Tach problems
 

Recently the tachometer on my '87 300 SDL has been working intermittently. There does not seem to be any rhyme or reason as to when it will or will not work. I did recently have the engine mounts replaced, what a difference in the smoothness of the engine, but the problem was happening before the engine mount repair. It sounds like a loose connection somewhere and I do not know where to begin looking. I dont want to start under the dash because it is a pain to get to. Where on the engine are the wires/sensors to check? Any other suggestions?

Houckpa - There are a couple of things that could cause your problem. Search this forum on tachometer and you'll find several things to check.

I fixed the tach on my '82 300D by doing the "cigarette butt" fix to the amp.

Wes

The rpm sensor is behind the oil filter and reads triggers on the flywheel. From there wires go to the Klima relay just inboard of the brake booster. Possibly goes through the EDS controller then to the tach.

If the AC compressor is not engaging when the tach isn't working, I'd check the Klima relay.

FWIW, the 603 doesn't have the tach amp arrangement of all but the 85 and some 84 617s so the cigarette butt/foam peanut fix doesn't apply.

Sixto
95 S420
87 300SDL