[jcsbrotha]

I have the tach in my 66 Mustang hooked up to the coil but when I start the car it jump to 500 RPMs and stays there no matter what RPM I really am at. Any ideas of what the problem may be?

[hypostang]

A few things to double check ,
1. make sure the tach is grounded properly (black wire )
2.make sure the green wire is going to the negative (-) side of the coil
3. make sure the tach is set to the proper amount of cylinders that your car has ( they usually have a switch marked 4/6/8)