Hi; I am not sure about the tack yet, But the Other gauges on peg if they are grounded, so you must have a wire that is grounded that shouldn't be in the harness.
I tried to hit the reply before but did not allow for response. A ground problem is the first thing that everyone tells on auto electric auto to look for, I had done that. you . That was not the problem, it was a power problem to all the gauges. once the motor is reinstalled and fired up I think the tach problem will be resolved as well. Your answer was not helpful so I don't acept any charge to my credit card.