I’m having trouble with an intermittent idle surge up to 2000 RPM. Replaced OVP (FastLane) and fuel pump relays. Thought Idle Speed Controller and Slide Valve were next but I have another thought.

I see the Tachometer shows what appears to be twice the RPM. Just over idle it shows 2000 RPM, which I can tell is way off. Also happens at highway speed.

Wiring diagram (on 107 CD) shows the tachometer is driven form a proximity sensor on the input side of the transmission and that the engine management controls get the RPM from the tachometer. Another place it appears that the RPM comes off the ignition distributor. I am not too great at reading the diagrams. Which is correct?

Is it possible a problem with the wiring harness, 6-pin connection or something is bad in the Tachometer that could cause ignition or idle problem?

I appreciate any help.

Steve
’85 500SL (Euro)

I can't recall off hand, but I think the tach gets its signal from the front crank sensor. This has nothing to do with other controls, I think.