Sometimes it's the little things that we overlook that can be a pain. Check the tension on the alternator belt and look for wear. I had this problem before, alternator was fine, battery was new but driving with the lights on, stereo, heater fan and cooling fan coming on at intervals it was enough to cause the belt to slip to the point of the alternator not charging. After bringing the alternator to a repair shop, the tech bench tested it and found it was okay but the pulley had some blue heat spots on it and there was some fine dust from the belt found on the fins of the alternator. He said that the belt was too loose and the load of all the electrical items drew off so much current that the alternator was actually not turning at one point. A new belt and the right tension solved the problem. He suggested if the belt wears out or the problem persists that maybe the pulleys could need changing. He was cool enough not to charge me for an unecessary rebuild or even his time to bench test it either.
Friday, October 30th, 2009 AT 7:20 PM