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 attract to the core. See Fig 1 . Replace the shorted armature.
Check the insulation between the armature coil cores and the commutator segments. See Fig 2 . They are normal if there is no continuity.
Check for continuity between the segments. See Fig 3 . The condition is normal if there is continuity.

Fig 1: Placing Armature In Growler

Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.

Fig 2: Checking Armature Coil Ground Test

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Fig 3: Checking Armature Coil Open Circuit

Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.