Audi coupe 1990 20v coupe abs problem

[Horning, Zain A]

Hi,

I had a similiar problem with my Coupe, though it was less extreme.  Usually the ABS would automatically kick off when a sensor acted up.  Isn't there a switch to turn off ABS in the center console?  Kill your ABS before it kills you (or someone else).  Fortunately, I was able to easily find my faulty sensor because my ABS started acting up soon after I changed a front wheel bearing (I bumped the sensor while pressing out the bearing).  I think that there were two styles of sensors used on the Coupe Quattro (early vs late).    

As you said, check all the connections for the ABS.  More importantly, make sure that none of the sensor wires have been cut or torn.  In general, sensors do not simply unplug because a metal clip holds the connector together.  If you want to check them anyway, they are fairly straightforward to inspect.  The front sensors are clearly visible and plug in near the left and right strut towers.  As I recall, the connections for the rear sensors are under the rear seat.  If you're going to pull out the CV joints, check the teeth on the CV joints for damage and also inspect the condition of each sensor in the strut housing.  The sensor should be clean and there shouldn't be any major dents or tears in the end of the sensor.  Sometimes when a sensor is reinserted into the housing, it is placed too close the CV joint and it gets chewed up by dirt or the joint.  

If you try to remove the sensors and they've never been out, plan on buying new ones because they'll probably come out in pieces (they rust into the housing).  Be aware that they are expensive (~$130ea for the early sensor from the dealer)!  Or buy one for a buck from a junkyard like I did.   

The sensors should be positioned as close as possible to the CV joint without actually touching it.  I've heard a rumor that the dealer sells spacers that will allow you to set the distance between the sensor and the cv-joint more precisely.  However, it sounds like a pain because you will have pull the cv-joint in and out twice (once to set it and once to remove the spacer).  I set the sensor position by pushing it in until it hit the CV-joint and then pulling it back out a little.  Gently spin the wheel after you've installed the sensor to make sure it isn't hitting the joint.

I've heard a couple different approaches to diagnosing a bad sensor.  If you only have one sensor that is discombobulated, than you might be able to feel which wheel is awry when driving the car.  E.G. the car will pull in some direction.  However, if you're car has other suspension issues such as dead control arm bushings than this may not be a good approach.

On the other hand, sometimes if a sensor is bad, you'll hear a clicking coming from the wheel with the bad sensor.  As a result of the bad sensor, the ABS control module will apply and reapply pressure to caliper with the faulty sensor.

Alternatively, I read somewhere that someone jacked their car up and connected an oscilloscope to each sensor (test individually).  When each wheel is hand-spun, the sensor produced a square wave output.  If the sensor produces some sort of irregular waveform or doesn't do anything, then you know it's dead.

Unfortunately, unless you have a scope, most of these techniques still involve some guess work.  Your best bet is to take it to garage with the proper testing equipment.

Zain Horning