I installed an aftermarket voltmeter this weekend on my '73 Mach 1, 351C and upon starting the car, the needle is jumping all over the place. In accessory position (car not running), it shows a constant 12V, and at higher rpms, it smooths out at 13/14V. Any help on this one?

 

Are you confident you've got a solid ground connection?

 

Sounds like the voltage regulator for the alternator may be going bad. I would start by using a multimeter at the bat and see if it's bouncing there and if not check your connections to your volt meter in the car, you may have it wired wrong.