Komatsu PC120-6 Hydraulic Power Loss After Warm-Up

[MikePhua]

The PC120-6 and Its Hydraulic System Design
The Komatsu PC120-6 hydraulic excavator, introduced in the mid-1990s, was part of Komatsu’s sixth-generation lineup aimed at balancing fuel efficiency, hydraulic precision, and operator comfort. With an operating weight of approximately 12 metric tons and powered by a Komatsu S4D102E diesel engine, the PC120-6 was widely adopted for utility excavation, trenching, and site preparation. Its hydraulic system features a load-sensing main pump, pilot-operated control valves, and electronic pump control via solenoids and pressure sensors.
The machine’s load-sensing system is designed to maintain a constant differential pressure—typically around 300 psi—above the working pressure required for each function. This ensures smooth operation and efficient power distribution across boom, arm, bucket, and travel circuits.
Symptoms of Power Loss After Warm-Up
A recurring issue with aging PC120-6 units is a noticeable drop in hydraulic power after 30–45 minutes of operation. The boom lift becomes sluggish, and other functions slow down, though the engine continues to run smoothly without bogging. This behavior suggests a hydraulic fault rather than an engine or fuel system issue.
Key symptoms include:

• Boom lift weakness under load
  
• General slowdown of hydraulic functions
  
• No fault codes displayed on the monitor
  
• Normal engine RPM and response
  

These symptoms point toward internal hydraulic bypassing or pump control malfunction once the system reaches operating temperature.
Potential Causes and Diagnostic Path
Before assuming pump failure, several components should be inspected:

• Hydraulic fluid condition: Ensure the oil is clean and of correct viscosity. SAE 10W is standard, but in high-temperature environments, a thicker oil like SAE 30 may reduce bypassing.
  
• Suction strainer and filters: Contamination or clogging can restrict flow and cause cavitation.
  
• Pilot pump output: Low pilot pressure can affect valve actuation and pump control logic.
  
• Load-sensing relief cartridge: Located on the main control valve, this sets the upper pressure limit. If faulty, it may prematurely dump pressure.
  
• Pump solenoid control: Disconnecting the solenoid harness forces the pump to full stroke, bypassing electronic modulation. If performance improves, the issue may lie in the controller or sensor feedback.
  

One technician noted that broken paint around the pump’s adjusting screws may indicate prior tampering. Another suggested checking for metal debris in the suction strainer before committing to a pump rebuild.
Flow Testing and Dealer Limitations
Flow testing the hydraulic pump is essential before replacement. However, not all dealers perform this test accurately. A proper flow test should measure:

• Pump output at full stroke
  
• Pressure differential across functions
  
• Pilot pressure stability
  
• Temperature-related performance drop
  

If no local mechanic is available, consider a service call from a dealer rather than trailering the machine long distances. A knowledgeable technician can assess pump stroke, valve behavior, and sensor feedback on-site.
Recommendations Before Replacing the Pump

• Inspect and clean all filters and strainers
  
• Check pilot pressure and reducing valve output
  
• Test the LS relief cartridge and main control valve
  
• Disconnect pump solenoid to force full stroke
  
• Perform a flow test with temperature monitoring
  

Avoid “shotgunning” the pump—replacing it without confirming failure. A rebuilt pump may cost $6,000, but if the issue lies in control logic or valve behavior, the problem will persist.
Conclusion
Hydraulic power loss in the Komatsu PC120-6 after warm-up is often misdiagnosed as pump failure. With careful inspection of fluid condition, pilot pressure, relief cartridges, and solenoid control, the root cause can be identified without unnecessary expense. The PC120-6 remains a reliable excavator, and understanding its load-sensing hydraulic architecture is key to maintaining peak performance.