Originally Posted by techguy512

After struggling with my tachometer for over a year, and trying all the tricks including contact cleaning
and cigarette butts, I decided that if I wanted a functioning tach I would need to take matters into
my own hands and tackle the project of designing a cost effective replacement Tach Amp.

The results of the project are shown below in photos.

1) Tach Amp installed in the connector socket on the left front fender
2) Cover (minus the old electronics module) being replaced on the connector socket

This tach amp should be compatible with the following vehicles equipped with the OM617A
engine built between 1978 and 1984:

W116 Chassis 300SD 1978-1980
W126 Chassis 300SD 1981-1983/1984 (it's unclear exactly when the tach amp was phased out)
W123 Chassis 300D/CD 1982-1984
W123 Chassis 300TD 1981-1984

My minimum order quantity on PC boards was 40pcs, so I have about 35 left, and I want to offer
them to anyone that is tired of having no tach. Cost is $42 with $4 S&H.

I have attached the User Guide (with photos stripped out to reduce file size) for anyone interested in
learning more about the operation of the Tach Amp.

During this project I learned a ton about the operation of the tach in these vehicles, as well as how
to troubleshoot tach problems. If anyone has any questions, just fire away....