JohnMetro

Hi all

I'm hoping someone here may be able to help with my problem? I have a 1990 C4

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...

NineMeister

Measure the circumference (to calculate rolling radius) of the front and rear tyres before you do anything else. They have to match exactly - or within 1% or so.

JohnMetro

Thanks Colin. Will do. That's an easy test. You've got me scratching my head now, trying to remember if symptoms started when I got the new tyres a couple of months ago? Running Cup 2's x 17" at both ends, so you would assume they should be same circumference! But you never know...

Ps I figure the easiest way is to chalk both ends at the same spot, mark the ground, then just roll forward one revolution ?

Terpenator

I solved my ABS problems 4 weeks ago by changing abs relay. maybe you have a mate with a 964 and try his abs relay for a while... Good Luck!

FeralComprehension

Make sure that your brake fluid res is full (check with engine on and ABS pressure pump *off*).

boxsey911

+1. I know a number of people that solved this problem by changing the relay.

JohnMetro

Thanks for all the suggestions. Looks like it could be any number of problems, and unfortunately the symptoms don't point to any particular area!

Oh well, process of elimination I suppose. I'll start with the easiest and then progress towards the hardest!!!

Thanks again. PS. Case is NOT closed, please fell free to offer more suggestions if you have them

c2dweller

Might be a curve ball but ...

In my car the ABS illuminates when system 12V voltage is low. I believe this is a deliberate feature of the system.

If my alternator is not working and the battery is low I get this warning lamp.

Henry964

I am having a similar problem with my ABS light and beep coming and going intermittently and it is driving me and my mechanic nuts because nothing seems to be wrong and the lights are still coming and going especially at speed

Pak_37

When's the last time you changed the coils!!

We just had exactly the same symptoms you outlined about a month ago and after we changed the coils the lights and alarm never came back on. We were scratching our heads -- we were concerned and re-bled the entire system but no success until we changed the coils -- it was magical!!!!!

JohnMetro

Sorry to dredge this up, but I did some testing tonight.

After start-up, and before warning lights come on, ABS works perfectly. After the warning lights come on, ABS no longer works (skid marks outside my office to prove it).

On someones else's suggestion, I checked all the connections to the green metal ABS cylinder unit (sorry, unsure what this is). Seemed OK. Took it for a test drive, lasted about 5 minutes (2 miles), then pop, warning lights came on, and ABS not working

I'm just going through the 964 bible (Adrian's book and I'm blown away by how many interconnecting systems and fail-safes are built into the ABS!