One corner drops overnight - to bumpstops

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Originally Posted by paddy119

Hi Leo,
I had the exact same problem with my D4, it ended up being a faulty height sensor in the front passengers side air suspension. when you would turn the car off, the front left would not have a height reference and it would push up which in turn lowered the rear right. That's how the service manager at Newcastle explained it. Repaired under warranty Cheers Pat

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That fault is an interesting one. It relates to the wake up function built into the air systems logic where after engine shut down, on some random schedule, the system wakes up, decides if the vehicle is level or not and then reacts appropriately, or in your case, inappropriately.

What I do not understand is how does it know it is level, or not level?

My understanding is that there is some communication between the wheel sensor signals and the yaw sensor for the anti skid function. I accept that it cannot be just the wheel sensors as that would mean the vehicle would have to be parked on flat ground all the time with never a wheel(s) down in a rut or the like, or a wheel(s) higher on a lump.

I have looked at the wiring diagrams related to the yaw sensor and cannot see any direct wiring connection between the yaw sensor and the air system computer but I suppose that does not mean much given that data seems to "secretly" flow over the various computer busses at all hours of the day and night.

That is why when trouble shooting the air suspension, one has to disconnect the power to determine if the problems are solely mechanical or somehow also electrical related. In the case of the above example, an electrical problem with a wheel sensor sends wrong signals after engine shut down to the computers and hence bad instructions are then sent to various air valves. That tends to suggest it is just wheel sensors that provide level information, however I do not think the sensors are the sole source of data.

Hence the question, how does the car know is it not sitting level?

:eek: too clever for its own good!

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Originally Posted by bbyer

Hence the question, how does the car know is it not sitting level?

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The 4 height sensor values for when the vehicle is level with respect to the ground (not the horizon) are stored in the suspension ecu. These are stored at height sensor calibration time. The ecu therefore knows how far each corner is from the correct height. The suspension ecu will level downwards at specific intervals after the ignition is turned-off even though the canbus is not operating, ie the suspension ecu does the periodic relevelling without regard to any other ecu or sensor not connected directly to it.

Edit: I would have expected an air leak at 1 corner (bag or air-line) to cause the others to also be lowered due to the periodic relevelling unless the suspension ecu main fuse was removed or the battery was disconnected to prevent that from happening. A height sensor giving plausible but incorrect signals would cause diagonal inconsistancies as the ecu relevels using faulty data. But I have experienced neither an air leak nor a faulty sensor.

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Originally Posted by Leo

Been ok since I made the post on Tues. .

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Five weeks on , and this is still OK ?

Yup, been ok since then. ( written while touching wood!!)