[Shazaam!]

The crankshaft rotation sensor is located near the water pump where the alternator leads come out of the casing. It's shimmed out to a specified clearance from the flywheel. If the gap is incorrect or fluctuates due to a loose flywheel or cracked engine casing the rpm signal will be erratic.

A too-large sensor gap usually results in a cut-out at high rpm similar to hitting the rev limiter. This is usually accompanied by the tach readout simultaneously dropping to zero.