It would indicate properly until 1400 rpm and them drop to zero. I was sure it was the tach.

Of course VDO doesn't make the tach anymore even though it is less than 10 years old. Through VDO I found a NOS tach from a dealer in Florida. Put the "new" one in and it didn't work either. I was NOT a happy camper. Called the dealer in Florida - he says send him both tachs. He calls me the day he gets them. Says: both tachs are good but the calibration on my old tach is very unusual. He sends me my old tach and refunds my money, less shipping. Aren't I the A-hole - assuming that I was getting ripped off. I got very good, courteous service from these guys at a more than fair price (tach check out for free).

Turns out that the wiring harness connector close to the engine was bad. Eliminated this connecter entirely and all is well.

Morals of the story. There are good companies out there and It isn't always what you think.

Your alternator could be fine but the diode tap for the tack in the alternator could be bad or your wiring could be bad. Try putting an analog voltmeter on the tach lead from the alternator and see if it changes when the tach quits. Digital Volt meter won't work in the case.