Rebuilt mine, but after a thorough clean up (bearings, field coils etc) and turning down the armature, just had to replace the brushes. Did a bench test by using a heavy duty set of jumper cables. Here's how: Attach your negative cable (-black in my case) to the negative post on a charged 12 volt battery (6 volt battery if it's a 6 volt generator) and clip the other end of that cable on the the generator case. I used the mounting points on the halftrack generator. Attach the positive jumper cable (+red in my case) to the positive battery post and touch the other end to the armature post on the generator for a few seconds. If your generator is rebuilt correctly it will spin like a motor and you've now bench tested your generator. On the case of my halftrack generator, I have the original shields in place, so I used a heavy duty screwdriver clipped into the positive cable clamp and touched the tip of the screwdriver on the armature bolt. DON'T HIT THE SHEILD WITH THE TIP, BECAUSE IF YOU DO, YOU'LL BE WELDING!!
