Diagnosing Hydraulic Failure After Pump Replacement on a John Deere 120C Excavator

[MikePhua]

The John Deere–Hitachi 120C and Its Dual-Pump System
The John Deere 120C excavator, built in collaboration with Hitachi, features a dual swash plate axial piston pump system—specifically the HPK055AT model. This configuration allows independent control of hydraulic flow to different circuits, improving efficiency and responsiveness. The machine’s hydraulic architecture relies on synchronized operation between Pump 1 and Pump 2, with pilot pressure and solenoid modulation governing swash plate actuation.
When one pump fails, especially catastrophically, the consequences ripple through the entire hydraulic system. In this case, Pump 2 suffered piston disintegration due to low hydraulic oil levels caused by a leak. The pump was replaced with a rebuilt unit, but afterward, the machine exhibited complete loss of hydraulic function.
Terminology Annotation
- Swash plate: An angled plate inside an axial piston pump that controls piston stroke and thus fluid displacement.
- Pilot pump: A smaller pump that supplies low-pressure hydraulic fluid to control valves and actuators.
- Speed sensing solenoid: An electrohydraulic valve that adjusts pump output based on engine speed and load demand.
- Main control valve: The central hydraulic manifold that distributes flow to various actuators like boom, arm, and travel motors.
- System contamination: The presence of debris or metal fragments in hydraulic fluid, often resulting from internal component failure.
Initial Troubleshooting and Pressure Verification
After installing the rebuilt pump, the operator verified pilot pressure at approximately 725 psi—within normal range for control actuation. Speed sensing solenoids were swapped to rule out electrical faults, but no change occurred. The tracks, which typically operate on basic flow, remained inactive, suggesting a deeper issue than solenoid miscommunication.
This points toward a mechanical failure in the rebuilt pump—most likely the swash plates not actuating. If both pumps are stuck in neutral, no displacement occurs, and hydraulic flow ceases. While rare, simultaneous failure of both swash plates could result from incorrect assembly, blocked internal passages, or improper break-in procedures.
Contamination and System Cleanup Protocols
The original pump failure introduced metal debris into the hydraulic system. Although filters and fluid were replaced, this may not have been sufficient. Without thorough flushing and inspection of the main control valve, residual shrapnel can block critical orifices, jam spools, or damage seals.
Recommended cleanup steps after catastrophic pump failure:
• Drain and flush all hydraulic lines and cylinders
• Remove and inspect the main control valve for debris
• Replace all inline filters and strainers
• Use magnetic probes to detect ferrous particles in the reservoir
• Pressure test each circuit individually before reassembly
In one documented case, a similar pump failure on a Hitachi ZX120 resulted in debris lodging inside the travel spool, causing intermittent movement and eventual total failure. Only after disassembling the valve block and manually cleaning each passage did the system recover.
Pump Rebuild Quality and Vendor Reliability
Not all rebuilt pumps are created equal. If the replacement unit was sourced from a non-OEM vendor, tolerances, seal quality, and swash plate calibration may vary. Some rebuilders reuse worn components or fail to test under load. A pump that appears functional on the bench may not deliver flow under pressure.
To verify pump integrity:
• Bench test the pump with a hydraulic test stand before installation
• Confirm swash plate movement via inspection ports or actuator feedback
• Check for audible cavitation or abnormal heat during startup
• Monitor case drain flow—excessive leakage indicates internal bypass or wear
If the pump fails these tests, replacement or warranty claim is necessary. In high-risk applications, OEM remanufactured units are preferred over third-party rebuilds.
Electrical and Control System Considerations
While mechanical failure is likely, electrical faults can’t be ruled out. The John Deere 120C uses a combination of analog and digital signals to modulate pump output. A failed controller, broken wire, or misconfigured sensor can prevent swash plate actuation.
Checklist:
• Verify voltage at speed sensing solenoids during startup
• Inspect harness connectors for corrosion or pin damage
• Use diagnostic software to check for fault codes or inactive modules
• Confirm that engine RPM signal is reaching the hydraulic controller
In one instance, a faulty engine speed sensor caused the pump to remain in standby mode, disabling hydraulic output despite full mechanical readiness.
Conclusion
Loss of hydraulic function after pump replacement on a John Deere 120C excavator is most likely due to internal failure of the rebuilt pump or residual contamination from the original failure. Proper system flushing, valve inspection, and pump testing are essential before reassembly. Electrical diagnostics should also be performed to rule out control signal interruption. In hydraulic systems, a single oversight—whether mechanical or electronic—can silence an entire machine. Thoroughness, not haste, is the key to restoration.