Hydraulically actuated aerial lift trucks typically have a primary
hydraulic pump system driven by the vehicle's engine and a backup pump system driven
by a direct current motor. Due to infrequent use of such motors, they are vulnerable
to failure due to corrosion of the brush/commutator interface or seizing of the
motor bearings. A remote power unit is provided to periodically run the back up
motor for brief periods to maintain the motor in running condition. Voltage levels
across the motor terminals are monitored for correspondence to failure indicating levels.
