After replacing my OVP, which caused the ABS light to constantly stay on, the light is off everytime I start my car. However, as soon as I start moving forward, the light comes on and stays on until I shut off my car and restart it, and then as soon as I start moving, it comes on again. Therefore, I think its a sensor problem. I've unplugged both front sensors and rinsed them off with brake cleaner fluid, and they were both already pretty clean except for a little grease. Cleaning the front sensors did nothing. Could it be the rear sensor that's causing the trouble? I have a multimeter, but how do I use it to test the sensor to see if it's operational? I checked the diode measurement in the front ones at the plugs and they both read about the same number, but am I even supposed to use the diode function of the multimeter when testing the sensors? Also, where would I find the rear sensor's plug to test it? And could it be a different problem altogether and have nothing to do with the sensors? Thanks.

It sounds as if the signal from one sensor at least isn't getting to the ECU. This could be a bad/dirty sensor, or broken wire.

Do you have a staggered wheel/tire set up at all?

An imbalance between the rolling size of the wheels can also cause this.

With regards to testing the sensors, I think you can check the resistance of them, but the definitive way is with an oscilloscope and spin the wheels. And correct me if I'm wrong, but I think the sensor for the rear is on the output of the transmission.

My tires are all michelins, same size. However I think I might need a slight alignment because the car kinda drifts to the right whenever I apply the brake. But does the alignment affect the sensors to send a bad signal? I'll check the resistance on the plugs when I get a chance. Where is the plug that I could check the resistance for the rear sensor?

Sensor could be bad, but you'd need a scope to check it - or you could try backprobing the wires with the key on, wheel jacked up & spinning and the DVOM set to A/C. Do all 4 and compare notes.

Alignment would have no impact on sensor input.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.

There is only 3 sensors on these early cars... The two fronts and one on the rear.

the plug for the third sensor is under the rear seat.you can compare the resistance of all three.measure on a 2kohm scale and you will find that one is very different from the others.this is causing the light to come on and the system to shut down.

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David S Poole
European Performance
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"Fortune favors the prepared mind"
1987 Mercedes Benz 420SEL
1988 Mercedes Benz 300TE (With new evaporator)
2000 Mercedes Benz C280
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Thanks, I'll check the resistance on the sensors in a little bit. I'll update on what I find.

I've checked the resistance on both front sensors at 2k ohms, and they both read around 1300. However, I couldn't find the plug for the rear sensor. I took out the rear seat bottom, and I only found wires and vacuum hoses running under the insulation padding. Does the sensor plug look different than the ones for the front wheels and I just missed it, or could the plug be somewhere else in my car?

It's probably one of the front wheel sensors. Look for broken or frayed wiring. My wiring on an 85 380SL was not routed correctly and was with great difficulty repaired. The assemblies are expensive.

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The rear sensor on my '87 560SEL and SEC are in the right (passenger) side of the differential, up front. It reads a ring mounted on the pinion shaft.

I'd clean my front rotor sensing rings (don't remember the right name - the grooved piece the sensors read) before doing anything else. IIRC, a compressed air blast will blow the crap (pun intended) out of the grooves.

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I'll give the compressed air blast on that grooved piece a try today and report back on my findings. Thanks.

Well, I blasted all the dirt out of the grooves, but the light still comes on as soon as I go over 3 mph. I checked the resistance on both front sensors, and they read about the same. Therefore, the only other options I see that could be causing the problem is that either the wiring from the front sensor plugs to the unit is not carrying the signal, or it is the rear sensor. Is this a correct assumption, or could it be something else entirely?

Could be a bad sensor. The only real way is to check the signal with a scope; the resistance test only locates "obviously" bad sensors.

Still, you should ohm the rear one before tracing a bunch of wiring. If it's "good", then you should test the inputs for wire continuity at the controller from the sensors. If the wires are good, it's either your controller or one of the sensors...

How does it brake?

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.

Where do I find the plug to ohm the rear sensor? I couldn't find it under my rear seat. And when you say controller, do you mean to check for continuity in the wires at the plug going into the main ABS unit? Also, do you know which wires to test in the plug for continuity, like the numbers?

With regards to how it brakes, my friend's 2000 Acura brakes feel a lot more sensitive to the touch, whereas mine kinda have to be pushed down harder for effect. However, the brakes still feel sufficient enough to stop my car when necessary. But even in rain, if I push down too hard, it'll slide. So with winter coming, I'd like to have the security of ABS for the snow and ice we get in Pennsylvania...

It's probably mounted in the differential (underneath the car). It should be the only part of the drivetrain past the tranny with wires going to it.

Yes, that's exactly what I mean. I don't know the numbers, but the wires *should* be colored the same at each end. I'm sure someone else can help as far as that goes.

Don't push down so hard, then. People drove (and still drive) just fine up there without ABS. ABS is a safety feature, not a crutch for poor braking habits; the more you use it the quicker it'll wear out.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.