ABS light on

[melsm3]

My abs light stays on after I start my 95 M3. I've read resistence on all sensors and they all read around 1125 ohms. Does anyone know how to read out the 2 abs relays? I've read in the archives that the battery / alternator can cause this. Has anyone actually confirmed this? Where do I measure the alternator voltage? One probe on the main alternator output voltage line and the other to ground, or from the battery? I don't want to fry out my alternator or voltage meter. ghead

[Mattitjah]

Yes, a bad alternator can cause this. The computer shuts the ABS function off to save power. Take the car to a parts store. They will check the alternator for free.

[melsm3]

In the next chapter of the ABS light saga: The light was off this morning driving to work. Since the roads were wet I decided to check if the ABS actually worked. The system worked for about 1 second, (slight pulsation felt in the pedal), then the light came on and then front left wheel locked up. I tkink that I may be able to rule out the alternator theory because the light came on when I activated the sytem. Does anyone know of a test for the pump or the relays?

[Furious]

i dont belive the light should come on at all. i was the one who wrote about the abs light comming on if the alternator is going. matt is right that it comes on to save power. however in this case i think it can be two things, the most probable one is that one of your speed sensors is faulty and the other is that your getting early signs of the alternator going. if i were you id check the voltage to see and or rule out the alternator theory. next i would scan for the code which should be there even if the light isnt there. i think the code is stored for a X amount of time.

[melsm3]

Can I scan for the code myself by using the gas pedal stomp test (it's a 95 M3) or do I have to go to the dealer? The car has 50K on it, is it possible that the brushes will wear so fast? My E30 318 alternator brushes lasted for over 100K.

[Furious]

check the FAQ forum and see if the code matches a code that is pull-able.

[thekid]

I had the same problem when I bought my 95 318i. I was told by my friend/proffessional mechanic that the code that came back was a speed sensor. The ABS still worked, he just reset the code so that it wouldn't come on all the time until I can get back intown to fix it.

[melsm3]

Another entry in the ABS light saga. The problem has appeared to fix itself! At least for 2 days now the light has not come on. The only thing that I did different was rotate the tires when I first had the problem. I'm thinking the ABS system is sensitive (at least on my car) to outer tire diameter. Orginaly when the light came on, the rears had 1/2 tread remaining while the fronts looked new. Now that the rears had a chance to wear down some, the system may have come back into spec. "Hows that for off the wall thinking" I'm keeping my fingers crossed.

[TECH]

You'd need to get the code for a place to start.

What you can do if you have some time to kill is pull out all 4 wheel speed sensors, and clean the ends of road grime/brake dust that accumulates on them. Take some brake wash spray, and spray into the bore of the sensor, while spinning the wheel at the same time, and that'll clean the pickup teeth as well.

You could also be looking at a bad hydraulic unit, relay, wiring, or control module as well. Guessing is pointless, really, you need to at least know what code is thrown.

I doubt it's an alternator issue.

[007ben]

My friend had a 94 325is. his ABS light came on from time to time. He belives this was caused by crap on the ABS senors. Road grime gravel magnetic metal bits and normal road stuff. He had been told this by a couple mechanics and just getting the sensors cleaned can help or replacing them.