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....

Originally Posted by Marshall Welch

A little off topic, but I might need a new one, too. I’ve had one of these for probably 10 years and it’s worked great.

It might have been damaged when I hooked up a Noco Genius Boost GB40 to jumpstart my car. The tachometer was then pinned all the way to the right (>6,000 rpm) afterwards. I removed the unit from the plug after that.

Originally Posted by math

I found something


https://www.ebay.com.au/itm/284269080366?hash=item422fc1ef2e:g:tTEAAOSwHtxgQ~Us

Originally Posted by Stlerman11

did anyone ever find this guy or find another option for making them new?

Originally Posted by ykobayashi

Yeah that’s mine. I designed that tach amp in March 2021. It runs on the same principles as Techguy512’s. He passed away many years ago.

Originally Posted by ykobayashi

Oh, I guess that would be me. I make these.

The original shown in this thread from years ago was made by Bob Myers “Techguy512”. It’s my understanding he passed on in 2015.

Math, it looks like your MB Electronics Upgrades amp is damaged. I think the program memory is corrupted and it gets stuck in the loop that generates the needle sweep. The only other explanation is it could be kicked into some diagnostic mode or it’s pickup circuit is receiving high frequency noise.

Originally Posted by ykobayashi

Yeah I don’t know. It isn’t my design. I’m not sure what is going on. Sounds like you have power and a good sensor.

Maybe you should oscilloscope probe the board at PIN 3 the output and PIN 9 to see if you have a solid input and output signal. The input should be the crank signal and the output should be a square wave. Both should be roughly 14Hz at idle.

Somebody else had this issue with his board. See my post #137 above. Same problem. Exactly the same. That’s why I suspect corrupted program memory. Pinned at 5000. I suspect the problem is related.