Hitachi EX120-3 Hydraulic Pump Components and Troubleshooting Insights

[MikePhua]

The EX120-3 and Hitachi’s Excavator Engineering
The Hitachi EX120-3 hydraulic excavator was introduced in the late 1990s as part of Hitachi’s third-generation lineup, combining mechanical durability with electronic control enhancements. With an operating weight of approximately 27,000 pounds and powered by a six-cylinder Isuzu diesel engine, the EX120-3 was designed for general excavation, trenching, and utility work. Hitachi, founded in 1910, became a global leader in hydraulic excavator technology, and the EX120 series remains one of its most widely deployed mid-size models.
The hydraulic pump in the EX120-3 is a dual-displacement axial piston pump, responsible for powering the boom, arm, bucket, swing, and travel functions. It is electronically modulated via displacement solenoids and angle sensors, making it sensitive to both mechanical wear and electrical faults.
Terminology Notes

• Main Hydraulic Pump: A variable-displacement pump that adjusts flow based on demand, typically mounted directly to the engine bell housing.
  
• Displacement Solenoid: An electrically controlled valve that adjusts the pump’s swashplate angle, regulating output flow.
  
• Angle Sensor: A feedback device that monitors swashplate position and communicates with the controller to optimize pump performance.
  

Common Hydraulic Pump Issues and Symptoms
Operators may encounter:

• Loss of hydraulic power during multi-function operations
  
• Pump failing to respond to control inputs or remaining in low displacement
  
• Travel speed stuck in low range despite proper engine RPM
  
• Intermittent or complete loss of swing and boom functions
  
• Diagnostic readings showing low voltage at solenoids or erratic sensor feedback
  

A technician in Queensland reported that an EX120-3 lost hydraulic responsiveness after startup. After replacing the displacement solenoids and bypassing the controller, the pump regained full pressure, confirming an electrical fault in the harness.
Root Causes and Diagnostic Strategy
Hydraulic pump issues often stem from a combination of mechanical wear and electrical control failures. Common culprits include:

• Worn Swashplate or Pistons
  • Reduces volumetric efficiency and causes pressure loss
    
  • Diagnosed via flow testing and internal inspection
    
• Faulty Displacement Solenoids
  • Prevent proper modulation of pump output
    
  • Confirmed by voltage testing and manual activation
    
• Damaged Wiring Harness or Relay Failure
  • Causes low voltage or intermittent signal to solenoids
    
  • Diagnosed by continuity testing and relay inspection
    
• Angle Sensor Malfunction
  • Provides incorrect feedback, leading to poor pump response
    
  • Requires sensor calibration or replacement
    
• Contaminated Hydraulic Fluid
  

• Accelerates wear and causes valve sticking
  
• Detected via fluid analysis and filter inspection
  

Recommended diagnostic steps:

• Measure voltage at solenoid terminals during operation
  
• Check pump pressure at test ports under load
  
• Inspect harness for abrasion, corrosion, or loose connectors
  
• Test relay output and controller ground path
  
• Use scan tool to read sensor values and error codes
  

Repair Procedure and Component Recommendations
To restore hydraulic pump function:

• Disconnect battery and relieve system pressure
  
• Remove pump and inspect swashplate, pistons, and valve plate
  
• Replace displacement solenoids with OEM-rated units
  
• Clean or replace angle sensor and verify calibration
  
• Repair or replace damaged wiring harness sections
  
• Flush hydraulic fluid and replace filters
  

Recommended components:

• Hitachi EX120-3 hydraulic pump rebuild kit
  
• Displacement solenoid set with mounting hardware
  
• Angle sensor compatible with pump controller
  
• Wiring harness repair kit with sealed connectors
  
• Hydraulic fluid rated ISO 46 with anti-wear additives
  

A contractor in British Columbia rebuilt their EX120-3 pump after experiencing slow boom response. By replacing the swashplate assembly and installing new solenoids, they restored full performance and reduced fuel consumption during trenching operations.
Preventive Maintenance and Long-Term Solutions
To prevent future pump failures:

• Replace hydraulic fluid every 1,000 hours or annually
  
• Inspect solenoid voltage and sensor feedback quarterly
  
• Clean electrical connectors and apply dielectric grease
  
• Monitor pump pressure and flow during scheduled service
  
• Use fluid analysis to detect early contamination or wear
  

Some fleet managers retrofit older EX120-3 units with upgraded controllers and diagnostic ports for easier troubleshooting. Others install auxiliary filtration systems to extend pump life in dusty or high-duty environments.
Recommendations for Technicians and Fleet Owners

• Maintain a hydraulic system log with pressure readings and service intervals
  
• Train operators to recognize early signs of pump imbalance or control lag
  
• Stock spare solenoids, sensors, and wiring kits for field repairs
  
• Partner with Hitachi dealers for updated service bulletins and retrofit options
  
• Consider proactive pump rebuilds during major engine or hydraulic service
  

For machines operating in extreme climates or multi-shift environments, enhanced cooling and electrical shielding are recommended.
Conclusion
The hydraulic pump in the Hitachi EX120-3 is the heart of its performance, and when electrical control and mechanical integrity align, the machine delivers smooth, powerful operation. With disciplined diagnostics, targeted repairs, and preventive care, technicians can keep the pump responsive and reliable. In the rhythm of excavation, pressure is more than a number—it’s the pulse of productivity.