The alternator has to be out of the car for that, and they cannot test for certain problems with it. That, and they're morons. I spent WAY more time than I would have liked to arguing with a guy at Murrays taht an alternator was bad, when his "machine" disagreed (wouldn't warranty it). Finally I had to practically educate him on how the damn thing works, and after going over my diagnosis step by step I think he may have been overwhelmed by big words and stuff that he gave up. Trust me paco, I did WAY more than load test the damn thing before calling it a bad alternator.