From the tach's housing (or cluster) to any metal on the car, battery's negative is the best. When I tested mine that's how I did it, however my tach's were loose and out of the cluster which made it real easy.
I used the cluster body for the ground to battery. Started the car and then laid the coil jumper onto the coil and the engine died. I then put the ground jumper on the tach body and the engine did not die.
The tach when installed in the car would ground through the cluster. So since its apparently shorting out, the internal wiring is somehow not insulated properly. I called the guy I got it from and he's sending a replacement.
Btw, I verified that it should be grounding through the cluster body by using my old set.
Thank you very much for your assistance !!
