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2001 Audi TT Quattro CAN Bus FaultBy Mark Banks A 2001 Audi TT Quattro arrived in the workshop with a complaint from the customer of an intermittent ESP light. At first the fault was not obvious at all. We had heard that the ABS modules could be a common failure but I had to be sure before fitting. A quick scan of the fault codes revealed CAN Bus comms errors: not a lot of assistance, but as usual a place to start. The fact that the fault was intermittent didn’t help either. If the fault was present, the ESP light would go out on initial start up but then come back on as soon as the car was driven down the road. When the ESP warning light was on, the bus line signals looked like this:  See how the signals go from opposing each other to pointing the same way but still trying to maintain the voltage differential (almost).  After a good visual inspection of the wiring, following the twisted pair as much as possible, and the high-to-low bus resistance checks, I made sure the fault was present. Then, thinking that it was transmission-related as the car had to be driven to produce the fault, I scoped the bus lines with the ABS module diconnected. Unfortunately the scrambled signal was still present. I reconnected the ABS and took a logical step along the transmission to the Haldex module on the rear diff. (Remember, this is the four-wheel-drive Quattro version of the TT). Disconnecting the module and scoping the twisted pair finally displayed a good set of bus signals.  With ignition on and the Haldex diff controller unplugged, the bus line signals were fine. I stripped down the old module to check the circuit board with a magnifying glass and could not see any obvious water ingress, dry joints or meltdown problems. Ah, don’t you just love electronics! A new Haldex controller (about £600/$950/€880) cured the problem!
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