Re: Tach signal negative volts??
That the tach will respond when connected directly to the alternator and, that you have no variation in voltage at the battery would indicate that your rectifier is dead.
A rectifier has diodes that cannot stand current coming at them from the wrong direction. Loose battery cables, cutting the ignition off and on quickly while running, switching batteries with a switch of the wrong type while running, reversing battery polarity, disconnecting battery cables while running, and testing rectifier wires with voltage can all destroy the rectifier.
Excessive heat can also destroy the type of rectifier that you have, and will happen quickly if the engine is deprived of cooling water for even a short time.
I cannot, with any degree of certainty, establish the condition of your rectifier. I can recommend that you contact the manufacturer for an adjustment if you feel that the problem may have been their fault in the least degree.
