I have a 87 300E and the antilock light is steady, it appears that the charging system is working properly which is one cause, I have not yet looked at the wheel sensors, can anyone give me the troubleshooting steps to find the problem. thanks in advance

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2004 SL500 Black 55K Miles
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1966 Twin Cessna 320D
1989 Jeep Cherokee 233K Miles
1994 Ford Van E150 399K Miles

First quick check would be the fuse on top of the OVP relay.

I had the same problem and realized that I had new tires on the front and old ones on the back. The size difference made the light come on. As soon as I replaced the tires, the problem went away. I had the same thing happen when I had a spare on for a few miles.

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I have a 1986 300e with the antilock brake light that comes on at about 25 to 30 mph. It then stays on until the car is turned off. The OVP relay and fuse looks ok and the voltage while the car is running is at 13.3 volts. I know the ABS is not functioning since I can now skid and lockup the wheels for the 1st time in 20 years. After looking at many other posts my best guess is that it is either one of the two front wheel sensors or the rear differential sensor. Without having a computer to plug in to the car and tell me which one is bad, how can I diagnosis the problem? One post said to use a ohm meter but gave no details on how or where to plug in the leads. Is there a better way to diagnosis the problem? Please be specific. Any pictures would also be appreciated. (And yes this is my 1st post)

Thank you

On the left and right side of engine bay are the plug connectors. Just look for two big cables with big connectors that look exactly the same right next to the suspension top hats on either side.
The female side of the connector goes to the ABS sensor I believe.

Thank you for your reply.

On the ohm meter am I looking for an open or closed circuit or a specific resistance to show which sensor is bad. How would I check the rear sensor in the differential?

Here you go, this thread also has a picture of what the connector looks like...

ABS Sensor thread- click me

Thank you, I was able to locate and test the sensors at the connection from the link you posted. Unfortunately, both front sensors tested the same at 900 ohms. I would assume that means they are ok. Any ideas as to what the next step would be to isolate the problem and how to fix it?

you need to check it while spinning the wheel as well. flip over to ac voltage and it should be reading some there as well.

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If they ohm out as OK, remove the sensors and see if there are metal filings on the tip of the sensors..common for this to accumulate on old sensors.

The only true way to test these is to look at the sine wave specs with a scope.

Thanks for both your answers. Does both sensors being at 900 ohms indicates that they are ok for the ohm test? I have not tried to spin the wheels or even examine and remove the sensors yet. It is the only car I have and I am sure it will take a few hours to perform. I assume that it will take the same amount of time to do a brake job since to get to the sensors, I have to remove the calibers and rotors. Is this correct? What would the specs be I were to hook the sensors to a scope? Would it be worth the expense to buy/rent/borrow a scope and perform the test?

Sensors are easily accessed with an allen wrench. No brake parts have to come off. I would simply remove them and see if magnetic crap is on the tips.
here is a site that explains the various mag sensor test procedures.
http://www.labscopes.com/pg13.htm#a

Thank you again. I did clean off a lot of black metal filings at the tips of both front sensors . I also got a maximun of 0.4 AC volts (with a normal meter not a scope) when spinning the wheels by hand. Unfortunately, The light still comes on. I noticed that when I accelerate quickly the light comes on at 18 mph. Should I check the rear sensor? And if yes, where would I find the connection under the hood? Should I also check the sensor at the differential for metal filings or is it more difficult? I doubt that this has anything to do with the antilock light but for a few years ago the cruise control got jitttery, the car lunges a little whenever it is engaged. I assume cruise control uses another sensor. Any other ideas would be appreciated to fix the ABS problem.

Did you check for equal PSI/Size ??

All 4 tires are the same size. The 2 front tires have 80% tread and the 2 rear tires have already reached the tread depth indicators and beyond (almost bald). To compensate for the decreased wear, I over inflated the rear tires to 42 psi and left the front at 29 psi. The antilock light still came on during the test at 18 mph. After the test I restored the rear tires to the correct preasure. Any other ideas?