I'll post a full VCDS scan in the next entry.
Symptoms:
- Air suspension compressor does not turn on
- Auto and Dynamic are the only modes selectable from MMI, but obviously they don't do anything
- Yellow and green suspension lights on dash are flashing and/or illuminated
- After a few days, the ride quality of the car is horrible. There is no dampening when going over bumps and the car bounces all over the place.
- Main VCDS codes
- 01770 - Temperature Sensor for Level Control Pump/Compressor (G290)
- 01577 - Turn-Off due to Over-Temp
- 01583 - Leak in System Detected
- 01400 - Suspension Level Control | 002 - Lower Limit Exceeded - Intermittent
- 02250 - Function Deactivation; Valve Duty Cycle | 001 - Upper Limit Exceeded - Intermittent
- 02214 - Tire Pressure Warning
- Sounds very much like this issue: https://forums.ross-tech.com/index.php?threads/14345/
- When looking at the VCDS measuring blocks for Level Control, the relay is in the ON position, and the temperature sensor goes from ambient all the way up to like 240C, then the relay clicks off, and it drops quickly down to 100C and stays there (Clue #1 !)
Troubleshooting so far:
- Cleared codes and tried setting the suspension mode again, just in case the temperature sensor issue needed to be cleared (works in some people's cases, I think there is a TSB about this too)
- Checked 40A fuse underneath the driver's seat. Tested continuity with multimeter. OK.
- I took the entire driver's seat out, but then realized it's not necessary in order to reach the fuse. Just take off the carpet trim, and push the seat all the way back.....you can get at it.
- Checked for voltage at the left side of the 40A fuse. 12V OK.
- Inspected 643 relay under driver's kick panel. Yes, I know it should be replaced with a 645, we're getting there. Relay inspection is OK.
- Swapped 643 relay with another relay in that same block. No change in symptoms.
- Removed under-body panel from passenger side to access the air compressor and wiring. Wiring OK. Checked voltage to large connector while car is running, 12V OK. So power is getting all the way down to the compressor.
- Unplugged 4-wire blue connector from compressor (I think this is temperature and pressure). Car threw many more error codes at this one, and was generally unhappy. No change in symptoms.
- Unplugged 2-wire 12V main line to compressor. Turned car on and watched VCDS cycle the relay on and off. Temperature sensor exhibited same behavior as above....quickly goes up to about 240C, then relay shuts off and it comes down to 100C and stays there (Clue #2 !)
- TODO: I will bench test the compressor when it is out of the car.
Assessment and Theory:
- Since the temperature sensor went way up and out of range while the compressor power was unplugged, this seems to point to a bad temperature sensor....since the compressor is obviously not running, yet the sensor displays wacky values. I'm wondering if it's a type of sensor that only has voltage applied to it when the compressor relay is on? Or maybe only polls when the control module thinks the compressor is on?
I'm going to get the car on a lift and replace the G290 temperature sensor (7L0971291A 7L0-971-291-A), which is normally integrated into the compressor package and not available separately. I think Audi also lists the part as pre-2011, so it might not show up in a parts search.
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