If this is an 81-83 300SD, do an archive search on 'cigarette butt.' It's the secret handshake of MB Diesel owners Seriously, the likely problem is that the tach amp isn't making good contact with the connector. That round tethered cap behind the left headlight contains the tach amp. Remove the cap, pry the tach amp board out of the cap, carefully scrape the silicone off the board, then fit cigarette butts, furniture foot pads, foam peanuts or whatever you have handy so when you refit the tethered cap there's good pressure on the board into the contacts. If you leave the silicone in place, the silicone will absorb the pressure of what you stuff in the cap and you'll probably have the same tach problems. Hard to visualize so get out there an have a look for yourself.

Sixto
95 S420
87 300SDL