Computamedic

Hi all,

As some of you may recall from earlier in the year, I've been chasing around a problem with my ABS/PDAS alarm ever since I got the car (an '89 C4 Coupe) back in January.

I really don't want to bore you all but I'm fast running out of ideas but would desperately love to find the source of this problem.

The thing that really puzzles me is that I can start the car from cold and drive 40 mles or more before the two warning lights and the buzzer come on. From then on, resetting it (by restarting the engine) just delays the problem by only a few minutes although often it will stay off - sometimes for hundreds of miles if I'm going that far.

Last week I had the front wheels changed (from 7.5" to 8.5" - albeit with the same 225 section tyres) and after that the ABS/PDAS alarm would come on almost immediately and after a reset, immediately on again. I had my garage go over it with a fine toothed comb last week and they found a few corroded connections and maintained that they couldn't get the problem to occur. The alarm sounded for me twice on my 20 mile journey home!!!

I convinced myself that the problem could be something to do with the transverse accelerometer (it's a C4) so took that out today, opened it up and cleaned it, blew it out, put it back, made sure nothing was interfering with it and gave the car a test drive. 40 miles or - alarm on again!!!

Curiously, when it's of a mind to alarm it will often alarm when the car is standing still so that would appear to eliminate much of the dynamics although I'm beginning to suspect things such as brake hydraulic pressure (or the pressure sensor) or something in the electronics that just gets confused.

I've had the wheel speed sensors checked - one was, in fact, changed since it was showing a low output compared to the other three. But that made no difference either!! Because of the instances where the alarm has come on while the car is standing still I'd pretty much discounted possible problems here.

Since the last visit to the garage I seem to be back to the initial 40 miles or so when the car is cold, then sporadic repeats quite quickly then, once left overnight it's back to the 40 mile trip again.

I've been struggling to imagine what sort of environmental factors might arise in the initial 40 mile drive that would affect the ABS/PDAS alarm. Issues such as temperature, battery voltage, hydraulic pressure etc. etc.. could all vary in such a distance and all may remain that way until the car is left to cool.

Over the past few months the car has had a new ABS relay (on the ABS unit - not the central electric), 1 new wheel speed sensor, a cleaned lateral accelerometer and countless contact cleans. It seems the Hammer diagnosis is useless since it doesn't indicate anything conclusive.

There has just GOT to be a way that I can identify what parameter is causing the alarm to activate. Any ideas gratefully received.

Best regards


Dave

garrett376

Did you also clean the longitudinal sensor - that is usually the one that I read most often triggers an alarm you describ when moving along (and if it's stuck, possibly when the car's not moving as well)??

If not, clean that one to rule out that possibility.

Computamedic

Garrett,

No I didn't clean that one - coz it's a PITA to get out!!! Maybe I'll just have to bite the bullet and get in there somehow.

Thanks for the advice.


Dave

DaveK

I think it's the longditudinal one that usually screws the ABS. To get mine out, I have one of those cheapo screwdriver sets where you can change the screwdriver bit - these were short enough to fit into the screws within the transmission tunnel - and an 8mm or 10mm spanner was good for turning them.

It was the longditudinal sensor which caused me ABS problems for 18 months - although I didn't get warning lights. Just dreadful brakes.

lunaboy

Hi Dave

I have got the exact same symptons on my 1990 C4.
When cold you can travel for say 30 mins with the ABS and PDAS working, then the warning and lights go on. Cancel the "buzzer" using switch on centre console and the lights stay on. Now you get sporadic buzer events and lights occassionally flash on and off. You can hear a "relay" switch just before the buzzer sounds. From now on warnings initiate as soon as ignition is turned on.

Park car for a couple of days, and you are back to 30mins or so before the cycle starts again.

Agent and Bosch people tell me the "ABS Brain" ECU is faulty and needs to be replace. But at NZD4000 I want to be sure!

Hope you find an answer, if you do please let me know so I can try it too!!

Find it hard to believe that if ECU works for 30mins or so, that it is faulty. Must be an input issue, surely