Mystery Tach Problem

[6.5 Detroit Diesel]

Hoping someone has an idea what would be causing this.
Recently the tach stopped reading correctly. It will read anywhere from 200-400 revs lower than it should. Occasionally when I floor it, the needle will jump up to where it should, but as soon as I back off the pedal, it drops down low again. I have pulled off the cluster and cleaned the contacts. Anything else that could be causing this?

[More Power]

The tach derives its signal from the alternator.. If the alt is having a problem, it could affect the tach.

Jim

[6.5 Detroit Diesel]

Any ideas as to what kind of problems? It has done this before and then it was fine again for some months.

[More Power]

It is entirely possible the tachometer itself or the gauge cluster is at fault, as could a wiring issue, but a flaky alternator is more likely. The alt's regulator is comprised of components that convert AC to DC. The AC part is also wave-shaped to provide a tach signal. The regulator in these alts is replaceable.

Jim