ABS System and Diagnostics

(Scott Justusson via 20v.org)  
If a sensor is bad, the light will stay off when the car is started, and then come on after driving the car from 10 feet to 2 miles. If the problem is elsewhere, it may turn the light on before moving the car. A new sensor should be US$5-US$20. To find the bad sensor, listen carefully after starting your car for unusual noises from one wheel. That's the one with the bad sensor. The noise is the ABS kicking in and out, until it decides to shut down. You can hook up a oscilloscope to the sensor outputs, raise the car and spin the wheel. You get a square wave (A/C) from the sensor. The actual value is irrelevant, matching them is. Basically, you want to get the same reading from all sensors spinning the wheel approximately the same speed by hand. Try removing the sensor, cleaning it, and reinstalling it until it rubs the hub. DON'T drive it like this -- you may damage it!! You must back it out slightly. It should not rub, but be very close. You can get a little plastic cap from the dealer for ~US$0.80 that will set the proper distance for you and keep the sensors from getting eaten. 

(quattro owners club forum)  
If the ABS warning light comes on after the car has moved a short distance, the most likely cause is that one or more sensors are out of adjustment. If they are the type with a single use ablative cap, the solution is to fit new caps, push sensors in as far as they will go and tighten. This should (but doesn't always) set the sensor/CV air gap correctly. The caps are approximately 80p each.

(20v.org)
The sensors too far from the driveshaft will give you pedal "boing" -- this is a groaning/spring sound just as you come up to a stop. The sensors too close will give instant ABS pedal kick in. Too far out tends to shut off ABS more quickly than too close. If you can't get the light to go out at all, you might have a fault in the centre differential lock switch, or one of the sensors is just bad. Worn or crudded driveshaft splines can do it too. There is an inspection/water drain at the bottom of each strut, you can sometimes get a brass bottle brush in there, though pulling the driveshaft out is easier, and about a 20 minute job. First check for proper sensor output, then check the differential switch, then the driveshafts. Rarely does that ABS box go bad. Sensors out of alignment usually allow the light to come on till the first brake app or 20-30 seconds, whichever comes first. 

(Phil Payne from Quattro Owners’ Club Forum)  
If the ABS kicks in just before the car comes to a standstill the problem is often low amplitude from one of the sensors - it's known as "the Aberdeen problem" because it's very common in cars from the east coast of the UK, especially the Scottish part. Sometimes - especially if the centres of the teeth are corroded - you will just never get a good signal. Replacement requires a new hub – but they are not that expensive. To check the signals, connect an oscilloscope; if the wheel is spun by hand, there will be only millivolts and up to an ocscillating +/-2.5V at speed. Since it's probably only one sensor malfunctioning, just measure them all and compare. 

Possible Causes ABS Sensors ABS switch on dash Diff locking system and switch ABS Relay (may have 8A0 part no prefix or 85 prefix) 

by Iain Atkinson, originally from the urquattro.de website