You have an intermittent problem so the testing has to be done while the problem is occurring. Brushes in the alternator are the most common source of trouble, and can be replaced separately for 9 bucks, but I would never do that without performing some tests first. When the problem is occurring, measure the resistance between the two smaller wires on the back of the alternator. If you find an open circuit, the brushes are worn.
Monday, October 22nd, 2012 AT 10:23 PM