• Step by step guide and via the process of elimination: Cleaning :
  1. First clean the com bars with 1000 sand paper and also the narrow grooves between bars using a blade. Then use a toothbrush to clean any remaining carbon deposit between the bars.
  2. Check the carbon brushes, if they are short, have chips or damaged, replace them with a new set. Also, inspect the comm bars for any damaged or out of the place bar.

* If the above didn’t help, you most likely have a shorted armature (sometimes field coils). To check the armature, first inspect it visually, if you see a burnt spot on wire coils or the spot where a wire is attached to comm bars, that armature is gone. If you don’t see any burnt spots, then you can investigate further using a multi-meter. With the multi-meter set on low ohm, check the resistance of each pair of the adjacent comm bars, the values must be very close, any deviation more than 0.1 ohm is bad. Next measure each pair that are at 180 deg of each other, the values must be close, any large difference points to a problem. You also need to check for a short between comm bars and body & shaft of the armature. There must not be any continuity.

• If the armature passed the test, the next step is the field coils. You have a pair of wire coils, check their resistance and inspect them for any burnt spot. As before, their resistance must be very close.