Is it possible for an alternator to test bad on the car after it's been running for a while and then test good after it cools down and taken off the car? My battery/charge light came on so I had the battery tested and the guy said the diodes in the alternator were bad. Took it off to get a new one and had it tested again to be on the safe side and it tested good... WTF? Can anyone shed some light on this? Thanks.

 

yes. although the diode going bad will also cause a drain when the car is off. this is why it is dumb to remove the alt to test it. just put a voltmeter on it and see what the reading is.

 

Well, that is what happened. I had to replace the battery too. I let the car set for a couple of days and before removing the alternator, I tried to start it but the battery was drained.

 

Any other opinions?

 

replace the altenator.... stop pulling your hair out.

If you replaced the battery and then battery ends up dying. And the altenator tested bad once. The next logical step is to replace the altenator. Its a 10 min job.

 

Just a lesson I've learned with many beaters i've owned while trying to buy my 7 is easier to replace battery and alt. together. Not the cheapest route, but avoids later headaches IMO.

 

Yes, I agree that you should replace the alternator. The reason it would have tested good when you took it off the car is because you had no load on it.
When it was in the car, you tested with the load it would normally sustain, and it was straining to keep up.