2001 Ford Expedition Alternator

We can't tell that without doing some tests. You have to catch it when the problem is acting up, and try to keep it that way.

Ford finally came up with a generator design that was fairly easy to diagnose, and I think yours is one of the last years before they saw their mistake and made it harder to work on. If you look on the back of it, you'll see the voltage regulator bolted on with four screw in a rectangular pattern, and two more screws along the bottom. As you face the rear of the generator, of those two screws on the bottom of the regulator, the right one might still have a plastic cap over it. The left one says, "Ground here to test". With the engine running, take a voltage measurement on that test screw and on the large output stud. If the cap is missing from the smaller right screw, take a measurement there too. These have to be taken when the problem is occurring. Post those readings here.