A beverage distribution warehouse reported that a Still RX60-40 4-ton electric counterbalance forklift had developed a progressive lift-system fault over a two-month period. When new, the truck would lift a full 4-ton pallet load briskly to full mast height. Over the past two months, lift speed under heavy load had degraded noticeably - full-lift cycles that took 8 seconds when new now taking roughly 14 seconds. During lift commands under heavy load, the truck's lift-pump motor contactor would emit a rhythmic "chattering" sound - rapid clicking as the contactor repeatedly opened and closed - and the cab display would intermittently show a "Lift System Performance" fault code. The truck's battery discharge indicator also showed faster-than-normal charge depletion during shifts.
The technician first verified the battery's state of charge and condition - the battery was fully charged and showed no significant cell-imbalance issues on the diagnostic tool. He then connected the Still diagnostic tool and retrieved live data during a loaded lift test cycle. The diagnostic tool showed the lift pump motor drawing 195 amps during loaded lift commands - far above the normal 130-amp range for this truck - and the contactor's coil voltage showing intermittent drops during the chatter events.
He understood the RX60-40's lift architecture. The truck uses a high-pressure "Lift Pump" driven by a dedicated "Lift Pump Motor" - a DC electric motor powered through a "Lift Contactor" that closes to connect the motor to the battery's power circuit when the operator commands lift. The lift pump draws hydraulic oil from the reservoir through a "Suction Strainer" mounted at the reservoir's bottom - a coarse mesh strainer that prevents large debris from entering the pump. The pump then delivers pressurized oil to the lift cylinders through the truck's control valve. When the contactor's electrical contacts pit or corrode, the contactor's electrical resistance increases - reducing the voltage and current delivered to the motor, forcing the motor to draw more current for the same work, and producing the contactor's characteristic chattering as the magnetic coil struggles to hold the contactor closed against the higher current load. When the suction strainer becomes clogged, the pump cannot draw sufficient oil, reducing lift speed and forcing the motor to work harder against a partially starved pump.
He inspected the lift pump motor's contactor first. Opening the contactor's cover, he found the main electrical contacts visibly pitted - the copper contact surfaces showed deep pitting, arcing marks, and dark carbon deposits, with one contact showing a portion of the contact material burned away. The pitted contacts had increased electrical resistance across the contactor, explaining both the excessive current draw and the chattering the operator described. He then inspected the "Hydraulic Pump Suction Strainer." The strainer's mesh was heavily clogged with fine metallic particles, hydraulic oil oxidation residue, and small plastic fragments - restricting oil flow to the pump inlet and compounding the lift system's performance loss. He further inspected the hydraulic pump itself. The pump's internal components (gears, side plates, and housing bore) showed moderate wear consistent with the truck's hours but were within service limits.
The root cause was traced to lift-pump motor contactor pitting compounded by suction strainer clogging. The contactor's contacts had pitted gradually over extended service from repeated high-current inrush during lift commands (the truck had approximately 5,400 hours), and the pitted contacts had increased electrical resistance, forcing the motor to draw excessive current for the same work. The clogged suction strainer had compounded the fault by restricting oil flow to the pump, further elevating the motor's current draw and producing the progressive lift-speed loss the operator described.
The repair involved replacing the lift pump motor contactor with a new unit (pitted contacts are not field-repairable), cleaning the contactor's mounting connections and verifying all electrical terminals were torqued to specification, and replacing the hydraulic pump suction strainer with a new element. The hydraulic oil was replaced with fresh specified oil, and the lift circuit was bled to remove trapped air. After reassembly, a loaded lift test showed motor current restored to the 130-amp normal range, full-lift cycle time under heavy load restored to 8 seconds, the contactor chattering eliminated, and battery discharge returning to its normal rate across a full 8-hour shift. Lift contactor inspection and suction strainer replacement were added to the truck's 500-hour preventive maintenance checklist.
