Ok, the 93 I just bought was an auto. Swapped for a manual, which was a 95 M6, by the previous owner. He had a MSD digital 6 in it, and the tach worked, both before and with the box. Before selling the car to me, he sold the digital 6. Tach has not worked since. Any ideas??

 

I guess that I should also add that the tach initially comes up to idle, but when the gas is pressed the tach drops down and bottoms out on the pin. Someone has to have some kind of an idea. What about the tranny/tach in-line signal filter? Could that be it?