A high-density distribution center reported a bizarre safety issue on a Crown FC4500 48-volt electric forklift. Whenever the operator raised the forks to pick a pallet from the top rack, the drive function would completely cut out. The operator would have to lower the mast, cycle the key, and then drive. The hydraulic lift function worked perfectly, but the truck refused to travel while elevated.
The Crown FC4500 uses a Curtis drive controller that monitors various safety interlocks, including mast height. The technician connected a laptop to the controller. The fault log showed a "Main Contactor Dropout" and an intermittent "Mast Height Sensor Out of Range" fault occurring simultaneously.
The technician first investigated the main 1A contactor (the high-current relay that connects the battery to the drive controller). The contactor was removed, and the contact tips were severely pitted and burnt. When the mast was raised, the hydraulic pump motor drew high current. This high draw caused a voltage drop across the damaged 1A contactor tips. The voltage drop was severe enough that the 1A contactor's holding coil lost power, causing the contacts to chatter and physically open the drive circuit, triggering the dropout fault.
However, the "Mast Height Sensor" fault pointed to an underlying issue. The FC4500 uses a string potentiometer on the mast to track fork height and enforce speed limits. The technician measured the resistance of the potentiometer while slowly extending the mast. At about 75% of the mast height, the resistance jumped erratically and shorted to ground.
When the mast reached the top rack position, the shorted potentiometer sent a massive voltage spike back into the Curtis controller's 5-volt reference circuit. To protect itself, the controller momentarily shut down, causing the 1A contactor to drop out.
The repair involved replacing the burnt 1A contactor with a new heavy-duty unit and installing a new mast height string potentiometer. The controller's 5V reference circuit was tested to ensure it wasn't permanently damaged. After recalibrating the new height sensor, the forklift operated flawlessly, driving smoothly at the correct restricted speeds with the mast elevated, completely free of drive cutouts.
