I looked around and saw different answers for this air suspension question but with the testing I did, it became confusing.
Problem: In "NORMAL" mode, both rear shocks drop about 7 cm (6.8 & 6.5 cm) while the fronts drop 1.5 cm. When in HIGH mode, no discernible drop at any tire is seen overnight. I can accept 1.5 cm as normal overnight drop but I wanted to troubleshoot the rear air shock drop.
NOTE: In each test, I turned off the air suspension switch in the trunk in the toolbox overnight.
Typical solution: Both rear air shocks leak or the line leaks. But how does this explain no leakage when in HIGH mode?
The only way this makes sense to me is if the HIGH mode actually changes the suspension to Firm Spring Rate and cuts off air flow between the main air chamber and the sub-air chamber in the shocks. And the sub-air chamber leaks in both rear shocks and a little bit in the front. In NORMAL mode, the 2 chambers would allow air to pass between them and be in Soft Spring Rate.
From the New Car Features book, The picture shows the 2 air chambers and has this text.
"Each pneumatic cylinder consists of a variable damping force shock absorber containing low–pressure nitrogen gas, a main air chamber and a sub–air chamber that stores compressed air."
The rotary control valve that changes the hydraulic shock absorbers firmness is coupled to the air valve control rod via a gear. The manual goes on to say
"The air valve is rotated by the suspension control actuator, via an air valve control rod, to regulate the amount of air flowing from the main air chamber to the sub–air chamber. The spring rate of the suspension is regulated in two stages as a result."
But I can't find anything in the manuals or online to suggest that the height control does anything to the spring rate control.
Additional information: I just received this 1990 Lexus LS 400 with 120,000 miles and a new engine 19,000 miles ago from my 70+ year old uncle. Fortunately, my uncle is OCD about keeping receipts and all maintenance records for the car were kept. After the engine was replaced at 101,442 in Nov 2003, I have a bill for the right rear shock at 102,390 May 2004 for $773 which leads me to believe it was a used shock (I didn't ask him yet). At 119,491 Aug 2011, the left front shock was replaced for $1889. That price makes me think it was brand new.
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I would agree with Yamae, there is only one height sensor for both rear air shocks,if this is faulty it may lead to the problems you are having.
Check the mechanical linkage on the sensor has not seized or broken.
The 1990 Lexus New Car Features Manual says the Height Control on/off switch "prevents compressed air in the pneumatic cylinders from being discharged and thus prevents the vehicle height from dropping." I guess it simply indicates it does not allow the exhaust valve to open for the system. All other leaks are possible.
The manual also seems to indicate there are 4 height control sensors - one at each wheel. If there is only one sensor for both rear shocks, where might it be?
Would this sensor affect the HIGH position, or does the system just go to maximum pressure?
My solution is to replace both rear air shocks (although only the left one really leaks) and all 4 rubber o-rings where the hose goes into the air shock. It turns out that when the car is lowered, the rubber in the air chamber folds over on itself like an "S" that is pushed over. In that lower position, the leak in the left air shock was evident after putting a lot of weight in the trunk after pumping up the air shocks to high.
When the car was in the high position, the rubber air chamber was straight instead of "S" like and it actually plugged the hole sufficiently that if there was not any weight in the trunk, it would look like it maintained the height.
This explains why, when the car was at "normal" height, it always leaked air and dropped to the lowest position. While in the high position, it only dropped sometimes.
I want to replace both air shocks even though only one leaked because the right rear air shock had been replaced with a used air shock and I don't really want to do this job twice. Although it is expensive to be lazy.