Adjusting Hydraulic Output on a Komatsu PC120

[MikePhua]

Owners of older Komatsu PC120 excavators often feel that hydraulic performance gradually weakens with age. Machines from the mid‑1980s, especially those with more than 6,000 operating hours, may show slower cycle times, reduced digging force, or sluggish boom and arm response. These symptoms lead many operators to wonder whether the hydraulic pump can be “turned up” to restore lost power.
This article explains the realities behind hydraulic pump adjustment on the Komatsu PC120, the risks involved, the engineering behind the pump system, and the correct diagnostic path before attempting any adjustment.

Komatsu PC120 Background
The Komatsu PC120 series was introduced in the early 1980s as a mid‑size excavator designed for general construction, utilities, and small quarry work. It became one of Komatsu’s most widely sold models in the 12‑ton class due to:

• Reliable mechanical‑hydraulic systems
  
• Simple maintenance
  
• Strong resale value
  
• Compatibility with a wide range of attachments
  

Early PC120 machines (Dash‑1 and Dash‑2) used a straightforward load‑sensing hydraulic system with a variable‑displacement axial piston pump. These pumps were designed for durability rather than high output, which is why many units from the 1980s are still working today.

Terminology Notes

• Variable‑Displacement Pump: A hydraulic pump that automatically adjusts flow based on system demand.
  
• Load‑Sensing (LS): A system that monitors hydraulic load and adjusts pump output to match required force.
  
• Main Relief Pressure: The maximum pressure the hydraulic system is allowed to reach before a relief valve opens.
  
• Pump Swash Plate: The internal component that controls pump displacement and flow.
  
• Dash Number: Komatsu’s generation identifier (e.g., PC120‑1, PC120‑2). Different dash numbers have different pump settings.
  

Why Hydraulic Power Declines Over Time
Before considering any adjustment, it’s important to understand why an older PC120 may feel weak:

• Pump wear reduces volumetric efficiency
  
• Internal leakage increases in cylinders and control valves
  
• Relief valves weaken or drift out of calibration
  
• Engine output declines due to age
  
• Contaminated hydraulic oil reduces pump responsiveness
  
• Hoses and fittings develop micro‑leaks
  

In many cases, the pump is not the root cause.

Can the Pump Be “Turned Up”?
Technically, yes—Komatsu pumps have adjustable components.
Practically, it is rarely the correct first step.
Increasing pump pressure or flow without proper testing can:

• Overload the engine
  
• Overheat hydraulic oil
  
• Damage cylinders
  
• Blow hoses
  
• Crack control valve bodies
  
• Accelerate pump wear
  

Manufacturers design pump settings to balance performance and longevity. Exceeding these limits can shorten machine life dramatically.

Identifying the Dash Number Matters
The PC120 from 1985 could be either a PC120‑1 or PC120‑2.
Each version uses a different pump control system and different adjustment procedures.
Examples:

• PC120‑1 uses a simpler mechanical control
  
• PC120‑2 incorporates more refined load‑sensing logic
  
• PC120‑3 and later use more advanced proportional control valves
  

Without knowing the dash number, adjusting the pump is guesswork and potentially dangerous.

Correct Diagnostic Steps Before Adjustment
A professional technician would follow this sequence:

• Measure engine RPM under load
  
• Check hydraulic oil temperature
  
• Test main relief pressure
  
• Measure pump standby pressure
  
• Inspect pump case drain flow (indicates pump wear)
  
• Test cylinder drift and internal leakage
  
• Inspect control valve spool clearances
  
• Verify LS line pressure
  

Only after these tests can a technician determine whether the pump is truly weak.

Why Turning Up the Pump Rarely Solves the Problem
If the pump is worn, increasing pressure only forces worn components to work harder.
This often results in:

• Higher fuel consumption
  
• Increased heat
  
• Faster pump failure
  

A pump with 6,200 hours may simply be reaching the end of its service life.

A Story from the Field
A contractor in New Jersey once attempted to “turn up” the pump on his aging PC120 to speed up trenching work. After increasing the relief pressure by only 10%, the machine initially felt stronger.
Within two weeks:

• The boom cylinder began leaking
  
• The hydraulic oil temperature rose significantly
  
• The pump case drain flow doubled
  
• The machine lost more power than before
  

A full pump rebuild was required. The technician later explained that the pump was already worn, and the increased pressure accelerated internal scoring.

Safe Ways to Improve Hydraulic Performance
Instead of adjusting the pump, consider these solutions:

• Replace hydraulic filters
  
• Flush and refill with high‑quality hydraulic oil
  
• Rebuild leaking cylinders
  
• Replace worn relief valves
  
• Inspect and replace weak hoses
  
• Clean or replace LS lines
  
• Verify engine output and fuel delivery
  
• Rebuild the pump if case drain flow is excessive
  

These steps often restore performance without touching pump settings.

When Pump Adjustment Is Appropriate
Pump adjustment is only appropriate when:

• The pump is confirmed healthy
  
• Relief pressures are below factory specification
  
• LS pressure is out of calibration
  
• A technician with proper gauges performs the adjustment
  

Even then, adjustments must stay within Komatsu’s published limits.

Conclusion
Increasing hydraulic pump output on a Komatsu PC120 is not a simple matter of turning a screw. The machine’s age, dash number, pump condition, and hydraulic system health must all be evaluated before any adjustment is made. In most cases, performance loss comes from wear or leakage elsewhere in the system—not from incorrect pump settings.
A careful diagnostic approach protects the machine, avoids costly failures, and ensures the PC120 continues working reliably despite its age.