I think it is as simple as checking for cracks, breaks and exposed wires. If you are familiar with electrical testers and how to use them it would be very helpful. I have a couple we tested and sure enough there were broken wires somewhere in the system. I've been thinking about doing a tutorial on rebuilding the motors but my camera doesn't do very well on close shots. I have 4 motors to rebuild. When the motor growls and complains or runs smooth then quits, or runs unevenly I check with the continuity tester to see if the wires are working. If not I clean up the motors. Once I got a machine someone had oiled. That one smoked and smelled and groaned like crazy until we got it cleaned up.