Today I put some front tires on my 87 300d and Orielys game me a new battery for it for free... got to love the exchange warranty program they have.... anyways. the idle was low and surging now and no tach. I thought ok OVP is bad. So I check fuse, it's fine. I pulled the relay and opened it up, it seems fine, it was marked as a unit from 2001, and showed no issues, but I went ahead and reflowed all the solder. no change to car.

The idle surge goes away if I disconnect the idle electronics from the IP, also the ELR does not work now either. no change to idle when moving it around. So this is obviously electrical.

It the only other way to trouble shoot it to order a new OVP and see if it fixes it, or is there another step I'm missing? I have gone ahead and disconnected the electronics for now and simply manually set the idle to normal. I guess I can live without tach if I have to .

I also have smoke at idle and cold starting... a lot at cold starting. on road it fine, and engine seems to be running fine... so that will be next. Just need the car running for a while as I tear into the wagon to start doing the long list of crap it needs.


This guy simply explains the idle adjustment on a 603. https://www.youtube.com/watch?v=MgH0fIWNcSc

Check the rpm sensor on the bell housing flange under the oil filter housing. IIRC it puts out a voltage signal proportional to rpm. The voltage reading should rise with rpm.

Sixto
83 300SD
98 E320 wagon