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