A beverage distributor reported a perplexing issue with a Crown RC 5500 stand-up counterbalance forklift. When the operator attempted to lift a heavy pallet of liquid while simultaneously traveling in reverse, the lift pump would momentarily cut out, dropping the load two inches before catching again. The truck operated perfectly when lifting while stationary. This specific set of symptoms pointed toward a systemic electrical overload rather than a simple hydraulic failure.
The Crown RC 5500 uses a traction motor for drive and a separate pump motor for lift and tilt, both powered by a 36-volt lead-acid battery bank. When traveling and lifting simultaneously, the current draw from both motors can exceed 600 amps. The technician first performed a battery load test using a carbon pile. The battery voltage dropped from 37.2V to 31.5V under a 600-amp load, which was slightly low but within acceptable limits for an older battery pack. The real clue came when monitoring the system voltage at the control panel under the operator seat while replicating the fault. During the combined travel-and-lift maneuver, the voltage at the panel plummeted to 24 volts-far below the battery's terminal voltage-causing the EV100 traction controller to initiate a low-voltage lockout and kill the pump contactor.
This massive voltage drop between the battery and the control panel indicated high resistance somewhere in the main power circuit. The technician bypassed the main power connectors to rule out burnt battery leads, but the drop persisted. Attention shifted to the main 1A contactor, which supplies power to the entire truck. Removing the arc chute cover revealed severely pitted and blackened contact tips. The contactor had been arcing under heavy load, creating a high-resistance barrier. When both motors demanded peak current, the resistance across the pitted contacts caused the voltage to crash.
However, the contactor pitting was a symptom of another underlying issue. A closer inspection of the truck's service history showed the traction motor had never been serviced. Removing the traction motor side cover revealed that the carbon brushes were worn down to their limiters, and heavy arcing had damaged the commutator bars. When the worn brushes made poor contact with the commutator, especially under the high torque of reversing, it created electrical noise and excessive current spikes. These spikes were causing the 1A contactor to chatter violently, which in turn accelerated the contact pitting.
The comprehensive repair involved replacing the main 1A contactor and installing a new set of traction motor brushes. The commutator was lightly stoned to clean the arcing residue, and the brush holders were cleaned to ensure free movement. Additionally, the main power cables were re-crimped to guarantee a solid connection. Upon reassembly, the combined travel-and-lift test was performed flawlessly; the voltage remained stable above 32V, and the hydraulic lift operated smoothly without a single dropout.
