Armature Coil Test

If the surface is dirty or burned, correct it with sandpaper (No.400) or a lathe.
Using an ohmmeter check that there is continuity between the segments of the commutator. See Fig 1. If any of the segments do not have continuity between them, replace the armature.
Check the commutator for ground. Using an ohmmeter, check that there is no continuity between the commutator and armature coil core. See Fig 1. If there is continuity, replace the armature.
Using vernier calipers, measure the commutator length. See Fig 2. If the length is greater than maximum, replace the armature.

Fig 1: Testing Commutator Ground & Continuity Between Segments

Fig 2: Measuring Commutator Length