A logging yard reported that a Case 621G wheel loader's boom chattered violently when lifting a full load of timber, and the bucket had lost its rollback holding force. After 30 minutes of operation, the hydraulic oil temperature spiked to 110°C, triggering a thermal derate. The engine and drivetrain operated normally under load.
The technician first checked the bucket relief pressure, which only reached 150 bar (spec is 250 bar). He then checked the boom raise pressure, which fluctuated wildly between 100 bar and 220 bar when the lever was released, synchronizing with the physical boom chatter. He removed the main control valve and isolated the "Boom Lock Valve," a pilot-operated check valve that traps oil in the boom cylinder to prevent drift. Its internal sleeve and guide piston were deeply scored with circumferential scratches. Metal debris circulating in the hydraulic oil had wedged into the valve bore, scoring the sleeve.
Because the worn lock valve could not maintain a seal, high-pressure oil from the boom raise chamber was leaking back to the tank during the lift. This continuous leakage meant the main pump constantly tried to compensate, dead-heading the pump at full flow and generating massive heat. The boom chatter occurred because the oil leakage caused the boom to drop until a pressure switch caught the drop and instantly re-fired the pump to raise it again, creating a rapid hydraulic oscillation. The bucket weakness was a secondary symptom: the overheated, thin oil was internally bypassing the bucket control valve spools.
The repair involved replacing the worn boom lock valve cartridge and honing the main control valve body bore to remove the scoring. The hydraulic tank was drained, flushed with a cleaning cart to remove metal debris, and refilled with fresh ISO 68 anti-wear hydraulic oil and new return filters. After reassembly, the boom lifted smoothly, the bucket restored full holding force, and the oil temperature stabilized at 82°C under heavy load.
