My 1998 Lincoln Town Car has decided to turn on its little battery warning light on the dash, which I assume would mean that the battery is not charging. The alternator is about 3 years old (purchased at an auto parts store by the previous owner). I removed it from the vehicle and took it to Napa and they said it tests good. I called my usual mechanic and he said it could still test good and have an internal diode burned out.
I measured the battery voltage when the car was off and it read about 13v. I started it and it actually went down to about 12v with just a little bit of meter shaking.
I also removed the thicker (single) wire from the top of the alternator, started the car, and read the voltage at this point on the alternator. I used the alternator metal case as ground. I read about 8 volts and, again the meter was shaking (a bit more), almost like there was some AC in there.
So before I buy a non-refundable alternator and install it, I'm just wondering if it can test good off the car and still be bad?