Strange brew

[jbach36]

Occasionally I'm sitting at a traffic light and the car is idling at 7 on the tachometer. Suddenly the idle speed will drop a lot but continue to run, and the tachometer needle goes to zero, and the ABS brake warning light comes on. 1 or 2 minutes later (or if the light turns green and I give it the gas) the problem goes away.

Any idea what is causing this? It might happen 2x per week.

jeff
1991 300d, 210k

[sixto]

Maybe the OVP relay is acting like an UVP relay? If it were just losing the rpm signal you'd lose the tach and AC compressor. Also cruise if moving. ABS warning is generally a low voltage indicator so I'd check for a tired voltage regulator or alternator. Can you check the OVP innards for failed components or solders? Do you have a known good OVP relay you can swap in to test? Tach and EDS failures are rare in these cars, intermittent or otherwise.

Sixto
83 300SD
98 E320 wagon

[Diseasel300]

As a confirmation - when the OVP relay is non-functional the ABS light DOES come on. All of the symptoms above seem to indicate a flaky OVP or other voltage issue.

[Mxfrank]

The only thing to do is to replace the OVP. That will fix the problem and done with it. It's a total waste of time to try to repair or test the OVP. The critical semiconductor components can't be tested in place.