ABS / Brakes problem '86 420SEL

While approaching a stop on dry pavement (at just a few miles per hour) the ABS unit kicks in. After this happens a few times, the ANTILOCK light on the dash illuminates. The car has new front and rear brake pads and I had the lines flushed about 6 months ago. Just prior to this problem the ABS was working fine. Could this be a dirty or mis-aligned ABS sensor? Or...worse yet...the ABS unit?

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Current: '86 420SEL, '81 380SL
Previous: '79 240D, '70 280SE, '81 300D.
Just about 600,000 miles on Mercedes products.

check for open circuit on front a.b.s. sensors,while tester is connected vigorusly move abs cables and watch for big changes in resistance. if open or large resistance changes occure replace sensor.

[Michael Haven]

Hi Ron you are describing a failed frt wheel speed sensors' symptoms exactly. It makes the abs unit think that the wheel has stopped turning. Thus engaging the anti-lock.
The home test is to check for continuity with a DVOM. Unplug the sensor at the wheel well, connect your DVOM to the sensor. Set your meter to a continuity test and then twist, and pull on the lead to the wheel sensor.
You will find an intermittent open in one or both frt sensors. Remove and replace the offending bit(s).

[stevebfl]

After you have tested the continuity as Michael describes, switch your meter to A/C volts and spin the wheel slowly. You should get a constant voltage only changing with speed. The amplitude of this reading should be within 10% of the same thing done to the other side.

I do this test with the Bosch tester while driving very slowly through our parking lot. By doing it while driving and switching back and forth between sensors its easy to see the differences. Bosch doesn't recommend driving with the tool on. For years we missed these diagnosis because it is too hard to get a good comparison when you spin one wheel at a time.

I doubt that you will find a continuity problem, as such a problem would turn the light on without driving and wouldn't care about speed as to whether the event occured. The reason it happens at low speed is that is the most critical time for sensor signal strength comparisons. You will notice that at some speed the amplitude will peak. This happens at around 2-4mph and its usually at lower speeds where full amplitude isn't achieved that the sensor differences trigger the fault recognition.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician