Start down a theory of operation discussion road with most techs, mix in some confirmation checks with a meter, add a pinch of common sense, stir well, and you may lose most dealer techs. And the ones that take it upon themselves to really understand are kind of stuck since pressure to produce can preclude the time it takes to follow up on a computer fault indication with actual confirmation. A lot of parts get replaced just because a problem revealed during diagnostic work is incorrectly perceived to also be the cause. No soaks on the folks that do this; the processes in place at most dealers can constrain their efforts. That, and the dealer's hourly rates become spendy quick, so it's rock vs. hard place.

Virtually everything to do with the suspension can be checked by the handy owner except perhaps the computer itself. Compressor operation, wheel sensors, actuating valves, dump valves, etc. Even the compressor head temp is just an analog voltage on a wire under the cargo floor. The question becomes do you want to take it on, and what's your own time worth?

Negative voltage?? ??? In reference to? The sensors have 3 wires, 5v source, return (let's just call it ground for discussion), and the signal. The signal is generally about 2.5v at normal ride height reading in reference to the ground wire. Now like anything else DC, if you read that against the source wire, then it looks negative. Same with your car battery - read it "backwards" and the meter duly reports that the voltage is "negative" with respect to the meter's common. Doesn't mean there is anything wrong with either part. The sensors are just a magnet that rotates inside a field. As the magnet's relative position varies (the arm rotates), there is a minute fluctuation in field current, much too small to be useful. So the sensors have a small microcircuit that scales this effect and provides an output that varies between .5v ~ 4.5v. Since there is no mechanical connection internally (rotating the arm just rotates the magnet), you can twirl the arm completely 360 degrees. If you monitored the output of a working sensor whilst doing that, you would see voltage peaks and valleys as it went around.

The diagnostic software graphically displays the source and signal voltage at each corner, and displays the interpreted relative position (height) as well. From there it is very straightforward to confirm 5v applied to the sensor across source and ground, and to confirm the voltage output of the signal wire - this is a discreet function that can be verified independently of the EHC computer. I'm not trying to bust any chops here at all - I want those in this group who so inclined to, at a minimum, understand enough to be able to inquire of whomever they are paying to do the work, or ideally be able to troubleshoot many of these faults with a modicum of confidence. This isn't magic; it really is just a logical arrangement of inputs, outputs and resulting effects. Ok, maybe you can slam me for using the word "logical" in a thread about the BMW self-leveling system. Let's all take a break whilst I go in the corner and beat myself about the head and shoulders.

Ok, I'm back.

In a post last week I gave the pin outs, wire colors, and functions for all 4 corners of the 2-axle air suspensions. The height sensors can be checked on the car (I'd at least unbolt the bracket with sensor so you can work with it in your hand), or checked on a bench. These stories about folks having techs swap sensors from side to side, or replace something electro-mechanical in the system "because the computer said so" are more than a little bit alarming.

I think what I need to do is a DIY on how to check the function of all the discreet components of the air system. The computer may incorporate speed, cornering forces, if a hatch is open, and so forth into it's processes, but the rest of this system is just a handful of inputs and outputs, or commanded actions re: the compressor, actuating valves, etc. Getting the diag out of the computer is useful, but that is just pointing to the symptom, not necessarily the cause. SlickGT hit it when he suggested there can be causal relationships between failures in one area that can result in symptoms in another. For example, the compressor has a temp sensor on the compressor head. Above a certain temp, the compressor is cut-off for a given period of time. (That temp signal is just a voltage on a wire under the load floor, by the way). Then it logs a code, and an example of a short-sighted part-chucking approach would be to say the compressor is failing.

I'll try to do a complete how-do-I for checking all of the discreet/mechanical parts of the system, but it'll be awhile. Just gotta find some free time.....

Great post Civdiv. Can't wait for your write-up. Let me know if you need some help in gathering anything. Car is in garage, so I can easily go out and check pretty much anything.

Thx slick. I will try but right now working approx 4 a.m. to 6 p.m and most of Saturdays. This will be a challenge to fit in. Right now on my list of things to post into this group:

How to read and verify the inputs and outputs, and verify the operation of the electro-mechanical components of the EHC system.

How to maintain the actual engine coolant temp closer to 200 (vs. 225) if you are so inclined.

How to electronically boost the main line pressure of the transmission under desired conditions (load, throttle, rpm, whatever) if you are so inclined. (you gotta do it "conditionally" or over time the EGS will learn around the firmer shifts and gradually go back to blue-haired granny shifting)

And so on...........

Exactly! Just throw out the air suspension garbage and switch to regular spring or coilovers.

cant wait, i got my snapon multimeter ready to go
my airide light has been on for over a year, no drop in ride height though
im pretty sure my right rear sensor is probly tripping