obviously handles both inflation and deflation. Therefore if it can inflate (which it can) then it can deflate, thus ruling out the height control valve circuit.

Edited by Mike Floutier
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Ok, I'm starting to make progress here.

Main thing is that I've finally found the Short Connector - for some reason it doesn't change sides with the ECU from RHD to LHD, that threw me. Anyway, I found it on top of my drivers junction box (passenger J/B for all you LHDers).

I did the shorting as described in the manual and soon had the ride height levels back to normal.

Not only is this a relief but it proves that the height level control valves are all working and also the compressor and the exhaust valve.

Before rejoining the short connector I once again pulled the air-sus 20A fuse to avoid the same thing happening again.

Ok, as I said, it's a relief to be back at the right height. However, I had expected that the very hard ride that I was also experiencing would also go away - NOT the case.

If you remember, we agreed that a logical "failsafe" response to a failed sensor/(circuit) would be to default to maximum ride height so as to cope with varying loads etc. However, if you think this through further, it also follows that, to avoid problems with rolling, squatting, bouncing etc, (ie. something the system is design to do), the "failsafe" response would also have to include ramping the absorber damping up to maximum. This may well acount for the remaining hard ride.

My next plan is to try to:

a) Find out what the absorber damping is currently set to, and

B) Manually soften the setting - presumably by applying battery voltage to the actuators in reverse order to the hardening sequence specified in the manual.

Finally I need to discover what is causing this "failsafe" scenario to operate and remedy it so I can return to automatic control.

As usual, any ideas are welcomed.



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Ok, back from church and flushed with yesterday's success I decide to have a go at the absorber damping as it's still just as hard as when the rear rose up like a dragster.

The front actuators came off in a few minutes and all the resistances were fine. I then twiddled it to see what it was set to currently and out of the 9 settings 1= soft, 9 = hard it was on 9, no wonder it was so hard.

Any I twiddled it back to 3 and lo and behold it's back to nice soft suspension. Ten minute job.

Now for the rears. I think that will take longer as they say that you have to remove the seats etc first. Maybe I'll leave that for next week.

This is wonderful progress, thank you Lord! And thank you guys for being there. I'd never attempt this stuff without the Forum.



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Ok, I didn't fancy taking out the rear seats and rear trim tray etc, etc to get at the absorber damping actuators so I decided to try out adjusting them by applying battery voltage to the relevant pins on the air-sus ECU plugs.

This worked well. I tested the idea on the fronts first as I knew that I would be able to verify the results by removing the actuator and checking it's new position. Having done that successfully I did the rears and was rewarded by a very observable change in the ride damping - ie. SMOOOOOTH.

So, I have the ride height and damping back to where I want it BUT there are two problems:

1. No anti-dive/squat/roll etc. I can live with that as I'm a very gentle driver, and

2. I have to leave the air-sus 20A fuse out because if I put it in,(I'm guessing) the ECU will again respond to whatever problem it's sensing, that presumably still exists, and simply undo all the manual adjustments I've made - ie. the ride height will skyrocket and the dampers will default to superhard. Problem with leaving the fuse out is that it stops the ECU sending signals to the headlight levelling ECU so no headlight levelling.

I guess I could always do the same thing and adjust the headlights manually to a satisfactory position and pull the fuse on that system as well - to hopefully get rid of that irritating dashboard warning.

Finally I'm left with trying to find the problem that triggered the ECU to default to the "failsafe" settings. What have I left to check that could be causing it? There are two things I suspect in the air-sus ciruits, 1. connector deterioration and 2. water ingress.

However, on the plus side, many parts are ruled out of the equation as by now I've done a lot of testing. The following components and associated circuits are ok - ie. have been tested for continuity in their complete circuits and have been tested in operation.

1. Compressor - works ok but I'm wondering about the terminals RM+ & RM- at the ECU and Compressor. They monitor the current flow to the compressor watching for abnormalities.

2. Exhaust valve - works fine.

3. Ride height control valves - all 4 let air in and out and remain closed when they should.

4. Absorber damping actuators - all 4 work fine.

5. Ride height sensors - another slightly grey area; all 4 sensors have been test and work ok. The circuit for the front sensors has continuity but I've yet been able to test the rear as the wiring diagram doesn't make sense and so I will have to resort to physically checking the relevant wiring for color-coding

So this leaves me with several possible areas that I've not even considered yet:

1. Skid control ECU input - surely if the Skid Control was bad then wouldn't it report itself, I don't know?

2. Steering sensor.

3. Absorber control switch.

4. Ride height control switch.

5. 2 seperate inputs from engine and ECT ECU.

So these along with previously mentioned:

6. Compressor monitoring circuit.

7. Rear ride height control sensor wiring.

When I was looking for the exhaust valve connector, (to test the exhaust valve) I unplugged it and water streamed out of the end of it's wiring's protective plastic shield. I think this water gets in when the valeting service squirt their high pressure water at the front of the car. It must hose in through the hood grille and somehow find it's way into the plastic shielding. Because the compressor monitoring wiring must be in this area too, I think I will start there.

Will let you know how it goes.

Kind regards,


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  • 4 weeks later...

I had a terrible time with this same scenario search the net hi and lo then discovered if you drive (very uncomfortably) FOR NO LESS THAN 45 MINS THIS SELF CORRECTED ie twicw i had this because of a low battery hope it helps Ive seen this all over the net.


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  • 3 years later...

Have 2001 LS430 with Air Supension not reponding. You can manually adjust height of each wheel, remove Glove compartment and locate blue connector. This is the connector to short out 3 pins at a time to raise or lower each wheel. I made a jumper with three wires so I could connect to 3 pins at the same time and turned igition on. When heght is where you want it, turn off ignition and then remove jumper. I will try to post manual connector pins and pic of connector.

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