Owners of some 1996-2005 Audi models may complain that the ABS activates under light brake pedal application. If conventional problems such as damaged wheel speed sensors, mismatched tires or out-of-spec speed sensor air gaps can be ruled out, Audi said, one likely cause for the condition is small cracks in the ABS tone wheels.
Depending on the model, the tone wheel can be found at the wheel hub, outer CV joint or the inside of the brake rotor.
Diagnosing minute cracks in a tone wheel is best achieved with a digital scope hooked up to the speed sensor, the car maker said. A healthy tone wheel will exhibit a smooth, round, uninterrupted sine wave that increases in amplitude and frequency as wheel speed increases. If there's a crack in the wheel, you'll see a notch or flat spot on the waveform that repeats itself at the same point for every revolution of the wheel.
Replacing the CV joint, hub or rotor as necessary is the only surefire remedy.
Models that are susceptible to the ABS malfunction are 1996-2001 A4s, 1998-2002 A8s, 1998-2004 A6s and 2000-05 Allroad Quattros.
"Factory Fixes" is written by Jim DePalma, a 30-year veteran of the auto repair business who has served stints as a service manager, parts manager and ASE-certified technician. His column provides vehicle manufacturers' authentic factory technical service bulletins (TSBs) that have been condensed for easier reading. Mr. DePalma advises that techs always check with a dealer or repair information system for the latest revisions before starting to work on a vehicle. He can be reached via email at [email protected].