The motor runs erratically.

Turn the power off at the tester. Disconnect the tester power cord from the power source. Remove the motor brushes. The brushes are located at the bottom of the motor. Remove the two plastic caps with a screwdriver on the right and left had sides of the motor. Inspect for even wear and the brushes are dry. If the brushes show signs of oil, the motor shaft seal at the gearbox is leaking. The motor must be replaced. If the brushes are worn to or past the minimum brush length indicator marked on the brushes, replace the brushes.