JohnMetro

Hi all

I'm hoping someone here may be able to help with my problem?

I have done a search, but I think I am now MORE confused than before.
I have read many of the posts, but my symptoms are a little different to others I have read about, so I'm hoping someone here can shed some light on where I need to start investigating? eg. Electrical connectors? Mechanical? etc

My symptoms;
- upon start-up, ABS warning light comes on as normal, and stays lit for the first 15-20 seconds, then the alarm sounds and lights go out. All normal and as it should be
- after driving about 5-15 minutes (mostly stop / start traffic), ABS and 4WD symbol light-up, and alarm sounds. Light will stay lit until I turn off ignition.
- if I turn ignition off and restart car immediately (eg at the traffic lights), light stays off for about 1 minute, then wham, light and alarm again
- if I park and let car sit for a length of time (eg. overnight), it takes much longer before the first "false" alarm, but then, same as before, turning ignition off/on and it triggers again with minutes.

Today was a little different. Have not driven the car for approx 2 weeks (have it on a trickle-charger, so battery fully topped up). Lights and alarm did not trigger for at least 30 min, then same as before. The only difference was, this drive was on freeway like roads, so not much stop / start or braking.

Brake pressure feels perfectly normal. I am considering "testing" the ABS ie. MASH the pedal to the floor and see if wheels lock or if the ABS kicks in! I will do this both before the "false" alarm, and once it has triggered. Suppose I am just risking getting a flat spot on my brand new $2.5K tyres!!! arrgghh

Any suggestions or help on where to start looking would be greatly appreciated...

EDIT: Whoops, just realized I posted int he Turbo form! Sorry, mine is a 1990C4. I'll leave here if that's OK and repost in non-turbo as well

Metal Guru

My guess would be the booster pump is going bad since you are seeing both the ABS light and the 4WD light.

fritz k.

hi,

i`ve had a problem with the pressure switch recently, seems to me the same behavior. did you face actually the ABS light or perhaps the pressure light (with exclamation point in brackets) ?

JohnMetro

AHA... looks like we are narrowing this down...

Yes Fritz, I got the " (!) " symbol
+ "ABS"
+ 4WD symbol (which looks something like this> = – =

fritz k.

hi,

well, turbo and C4 brake system are most similar. there is a boost pump with a pressure switch on it and a pressure accumulator in the in the front trunk. the pressure switch controls both pump and warning lights.

the pump generates up to 180 bar pressure, captured in the pressure accumulator. you´ll find a measurement connection at the accumulator and a checking procedure in the workshop manual.

on my car it turned out that the pressure switch failed. i had nearly the same symptoms as you described, after changing the pressure switch everything was fine.

this is the switch:

JohnMetro

Thanks Fritz. The pic is a great help as well. I'll try that next....

993MAN

Having been through this recently on my TT I can offer this.
Yes the braking system is simple, but it is also fairly reliable. We spent hours trouble shooting this issue and I even had the pump rebuilt. A waste of $450.00Take the brake sensors out and clean them. This is a very easy to do and a common thing to cause the ABS light to come on.
Mine were filthy. Trouble is it never solved my issue.
After weeks of playing around I had my brakes bled by the shop with the Bosch hammer, and it solved the problem.
Hard to believe I know, but that fixed it. They did it in some type of sequence and it took about an hour.
Just thought I would mention this.
Cheers

JohnMetro

Thanks 993man

I just got back from 3 weeks holidays. Car has been sitting in garage on a trickle charger. Took it for a spin (I was having withdrawals whilst away!), and it must have been at least 30 minutes before ABS lights came on! Seems the longer I don't drive it, the longer the delay before fault occurs. Has me stumped, so I think I'll pop it into the shop for a professional peek.

Appreciate the advice about chasing shadows, hopefully they can identify and nail it first go....

993MAN

John
Whatever it is, it will be something simple to sort out. These cars are just too basic for it to be a major.
The first thing I would do is take it to a garage with the Bosch hammer.
That will tell all, or at least it should.
Cheers