2005 BMW 4.8is AIR LEVEL PROBLEMS AGAIN

[SlickGT1]

Originally Posted by civdiv99

Negative voltage?? Cannot happen. The sensors have 3 wires, 5v source, return (let's just call it ground for discussion), and the signal. 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. There is nothing 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 180 degrees from each other.

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 voltage, 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.