ABS light is on

have you checked that all the brake lights work?

& we had one which did this, as the wiring from the sensor was picking up noise from the ignition, a set of OEM leads cured it.

This means the ABS system has detected an error. Someone needs to connect up a scanner to find the error.

I would suggest a local independant BMW service guy, not a dealer but someone who has done the BMW training and has the special tooling. You can try your local garage but many of the generic scan tools won't be able to get the error off the system.

Once you know what the actual error message is, then you can find out what is likely to be wrong and then how to fix it. If you're lucky it's just a bad wheel sensor somewhere.

--scott

Thanks all, appreciate the fast responses.

Booked it in for a scan. The mechanic I spoke to thought it might well just be a wheel sensor as you suggested. One symptom I mentioned to him that I missed from my post was that the light doesn't come on when the engine is started, only when in motion. (And I *think* not when I'm reversing out of the drive.)

the car cant tell whether the wheel sensor is giving a signal till it moves and abs is probably disabled in reverse. So wheel sensor or wiring is certainly high on the list to check.

The ABS light being on means the ABS will not work -- your tires will lock if you use the brakes in a panic, or on ice even if not in a panic.

ABS looks for a tire to stop turning as you apply the brakes. It it finds one then the brakeing pressure is removed from that wheel for a moment and then is reapplied. The result is that the wheel is allowed to turn so you retain directional control over that wheel. The light being on means the system is disabled and the tire will lock and remain so until you release the brakes.

This is not a fatal condition -- we drove around for decades without ABS systems. Indeed, in the early days of ABS, lots of people wanted information on how to disable them.

In any case, you have a failed speed sensor in one or more of the wheels. Odds favor just one failed sensor, but there are four of them and you have no clue which one. You need a diagnostic device that connects to the Data Link Connector (the round one by the right-side shock tower in the engine bay) to read which one has failed. Replacing a speed sensor is not a difficult task, and one that most people should be capable of undertaking at home.

[...]

No it doesn't.

It means the system "thinks" there is a fault which *might* mean the ABS won't work.

Chris

Chris Whelan gurgled happily, sounding much like they were saying:

I've never yet come across an ABS system that didn't fail-safe and turn off completely at the first sniff of a fault.

The wiring loom to the dash gets damaged; a short from a live wire connects to the ABS warning light...

Chris

Same thing.

If you cannot rely on the system working, then it does not work even if there is random functionality.

The only safe assumption is that if the light is on the system has disabled itself. If the system turns out to work when you need it, count that as Living Well and thank your lucky star.

Go on then ,when did you ever come across that?

How is a working system with a malfunctioning fault indication the same as a broken system?

Chris

I never said I did.

I have seen an ABS control module where the electronic switching for the warning light had gone permanently closed though.

Chris

[...]

Yep.

But the light being on doesn't mean the ABS *will not* work.

Only that it might not, so shouldn't be relied on.

Chris

Because it's malfunctioning, that means it's broken. Besides you can't usefully tell that that's the failure made so you're going to have to treat it as broken.

That's a corner case. You should consider the ABS system out of service if the light is on. And in the instance you supposedly saw, the controller had a short in it that turned the light on, and there's no way to ascertain out here in the trenches that the short hadn't taken out some other significant part of the controller function.

The system design is that if the light is on, the system is off.

You're not laboring under the illusion that because the car stops then the ABS works, are you?

Yep, but it still doesn't mean what the "The ABS light being on means the ABS will not work"

Chris