Observe whether sparks occur between the carbon brush and the commutator when the motor is running and the degree of sparks.

1. No sparks, indicating that the carbon brush and commutator are normal and need not be repaired.

2. There are only 2-4 minimal sparks. If the surface of the commutator is flat, it is not necessary to repair it in most cases.

3. There are four minimal sparks and one to three large sparks, so the armature need not be dismantled, and the carbon brush commutator need only be ground with sandpaper.

4. If there are four large sparks, it is necessary to grind the commutator with sandpaper, or even carefully add the commutator to the car. At this time, the carbon brush and armature must be removed. Of course, after a huge commutator is processed, it is necessary to change the carbon brush to grind the carbon brush.