A beverage distribution warehouse reported that a Crown FC 5200 series 48-volt electric counterbalance forklift developed intermittent jerky travel. When the operator accelerated with a 2-ton pallet load, the truck would surge forward, momentarily lose power, then surge again in a jerky, stuttering fashion. The fault was worse under heavy load and less noticeable when traveling unloaded. The hydraulic lift function operated normally.
The technician first connected the Crown InfoLink diagnostic tool and retrieved a "Traction Motor Phase Loss" fault code with a "Traction Controller Thermal Warning" sub-code. He installed a clamp-on DC ammeter on the traction motor's three output phases and ran the truck under varying load conditions. The ammeter showed intermittent phase-current dropouts on the U-phase-every few seconds, the U-phase current would drop to zero for 50 to 100 milliseconds before recovering, while the V and W phases continued conducting normally.
He then inspected the "Traction Motor Encoder Coupling"-a small, precision-machined coupling that connects the traction motor's rear shaft to the motor's angular position encoder. The encoder provides real-time rotor-position feedback to the controller, which uses this information to time the three-phase output precisely. Upon inspection, the encoder coupling had a cracked rubber insert, allowing the encoder to intermittently slip relative to the motor shaft. When the encoder slipped, the controller received an incorrect rotor-position signal, briefly commanding the wrong phase timing-producing the momentary power loss and jerk the operator felt.
However, the technician suspected a secondary fault, because a slightly slipping encoder would typically produce a smooth but reduced torque output rather than the sharp jerks the operator described. He further inspected the "Traction Controller's Internal Capacitor Bank." On the FC 5200, the controller uses a bank of DC-link electrolytic capacitors to smooth the DC bus voltage and supply the high peak currents required for AC traction. Upon opening the controller's access panel, he found one of the six DC-link capacitors had visibly bulged and was venting electrolyte-indicating the capacitor had failed internally.
The failed capacitor reduced the DC bus's ability to supply peak current during hard acceleration, causing the DC bus voltage to sag under high load demand. The combination of the sagging DC bus and the slipping encoder produced the pronounced jerky travel under load. The controller's thermal warning was the result of the failing capacitor's elevated internal heat generation.
The repair involved replacing the complete traction controller (since the internal capacitor bank is not field-repairable) and replacing the traction motor's encoder coupling with a new unit. The DC bus capacitor bank's cooling fan was inspected and cleaned. The forklift's phase-current was re-tested after the replacement-stable across all three phases with no dropouts. After reassembly, the truck accelerated smoothly under full 2-ton load, the jerking and power loss disappeared, and the controller temperature stabilized within normal operating range.
