My tach on '79 300SD has stopped working completely. I have all the service manuals but no test is listed for the tach system. I have the unit mounted on the left fenderwell. They call it a revolution counter test socket. I have the bottom part with three electrical wires, the top part that unscrews and has the male pins(the expensive part), and I have the long gray wire that plugs in to the bottom of the test socket. The other end of this gray wire has a sensor on it. It is bolted on to a bracket right below the vacuum pump and a pin hits it every time the engine goes around once. My pin looks like it no longer hits it. Is this adjustable to move the sensor closer to the pin or should I replace the sensor? Does anyone know of a test for the other unit? Thanks

THE Pin is just there to pass by the sensor, not to hit it.

9 times out of 10 the unit on top of the socket you mentioned is faulty (counter). Replace it. (we sell parts you know)

__________________
Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS

I had the top section of the tach. sensor replaced, and it cured the problem. Costs like $78.