2 weeks ago my car died, i thought it was the battery so i got some jumper cables and boom there it turned on again. but then it started to slowly run out of electricity. and ive had the alternator checked out before and it said it was overcharging the battery often having spikes up to 18 volts then going down to 10 then back to 20. so while changing the alternator tonight i take the old one to kragen so they can help me take off the pulley, when the dude asks me if i want to test the alternator just in case, and i do, and it passes the test 3 times. and i just bought a new one before that so, someone tell me that its possible that the alternator can pass yet still be bad?

 

Some of the gm altenators have a stupid cercuit breaker on the inside. If the battery is almost completly dead when you put the altenator in and you don't fully recharge the battery before strarting the car this can cause the stupid thing to pop. I am not sure of how to fix it without opening it up. But this happened to the one on my camaro and the altenator still tested good at autozone even though it wasn't charging the battery on my car. If you get a new atenator I suggest fully chargeing the battery before starting the car with the new altenator

I hope this helps

 

I'd also suggest that you check the alternator pigtail and make sure there isn't any rust on the connectors or rotten wires. I was having the same problem with my battery slowly losing a charge and I replaced the alternator pigtail last week and so far I haven't had any dim lights or any other signs of a low charge.

 

Also to find out if the alternator is working jump it and and get it started and when you do unplug the positive cable to the battery if it dies the alternator is dead do not turn the lights on b/c it will kill it immediately and yes check the connectors on the alternator also if you don't want to disconnect the batt. just use a piece of metal that a magnet can attract and tap the back of the alternator if it sticks it a good alternator. the problem could be a bad battery or a short somewhere.