A manufacturing plant reported that a Linde E25 electric forklift took 10 seconds to lift a 2-ton load (normally 4 seconds). When trying to lift, the hydraulic pump motor made a loud straining noise, and smoke poured out of the motor housing. The controller displayed a "Motor Overcurrent" fault.
The technician first removed the motor housing cover. The carbon brushes were severely worn, and the commutator was burnt and pitted. He suspected the motor was failing, but before replacing it, he checked the hydraulic circuit. He connected a pressure gauge to the lift pump outlet. When the operator commanded a lift, the pressure spiked instantly to 220 bar (relief pressure) before the forks even moved, indicating a severe blockage in the lift circuit.
He traced the pressure line to the "Lift Cylinder Counterbalance Valve" (a valve mounted directly on the cylinder that prevents the load from free-falling). He removed the counterbalance valve. The internal pilot poppet was seized shut by a piece of shredded O-ring material. Because the valve was jammed closed, the pump could not push oil into the lift cylinder. The pump was dead-headed against the relief valve, generating maximum hydraulic pressure.
The hydraulic pump is driven directly by the electric motor. Dead-heading the pump caused the motor to pull maximum current (400A). The controller's current limit tried to protect the motor by pulsing the power, which caused the violent straining noise. The continuous high-current draw burned the carbon brushes and scorched the commutator.
The repair involved replacing the jammed lift cylinder counterbalance valve, flushing the hydraulic tank to remove O-ring debris, and replacing the hydraulic filter. The pump motor was replaced with a remanufactured unit (the commutator damage was too severe to turn on a lathe). The forks then lifted a full load smoothly in 4 seconds with no overcurrent faults.
