MSD Fast Tach wiring.....only 3 wires ?

Ok.....so if I hook red to + and black to (-) the tach jumps from 1000 where it seems to rest when disconnected to about 1300.....

But when I touch the red to(-) and black to + on the battery the tach zeros out.....

I am assuming that the white is the signal wire.....

Which way is correct? It seems like the tach should zero out once the ignition is switched on before starting engine.

Signed,

Dazed and Confused