The tachometer on my 1991 940GLE at idle jumps between registering zero RPM and the correct value. Occasionally while I am driving it also jumps 500 – 700 RPM. All of this occurs with no change in the engine performance. I changed the crank angle sensor since the insulation was bad but unfortunately this did not correct the problem. I experienced the same problem about 30,000 miles ago but after I changed the Cap and rotor the problem went away. This time I changed the cap, rotor, plugs, spark plug wires and the problem is still persisting. Does anyone know what drives the tachometer on the 940 B234 engine? Is it the crank angle sensor or is it sensing from the coil?

I would appreciate any help in resolving this problem. After reading through all of the archives it appears that some other people have experienced the same problem but no resolution that I could find was ever posted.