A manufacturing facility reported that a Clark ECG25 48-volt electric forklift would refuse to lift whenever a load exceeded 2,000 pounds. The drive function worked fine, but when the operator pulled the hoist lever with a heavy pallet, the hydraulic pump motor would groan, the dashboard display would flicker and reset, and the lift would abort.
The technician connected a laptop to the Curtis motor controller to pull the fault history. The log repeatedly showed a "Controller Undervoltage" fault, meaning the voltage at the controller's power terminals was dropping below the safe operating threshold the moment the high-current hydraulic pump motor engaged.
To find the voltage drop, the technician connected a multimeter directly across the controller's B+ and B- bus bars. With the machine sitting idle, the voltage was a healthy 49.5V. When the hoist lever was engaged with a heavy load, the voltage plummeted to 28V, triggering the reset.
The technician bypassed the main power cables by running heavy-duty jumper cables directly from the battery terminals to the controller bus bars. The mast lifted the 4,000-pound load flawlessly, proving the controller and pump motor were fine, and the issue was high resistance in the main power cables or contactors.
Inspection of the battery connector revealed a frayed negative cable. The copper strands at the battery terminal lug had corroded and broken away, leaving only a fraction of the wire conducting current. Furthermore, the main hoist contactor (the heavy-duty relay that supplies power to the pump motor) was disassembled. The contact tips were severely pitted and covered in black carbon tracking from years of arcing. The high resistance from the frayed cable caused a voltage drop, which prevented the contactor coil from pulling the tips together with sufficient force, leading to further arcing and an inability to pass the massive current required to run the pump under load.
The repair involved cutting back the frayed negative cable, crimping on a new, tinned copper lug, and coating it with antioxidant compound. The main hoist contactor was replaced with a new unit. After reassembly, the forklift lifted maximum capacity loads smoothly, and the voltage at the controller only dropped to 46V under maximum hydraulic load, completely resolving the controller lockouts.
