708-3S-00961 708-3S-01850 Excavator Hydraulic Pump , Komatsu PC50 PC55 PC56-7 Main Piston Pump

The main pump for Komatsu PC56-7, part No. 708-3S-00961, is a variable displacement piston pump integrated with a tandem pilot gear pump, equipped with PC-EPC electronic flow control solenoid valve. Primary failure causes include contaminated hydraulic fluid, suction-side air ingress, defective PC-EPC solenoid valve, and seized regulator spool.

Normal operation at no-load; pressure drops rapidly under loaded conditions, and movements become slower when hot.

Internal leakage induced by worn valve plates and piston shoes; faulty PC-EPC solenoid valve; seized regulator spool and fatigued springs.

Blocked suction filter, air leakage on suction pipeline, metallic particulates in hydraulic fluid.

Excessive pump case drain flow indicates internal pump wear.

• High-pitched squeal (Cavitation): Inadequate suction or air entrainment in system; milky and foamy hydraulic fluid.
• Metallic tapping noise: Damaged internal bearing and ball joint, misaligned coupling.

Malfunctioning PC-EPC solenoid valve or seized power control spool. The pump remains at large displacement, and power consumption exceeds the engine rated output.

Cut off power supply to PC-EPC solenoid. If lugging disappears, the fault lies in solenoid or wiring circuit.

Fluid temperature rises rapidly after continuous operation, and actuator movement slows down gradually.

Severe internal pump leakage; blocked case drain circuit leading to elevated case pressure.

Degraded seal; high case pressure; seal lip scratched by misaligned coupling.

A damaged seal draws in air, inducing cavitation and accelerating pump deterioration.

Air trapped in hydraulic circuit; unstable regulator response; localized scoring on valve plate.

Unstable pilot pressure, abnormal main pump displacement modulation and weak joystick control.

Worn pilot gear, seal leakage and internal fluid bypass.

1. Inspect hydraulic fluid condition, fluid level, suction filter and suction hose
2. Measure maximum main pump pressure and pilot pressure, test PC-EPC solenoid output signal; cross-reference with service manual specifications
3. Take pump case drain flow readings
4. Disassemble and clean power regulator, inspect springs and spool; test PC-EPC solenoid and wiring
5. Verify coupling concentricity

• Minor wear: Lap valve plate, renew pistons, piston shoes, ball joint and full seal kit; clean regulator, repair or replace PC-EPC solenoid valve.
• Severe scoring: Fit new or remanufactured complete pump assembly.
• Post-repair: Replace all hydraulic filters and thoroughly clean the hydraulic tank.