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Old 10-23-2016, 08:37 AM
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Default Help Diagnosing (ABS/DSC/Airbag and now Transmission)

The car is a 2011 BMW 135i E82 Coupe (N55 Engine DCT Gearbox) with 36000 miles (~59000 KM) on it.

The car has also recently had a brand new battery which was registered with ISTA/P after installation.

This all started a month or so ago. It originally presented with
  1. ABS Light on (orange)
  2. DSC Light on (orange)
  3. Parking Brake light on (orange)
  4. Airbag light on (red)

I scanned it with ISTA/P and found a whole bunch of fault codes. All codes were in the DKG/DSC modules - all codes related to missing wheel speeds. There were codes stored for missing wheel speed from all 4 wheels. I cleared them then took the car for a drive. Now only the code for the FRONT RIGHT wheel came back. I confirmed in the live data read-out that there was indeed no wheel speed coming from the front right, So I ordered a replacement ATE OEM wheel speed sensor from Pelican parts and installed it expecting that'd cure the fault. But it didn't - same symptoms. So I checked the live data again, saw that now there is no speed data coming from the FRONT LEFT wheel. 2 sensors failing at the same time? Unusual but possible. I replaced the front left as well, then the fault went away. For the first time in a month I had a dashboard that wasn't lit up like a Christmas tree.


...until a about 20-30 miles later. All the ABS/DSC/Parking Brake orange lights are back on again (But no AIRBAG light this time ). So I scanned it, and now the REAR RIGHT wheel speed sensor is working intermittently. Sometimes it's sending wheel speed data, other times it isn't. What are the chances of 3 sensors going bad? At this point I was suspecting a busted ABS module or bad wiring to the ABS module/sensors.

I cleared the fault codes and took the car for a drive. After driving 20 minutes, I stopped for about 10 minutes to nip into a store. Interestingly enough at this point I had no dash lights. When I came back out and started the car to drive off, I put the shifter in "M" mode, turned on "Sport" mode and pulled out (Sedately!). Suddenly the car began bucking (as though you'd dumped the clutch) and an orange "Transmission Fault" message came up. Not even two seconds later a red message stating "Transmission Overheating" came up, then less than a second later went away. I stopped and checked everything but couldn't see anything wrong. I drove home and did another scan with ISTA/P, here's what it came up with:



I've never seen those gearbox fault codes before. Here are the freeze frames if they're relevant:

Gearbox overheating phase black


input-speed, sub-transmission 1: value implausible


input-speed, sub-transmission 2: value implausible


What do I do from here? How can all these seemingly unrelated systems come down at the same time? What is common to these systems?


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Old 11-07-2016, 12:24 AM
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Default

I've finally resolved this.

Turns out I really did have a faulty rear right wheel speed sensor that was sending data intermittently. A broken wheel speed sensor on the rear is a big deal for a RWD car with a DCT gearbox. The gearbox ECU needs accurate wheel speeds to decide shifting parameters. When the wheeel speed data is not present or the ECU thinks it's implausible, it will attempt to read the wheel speed using the output shaft which apparently is not very accurate, causing it to go into a fail safe mode? This causes the box to shift conservatively (and slip the clutch a lot I'm guessing). So the gearbox really was overheating.

I replaced the faulty wheel speed sensor and the fault has gone away completely and the gearbox has not overheated after over 300 miles of driving in all conditions (hard pulls, normal driving, heavy traffic driving and freeway cruising).

The front right and front left Wheel Speed sensors I pulled really are broken - when I put them back on the car I get NO wheel speed data at all. Maybe I part of a bad batch or similar. So there you have it. Turns out 3 wheel speed sensors can fail at the same time.
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