VehiGAZ

ABS light is on at start-up. Have brake fluid, no leaks, etc.

I told my mechanic about the common issue of mice chewing up the wiring to the ABS pump behind the front left wheel well liner, but he said he found nothing there. He jumpered the relay and the light went off, so we ordered a new relay, which will be in on Monday or Tuesday. In the meantime, I want to confirm the diagnosis...

Are there any other potential problems that would cause the ABS light to come on and would appear to be corrected by jumpering the ABS relay socket?

Thank you!

Mrmerlin

FWIW common issue with the ABS system is a failed relay on the CE panel.
NOTE take the old relay apart and see if there is a broken wire or bad solder joint.

a used ABS relay will work

So this is when the engine is started the ABS light will come on = failed relay on the CE panel.

If the light comes on after the car goes faster than 4 MPH then it s a wheel sensor or wire issue
NOTE the relays at the ABS unit rarely fail.

NOTE be cognizant of the HOT wire to the ABS junction post next to the PS reservoir ,
make sure it is routed properly,
and that the wire sheathing is good

VehiGAZ

Thank you Mr Merlin! Yes, the light comes immediately on startup, before the car moves. I will check the hot wire as suggested and post a pic here for verification.

John Speake

There are also two relays on the back of the ABS unit, under the inner wing liner.

These both have Bosch part numbers on them, and are not expensive. One of these failed on my car with the same symptoms as yours.

VehiGAZ

John, would the ABS light go off with the CE panel relay jumpered if one of the relays on the ABS unit itselfhad gone bad?

Speedtoys

Merlin:

Mine comes on after "a few moments to few minutes" after I start driving.

Sometimes..the first stop or slow down in the neighborhood in the AM you will feel the pump cycling, making harder to stop.

?

John Speake

No, all 3 relays need to be good for the light to stay off when just ignition is switched on and car stationary.

VehiGAZ

Thank you again John! I'll crack open the old relay anyway and fix it, and offer it up to the next rennlister that needs one.

SMTCapeCod

Mine did that for many months- intermittent light, then became more frequent. Very occasional pump activation diminishing braking power. Seemed like I could sense one wheel that was the culprit- I know I have a repaired wire pass front, but maybe that was a gut response to the pump noise and it was radiating in a weird way (given the pump is drivers side).

Eventually, I was on a looong drive and the pedal went to the floor. Wasn't firm then soft, just no resistance the whole way, and wouldn't pump up. Avoided catastrophe, and a few miles later pedal was back. Fluid level normal. Attempt to remedy was a new master cylinder and proportioning valve. Not long after, it did it again. Maybe maybe it was a huge bubble from the proportioning valve/ABS unit. But Pulled ABS relay for time being, no pedal problems.

Speedtoys

Pull relay = no more ABS light on dash too?

SMTCapeCod

Nah, pull relay=guaranteed light on dash.

I just wanted to isolate the ABS system as a potential culprit in the dead pedal, though most folks with more brains/expertise don't concur with that hypothetical correlation.

Speedtoys

Ugh..ok.

Mrmerlin

Just had another one today, 1988 S4 ABS light comes on when the engine is started.
The fix, swap in a new used relay on the CE panel,
more often than not its the relay on the CE panel that fails,
though if it tests good, then your off to fender liner removal to test the relays in the fender

Speed from your info you have a bad sensor or ground wire or bad connector connection\,
check the grounds in the spare tire well ,
and clean all of the sensors and wire connections,
inspect/test the ground wires in the barrels,
use deoxit 100 on all connections