A mining contractor reported that a Komatsu PC360LC-11 excavator's engine RPM dropped drastically (from 2000 to 1400 RPM) whenever the operator engaged a heavy digging cycle. Simultaneously, the main hydraulic pump emitted a high-pitched whining noise. Light-duty operation was normal, but any heavy bucket load caused the engine to nearly stall.
The technician initially suspected an engine fuel issue, but the engine ran perfectly under no-load conditions. He connected a pressure gauge to the main pump outlet. When the operator attempted a heavy dig, the pump pressure spiked instantly to 350 bar (main relief setting), and the engine lugged down. He checked the PC (Pump Control) solenoid current, which was normal.
He knew that in a closed-center load-sensing hydraulic system, the pump regulator relies on a small pilot pressure signal from the control valve to destroke the pump when the control valve closes. He removed the "Pilot Filter" element located near the hydraulic tank. The filter had completely disintegrated, and the pleated paper had turned into a pulpy mush. Pieces of this paper were flushed into the load-sensing (LS) circuit.
He disassembled the "LS Relief Valve" on the main pump regulator. The valve spool was jammed open by a chunk of filter paper. Because the LS valve was stuck open, the pump regulator was receiving a false "maximum demand" signal. The pump was locked at maximum displacement (swashplate at full angle) regardless of the operator's lever movement. When the operator commanded a heavy dig, the bucket cylinder moved slowly, but the pump was forcing maximum flow against the closed-center valve. This massive internal bypass at 350 bar created extreme mechanical resistance on the pump, which lugged the engine down and generated the high-pitched whine (fluid escaping through the relief orifices at the speed of sound).
The repair involved replacing the pilot filter element and the main return filter. The LS relief valve spool was removed, and the valve bore was cleaned with lint-free wipes and brake cleaner to remove all paper pulp. The pump regulator housing was flushed. Fresh ISO 68 oil was added, and the system was bled. Post-repair, the pump destroked normally at idle, and the engine maintained 1950 RPM during heavy digging cycles with no whining noise.
