Well I tried different power supplies and grounds, unhooked the dash light in the tach and still the same deal.
I saw somewhere that if you hook to the coil without the filter or adapter that it will fry the tach, don't really know.
As for the anti-hotrod spirit,Same Here. I've decided not to worry about the tach it works when the lights are off and thats good enough for me. I'd rather enjoy the car than worry about something so stupid.
If you figure it out let me know and then I'll fix mine to.

Rocky