A cold storage warehouse reported that a Linde E25 electric forklift's forks were stuck in the raised position and would not come down. The forklift also smelled like burnt electrical wiring.
The technician opened the "Controller Compartment." The "Lowering Contactor" (a heavy-duty relay that reverses current to the pump motor for lowering) had its contacts physically welded together by an electrical arc. He disconnected the battery and pried the contacts apart.
He then tested the "Dynamic Braking Resistor" (a large ceramic resistor used to absorb the regenerative energy during fork lowering). The resistor was burnt open (infinite resistance). The engineering logic: When lowering the forks, the pump motor acts as a generator. The generated current is dumped into the braking resistor to slow the descent. Because the resistor was burnt, the generated current had nowhere to go. This caused a massive voltage spike, which arced across the lowering contactor, welding it shut. The welded contactor kept the motor energized, but because the flow control valve was closed, the forks couldn't move.
The repair involved replacing the burnt dynamic braking resistor and the welded lowering contactor. The pump motor commutator was cleaned. The controller parameters were checked to ensure the "Lowering Current Limit" was set correctly to prevent future spikes. The forks then lowered smoothly and safely.
