Hey Don...

Did you ever get your fluctuating Tachometer situation figured-out???

Regarding Isolators: I doubt it matters what "brand" of Isolator one replaces the current "brand" with, as long as the number of POSTS is correct (3) and the minimum amperage is that of the alternator's output.

I am still having a fluctuating Tachometer.

Fluctuates about 0.6 volts up and down, rythmically, between 13.9 and 14.5 volts (I think those numbers are right) while the engine is running.

This voltage fluctuation is reflected in the up and down movement in the TACH.

When I turn-on the headlights, the TACH stops fluctuating.

It looks-like the TACH leads are connected directly to the alternator, thus, this makes me suspect something in the alternator(?)

I don't see how the Isolator could cause this to happen, but am willing to replace the Isolator, if indeed, it could be the culprit.

Isolator should be fairly easy to replace and the wiring is fairly straight forward.

Did you come to a conclusion to your problem???