Diagnosing Hydraulic Slowness in the Komatsu PC35MR-2

[MikePhua]

The PC35MR-2 and Its Market Legacy
The Komatsu PC35MR-2 mini excavator, introduced in the mid-2000s, was part of Komatsu’s MR (Minimum Radius) series designed for tight urban and residential job sites. With an operating weight of approximately 3.5 metric tons and a compact tail swing, the PC35MR-2 offered a balance of maneuverability and hydraulic power. It was equipped with a 3-cylinder Komatsu 3D88E-5 diesel engine and a variable displacement axial piston pump, capable of delivering up to 3500 PSI under load.
Komatsu, founded in 1921, has long been a global leader in construction equipment. By the time the PC35MR-2 was released, Komatsu had already established a strong foothold in the compact equipment market, competing with brands like Kubota, Bobcat, and Caterpillar. The PC35MR-2 sold widely across North America, Asia, and Europe, with thousands of units deployed in landscaping, utility trenching, and small-scale demolition.
Symptoms of Hydraulic Weakness and Flow Loss
Owners of the PC35MR-2 have reported persistent issues with slow hydraulic functions, particularly when attempting to operate multiple functions simultaneously. While single operations such as boom lift or bucket curl may perform adequately, combining movements—like tracking while swinging or digging—results in dramatic performance loss.
Terminology annotation:

• Hydraulic pump: A device that converts mechanical energy into hydraulic energy by pressurizing fluid.
  
• Swash plate: A component in variable displacement pumps that adjusts the stroke of pistons to regulate flow.
  
• Load-sensing (LS) valve: A valve that monitors system pressure and adjusts pump output based on demand.
  
• PC valve: Pressure compensation valve that balances flow between functions to prevent overload.
  

In one case, a pressure test revealed that the system could achieve 3500 PSI when the tracks were blocked, indicating that peak pressure was available. However, under dynamic conditions—such as driving and operating the boom simultaneously—pressure dropped and functions slowed. This pointed to a possible issue with the swash plate not adjusting properly or a malfunction in the LS or PC valve.
Pump Behavior and Swash Plate Control
The PC35MR-2 uses a variable displacement pump, meaning the swash plate angle determines how much fluid is delivered. If the swash plate fails to respond to load demands, the pump may remain in a low-flow state even when high output is needed. This can result from:

• Mechanical binding of the swash plate linkage
  
• Faulty pilot pressure signals from the control valves
  
• Internal wear or contamination in the pump body
  
• Electrical issues in solenoid-controlled displacement systems (if equipped)
  

Recommendations:

• Inspect the swash plate actuator for free movement and proper response to pilot pressure.
  
• Clean or replace the LS valve and PC valve to ensure accurate pressure modulation.
  
• Flush the hydraulic system and replace filters to eliminate contamination that may affect valve operation.
  
• Use a flow meter to measure actual pump output under various load conditions.
  

Gear Pump and Pilot Circuit Considerations
In some cases, the root cause of sluggish hydraulics lies in the gear pump responsible for supplying pilot pressure. If the gear pump fails to deliver adequate pressure, the control valves may not actuate fully, leading to weak or delayed responses.
One operator reported replacing the gear pump and observing improved performance, though the machine still lacked the responsiveness of newer models. This suggests that while the gear pump was part of the problem, other components—such as worn spool valves or aging seals—may also contribute.
Terminology annotation:

• Pilot pressure: Low-pressure hydraulic signal used to control main valve actuation.
  
• Spool valve: A sliding valve element that directs fluid flow within a hydraulic manifold.
  
• Hydraulic fluid viscosity: A measure of fluid thickness, which affects flow and pressure transmission.
  

To optimize pilot circuit performance:

• Use hydraulic fluid with the correct viscosity rating for ambient temperature conditions.
  
• Replace pilot filters and inspect pilot lines for kinks or leaks.
  
• Test pilot pressure at key control points using a calibrated gauge.
  

Heat-Related Performance Decline
Several users noted that hydraulic performance deteriorates as the machine warms up, especially on hot days. This is a classic symptom of fluid thinning and internal leakage. As hydraulic fluid heats, its viscosity decreases, reducing sealing effectiveness and increasing bypass flow within valves and cylinders.
Solutions:

• Upgrade to high-temperature hydraulic fluid with better thermal stability.
  
• Install an auxiliary cooler or clean the existing radiator and oil cooler to improve heat dissipation.
  
• Inspect cylinder seals and valve bodies for signs of wear that worsen with temperature.
  

Field Anecdotes and Operator Workarounds
One operator in Hawaii discovered that curling the bucket slightly while tracking improved left-side movement. This workaround suggests that engaging a secondary function may boost pilot pressure or redirect flow through a more responsive valve path. While not a permanent fix, it highlights the complexity of hydraulic interactions in multi-function systems.
Another technician in Connecticut noted that his machine ran flawlessly when cold but lost power as it warmed up. After replacing all filters and fluid, the issue persisted, leading him to suspect internal pump wear. He emphasized the importance of diagnosing with both pressure and flow tests, as pressure alone can be misleading.
Preventative Maintenance and Long-Term Reliability
To maintain optimal hydraulic performance in the PC35MR-2:

• Change hydraulic fluid every 1,000 hours or annually, whichever comes first.
  
• Replace all filters at regular intervals, including pilot, return, and suction filters.
  
• Monitor system pressure during operation, not just at idle or static conditions.
  
• Keep the cooling system clean and ensure fans are operating at full speed.
  

For machines with over 5,000 hours, consider rebuilding the main pump or replacing key valve components. While costly, these repairs can restore full functionality and extend machine life.
Conclusion
The Komatsu PC35MR-2’s hydraulic sluggishness under load is often a multifactorial issue involving pump displacement control, pilot pressure integrity, and thermal degradation. Through systematic testing, component inspection, and fluid management, operators can identify root causes and restore performance. In the world of compact excavators, understanding the interplay between pressure, flow, and control is key to keeping machines productive and responsive—even after years of hard service.