tack reads double

the tachometer in my 78 suddenly started reading double the actually RPM's. The car runs fine and I borrowed a tach from my other Vett. It has to be an electronic component (there are only a few components, and naturally the components are not stamped with part numbers). Does anyone have a schematic diagram or electrical parts list for the factory tachometer in a 1978 vett?

As I recall, all the components that are electronic are inside the tach. Did the other tach read double as well, when you hooked it up?

Alex

when I switched the tachs around the good tach still read correct and the bad tack still reads double except at the engine off position when both tachs read the same