A beverage distributor reported that a Raymond 4150 stand-up counterbalance forklift would cut all drive power after climbing a steep ramp to the loading dock. The machine would sit dead for 5 minutes and then work fine until the next ramp climb.
The Raymond 4150 uses a DC traction motor and a transistorized controller. The technician drove the truck onto the ramp and monitored the motor temperature via the dashboard display. The motor temperature climbed rapidly from 60°C to 110°C-the thermal cutoff limit. The controller was cutting power to protect the motor from burning out.
The drive motor cover was removed. The carbon brushes were worn down to their minimum length limit. Worn brushes create poor electrical contact with the commutator, resulting in high resistance and excessive arcing (sparking) inside the motor. This arcing generates massive heat, drastically reducing motor efficiency.
Additionally, the transistor controller housing was packed with a thick layer of dust and paper debris. The controller uses a large aluminum heat sink to dissipate the heat generated by switching the high current to the motor. The clogged heat sink could not cool the transistors, causing them to overheat and trigger a separate "Controller Overtemp" fault.
The repair involved installing a new set of drive motor brushes and cleaning the commutator surface with a commutator stone to remove the carbon glazing. The controller heat sink was blown out with compressed air. The technician also adjusted the drive motor belt tension, as a loose belt was causing the motor to work harder than necessary to maintain speed. Post-repair, the forklift climbed the ramp continuously for 20 minutes without overheating, and the motor temperature stabilized at a safe 75°C.
