ABS Diagnostics and testing

Ben Swann benswann at comcast.net
Wed Sep 8 10:01:49 EDT 2004

I have had a problem with ABS becoming disabled while driving.  The problem was intermittant until one day the ABS would become disabled just after starting the engine - no vehicle movement.  After taking the car to my local dealer which is usually pretty good, they were not able to properly diagnose the system because their analyzer had "dissappeared" and they pretty much told me they wouldn't get another because people don't bring these cars in anymore.  More on this issue below.

Anyway, I did some more checking and replaced the ABS relay(under dash) and then swapped the ABS ECU ( I'll call it the ACU from here on) out with one from my departed '87 tq avant.  Low and behold, the ACU swap appeared to fix the problem, until I drove for about a minute and the "ABS OFF" light went on again.  Now this happens about 1 minute after driving consistanly and does not seem to be triggered by bumps, braking or anything in particular.  I have confirmed that ABS does work just after starting to drive or re-enableing the ABS with the ABS switch, but always goes out in about 1 minute - have not couted the seconds, but it is around 30-60 sec.

So this is the problem with my '91 200q20Vtq avant, and some friends have similar problems with their cars - one is a 200q20Vtq sedan - ABS goes out when ignition on, and the other is an '87 5000 turbo quattro avant that has pretty much the same symptoms as my 200q20v tqa.

Question is twofold: (1) What other diagnostics can I do to narrow down and resolve the problem with my car?  With winter and rainy season coming, I don't want this problem outstanding for too long especially since today in the rain I experienced harrowing braking experience when someone cut in front of me, so fix is a priority.  The Bently indicates use of an anyalyzer that plugs in where the ABS Control Unit(ACU) goes.  I suppose this checks inputs and performs some output to the various ABS sensors and pump controls.

(2) Where, how, who can do this kind of testing and/or is there anyone who has one of these anyalizers floating around that perhaps collectively could be used/purchased that can be shared amonst the Audifans community.  I'm just looking for ideas, and since I know of 3 cars with this type of problem, I am sure there are others.  Perhaps collectively something can be done to resolve this issue.    If I have an ABS harness, how difficult would it be to build a simple tester for this if say I have access to an O-scope and/or other ancient test equipment?

Personally, this is one of the few times I actually took my car to the dealer to have it diagnosed or worked on. For one they should be able to do this in a few seconds vs. the time it would take me to check the system out.   Also,  this a safety issue that, no matter how old the car is, IMO should be supported by the dealer network.  Isn't this same ABS system shared by all  Audi ABS equipped vehicles to circa '92?  Should I beat on my local dealership further?  Ideas?

10 Audis in the driveway: 4-T44 avants and 1-sedan, 3-4kqs, 1-CGT, 1-UrQ.  Oops missed one - still working on that GT Coupe turbo conversion which is supposed to be in Germany by now, but project dragging on due to brake swap and other issues.

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