300E - ABS light stays on intermittently
 

The ABS light has started to stay on intermittently on my '86 300E after I start it. There doesn't seem to be any particular scenario where it happens more often, i.e. hot or cold engine, etc. If I turn the switch off and restart it immediately, it goes out. This has probably happened about 10-12 times over the last couple of months, so it's not a real problem or anything. Every now and then when the road is wet and slippery, I try out the ABS and it's never failed to work for me, although I haven't had the chance to actually try it when the light is on. I'm just wondering what might be causing it to come on. I seem to remember reading something about how the OVP relay could cause this. Can anyone shed some light on this for me? Is there any way to accurately test an OVP relay, or if it's suspect should I just replace it? (I think it's probably the original one). I've cleaned the pins and the socket with DeOxit 5% ( www.caig.com/ ), but it hasn't really made any difference. The 10A fuse on top is good.

Thanks,

Gary

I thought I read on another thread that the impending death of an alternator could cause the same light to illuminate...

...just a guess.:confused:

Hopefully not. I just replaced my alternator/regulator last year.

Gary

G,

Voltage drop below 10 vDC will trigger the SRS light, not sure if it will trigger the ABS as well.

A failing OVP can intermittently illuminate the ABS light. This used to happen on my 300SEL. On my 560SL, my ABS sensors needing replacing, but I don't remember whether the ABS light was intermittent or just on all the time.

Gary G-Benz is right , my abs was coming on at odd times yet the abs was working fine after much anguish trying to find the problem, it turned out to be the alternator. But it was charging fine at the correct volts and all, but the problem was that one of the exciter diodes was fauty in it and setting up a very strong rf field at was causing the abs sensor closest to the alternator to malfunction.
Had it repaired and all is good again.
Cheers George

You most likely have a speed sensor problem. An OVP problem is unaffected by recycling the key. Any intermittant capable of setting the light will also be reset with an ignition cycle, just as any such fault will always keep the light out till recycled.

Since the OVP powers the controller when it fails the controller sees it as a recycle. So the light can come off and on and not be viewed as a fault to the controller as it just thinks it is being turned off and on.

When the light is on the system is off and won't function.

Something else that comes to mind, though it hasn't happened for a while, is that the ABS would occasionally activate when coming to a stop at about 1 mph or less. In just the last 2 or 3 feet of stopping the system would activate and cycle for half a second or so. This hasn't happened for a while, but could it be related, and does it indicate anything in the way of a failing component?

Gary

Well, in that case, it absolutely is a wheel speed sensor.

Do these get dirty over the years and need to be cleaned, or do they just go bad and need to be replaced? If the latter, how can I determine which one is bad? Are they on all four wheels or just the front two?

Gary

I agree - sounds like the ABS sensors. I think they wear out and need to be replaced. I've never heard of refurbishing them. Usually the fronts go bad and I think they need to be replaced in pairs. They're pricey - around $100 each, but not hard to put on.

They are really easy to test with proper equipment, but since most don't have it I suppose replacing front sensors in a pair is a good procedure.

The 300e will have 3 sensors. The rear one is in the differential and the rear brakes are controlled as a pair. The diff sensor hardly ever goes bad.

if you really wish to beat the system you might try just the right front and see how it goes. It is so rare to find a left front snesor bad that it is a shop event when one is detected.

Thanks everyone for all the good info. Just a couple more questions:

Are the sensors made as left and right units or are they interchangeable? If the latter, I suppose I could try it on one side and then switch it to the other if that doesn't fix it. How easy are they to get at?

What equipment is involved in testing the sensors? Are they tested through the system or in isolation?

Gary

Gary,

Have you gotten anywhere with this? My car is doing the exact same thing.

No, it's not really causing a serious problem at the moment, so it's at the bottom of my priority list right now. When I rebuild the front end I'll deal with it. Might be a while.

Gary