Bad Alternator?

[chrismoore701]

I have a 1979 C10, I have the engine bay wired on my truck. I have a new painless wire harness hooked up. I have three (3) wires on the back of my alternator. I disconnected the battery while running and it died. I measured the voltage off the post while running and had just under 12 volts, i.e. battery voltage. I did paint the alternator case so I am wondering if I don’t have a good ground. I had it tested and they said it worked. I didn’t think to ask them what voltage I was getting, but it should be somewhere north of 13 volts correct?

Thanks
Chris