well the rev counter only has 3 connections, 12v, earth, and a connection to the coil negative.

It works sort of, so theres 12v and earth there, and im presuming the car runs fine so it wouldnt sending a flase signal to the rev counter.

u could check the continuity of the wires, check they arent giving eratic readings or something.

Im almost willing to bet my jam tart on it