I am still working on the tach on my 87 300sdl with no luck. It is fine when the engine is cold, but once the engine warms up over 80 the tach goes to 0. Let the engine cool down and the tach is fine again. I cleaned the connections on the OPV and Klima. I cannot find where the sensor is on the engine. I read somewhere it was behind the oil filter, but I cannot find it. Can anyone give me a better idea where the sensor is located or what to try next. I haven't fooled with the tach gauge itself only because it is a pain to get up under the dash. I dont want to start replacing relays, etc without know what is wrong. Thanks
Brad

Happened to me on my 124. It was long ago I forget what I replaced, It was a relay of some sort. My idle sensor relay or whatever was burnt and it would work if hte car was cold but as soon as the heat hit it it was shot, replace it.
It costs maybe around $80 and is a swap/plug and play.

Someone else should know the exact name of it, if they dont I'll look at my receipts and figure out what part i ordered.

Edit - The relay on mine was a PITA to get to, located deep beyond the to the driver's side of the battery underneath the single windsheild wiper

__________________
-Sam
93 300D
210k - Sold, but not forgotten.