I was going to post a very similar message as rustednuts did just above. I owned many cars with generators, and they are every bit as good as an alternator, but a generator becomes marginal if a person is running an air conditioner or other high load devices. The first thing to do, would be to disconnect the battery ground cable, remove the voltage regulator, remove the cover, and, with a diamond file carefully clean the points. This will solve tho problem in majority of situations. Even though the voltage regulator is sealed, the points will oxidize if there is enough ambient moisture. I probably still have a manual with the testing procedures if you need to go further. A lot of generators have a brush cover which can be removed to check the condition of the brushes.
Bob
