Been working to fix the tach on my 87 300d. Has not worked for many years. Checked the signal and power and both were good.
Finally removed the tach, noticed a burnt resistor.
Replaced the resistor, and the two capacitors on the board.
It now works correctly! RPM's jive with the idle speed adjuster setting.
The resistor and Cap specs are; Resistor 51ohm 1w
Caps 100uf & 1uf

I posted this for all who have searched for tach repair info and could not find it in the archives.