I'd suggest a thorough check of the physical condition of the generator first. Remove it from the vehicle, tear it down, and inspect the armature and field coils for damage or wear, install new brushes, clean up the commutator with a strip of non-metallic sandpaper, and be sure all the connections are clean and tight. Then do the motor test on the workbench, positive polarity to the armature terminal and negative to the generator housing. The armature should spin at a pretty good rate of speed if everything is in good shape. All it will cost you is a few bucks for brushes and a little labor time.
Polarize the generator after you reinstall it and check for voltage at the armature terminal with the field terminal grounded with a jumper wire. You should see a quick rise in voltage as the speed is increased above idle.
Jerry
