When I turn my key on and crank the engine over, the tachometer needle pulses downward, (counter clockwise),...but when it starts, the needle retrurns to the resting nuetral position and doesn't move at all.

What does this mean? What is the diagnosis?

 

Sounds like one of two things. Either the tach isn't compatible with whatever ignition system you have, or it's reversed polarity. Mine drops to below zero when the key is off, but as soon as I give it power it comes up to zero, and then to engine rpm when I fire it.