The tach does not work on my benz when I bought it. Do these things have a tach rely, if so where is it located?

if the tach is similiar to the tach on a 79 300sd, the tach amp could be faulty. you may want to check the pickup at the flywheel for gunk buildup as well. A trick i read about the tach amp is to take it apart and do the "cigarette butt trick". which is stuffing a cigarette butt or a piece of foam in the cap to create a better connection between the tach amp and its receiver.

i hope this helps

StraightAway:

I tried the "cigarette butt trick" on mine and now have a broken circuit board. The common problem with the tach amps is that the pins on the amp loose contact with the diagnostic socket. I have heard a number of folks correct this problem, with success, by taking a small gage strand wire apart and inserting one wire strand in each socket that receives a pin. The wire strand bridges the gap between the amp and the socket.

So where is all this stuff? See: http://web.infoave.net/~ontheriver/automobiles/tdcamp.htm

Let us know if this helps.

Sincerely,
Scott