Tachometer not working.
 

This is for my nieghbor's '80 300SD. His tach isn't working and we already tried the cigarette butt trick, but with no success. We also tried putting in a "new" tach from an instrument cluster that he bought to fix another problem. Anyways, his tach still wouldn't work. Does anyone know how the rpm sensor works and has anyone changed one before?

When i replaced my tach converter my tach worked fine. Mines on an 83 300D but im sure yours is the same. Little round cylindrical black plastic car that screws on and off easily, LIke $70. Drivers side top of wheel well youll find it~
try that, cant be anything else.
brett

So, when you replaced your tach sencor, that included the black cylinder on the wheel well, and the gray line that goes down to the crank pulley. How exactly does this system work? I always thought the tach got it's info from the distridutor. Of course our cars don't have one, so what exactly is the sensor above the crank? Does it run off a magnetic field princible? The jet that I work on uses this type of system to measure engine rotations. A sensor sends out a magnetic field. As the engine fan blades pass by the sensor, "breaking" the field, it creates a pulse which then is used to determine fan speed.