Armature Check

Check that the armature coil is not grounded.
Place the armature in a growler.
Hold a thin steel blade parallel and just above the armature while slowly rotating in the growler. A shorted armature will cause a blade to vibrate and be attracted to the core. Replace the shorted armature.

Fig 1: Holding Thin Steel Blade Parallel And Just Above Armature While Slowly Rotating In Growler
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
Check the insulation between the armature coil cores and the commutator segments. They are normal if there is no continuity.

Fig 2: Checking Insulation Between Armature Coil Cores And Commutator Segments
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
Check for continuity between the segments. The condition is normal if there is continuity.

Fig 3: Checking Continuity Between Segments
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.