Ur quattro ABS light on

[newsh]

This is an 84 Ur, first year of ABS, 3 separate relays in the aux carrier.
ABS light stays on, doesn’t go off when moving. Switch has no effect.
One of the relays, the one with aluminium cover and a big diode inside, gets very hot!
Everything worked before the car was laid up (8 years ago)
Does anyone have any experience to share?

Pic for encouragement 😊

[BackintheFold]

I can't help much except to say that relay shouldn't get hot, and as the ABS isn't working anyway, I'd remove it before it "ducks" up the car.

[newsh]

Yes the relay is removed! 👍

I’ve found that it’s energised with the ignition off so will flatten the battery.

[tractor Dave]

I have a big box of old Audi relays. If you let me know the number I will check for you John

[newsh]

Cheers mate
There are 3 relays
Square ali one 437 927 827
Tall ali one that gets hot 443 927 825
Small red one 431 951 253F

[tractor Dave]

Originally Posted by newsh

Cheers mate
There are 3 relays
Square ali one 437 927 827
Tall ali one that gets hot 443 927 825
Small red one 431 951 253F

I will check for you in the morning

[Hanuman]

I believe the same is true for earlier systems of the ABS, as the system was largely the same, but layout was refined over time, along with some system refinements such as the inclusion of an accelerometer later on.

The big relay that is being referred to is most likely the combination relay.
This relay monitors several inputs to give an output.

Inputs include (my notes and memory are all but absent at present)
System voltage good (10v+ rings a bell, but once on, it will stay on until the volts drop below 9v)
ABS switch not off (a faulty switch can cause issues)
difflocks not engaged
System (ABS) ok
Power on

There’s a bit more, but basics are there. If an input is not present, the relay won’t trigger. It’s the fail-safe condition.

When the car is operated normally, engine running, difflocks not engaged, the ABS light should be off.

ABS light comes ON In the following cases
-Engage a difflock, or press the ABS switch, and the system is disabled.

-Once moving, a poor wheel sensor signal will cause an error in the ABS controller which will no longer give an OK.

-System electrical supply fault


At this age, I’d feel confident that any error involving a relay has a high chance of poor solder joints in the relay itself.
Any relay running hot could be the electronic gubbins if it’s a complex relay, or simply welded contacts. I’ve lost count of the number of relays that I’ve repaired in my cars, mostly due to dry solder joints. That’s a joint that is no longer secure. Sometimes a simple repair involving reflowingbthe solder with a soldering iron is sufficient.
I’ve repaired relays for Fuel pump, lights, seat memory, etc etc.

[tractor Dave]

Originally Posted by tractor Dave

I will check for you in the morning

I have one close to the last number on your list. I will send you a pic

[newsh]

Fixed
Tidied wiring to allow all three ABS relays to sit together ( this car is an early one before they were “combined” and found possible short to ground. Relay stopped getting hot.
This left everything working ok apart from a lack of power arriving at the controller.
A look inside the “hot “ relay again, revealed a blown wire from the contact to the output pin. Replaced with a thin strand from a piece of 1.5mm ( I case it’s supposed to act as a fuse). Fired up the engine and the light is out 👍👍

[BackintheFold]

Nice detective work.