Diagnosing and Managing Pump Displacement Solenoids on the Hitachi EX120-2

[MikePhua]

The Hitachi EX120-2 and Its Hydraulic Control System
The Hitachi EX120-2 excavator was introduced in the early 1990s as part of Hitachi’s second-generation hydraulic lineup. Designed for mid-size earthmoving and utility work, the EX120-2 featured a refined hydraulic system built around twin variable displacement piston pumps. These pumps are electronically modulated to adjust flow and pressure based on operator input and load demand, improving fuel efficiency and responsiveness.
At the heart of this modulation system are the pump displacement solenoids—electromagnetic valves that control the swash plate angle inside the hydraulic pumps. By altering the angle, the solenoids regulate the volume of hydraulic fluid delivered per revolution, allowing the machine to scale power output dynamically.
Function and Behavior of Displacement Solenoids
Each pump on the EX120-2 is equipped with a displacement solenoid. These solenoids receive signals from the machine’s electronic control unit (ECU), which interprets joystick position, engine RPM, and hydraulic load. When energized, the solenoid shifts a spool valve that adjusts the swash plate angle, increasing or decreasing pump output.
Key characteristics:

• Voltage: Typically 24V DC
  
• Resistance: Around 10–15 ohms depending on model
  
• Response time: Milliseconds, allowing real-time modulation
  
• Mounting: Threaded into the pump body with O-ring seals
  

If a solenoid fails or loses signal, the pump may default to minimum displacement, resulting in sluggish operation or reduced digging force.
Symptoms of Solenoid Malfunction
Operators may notice:

• Slow boom or arm movement despite high engine RPM
  
• Inconsistent hydraulic response under load
  
• One pump performing normally while the other lags
  
• Diagnostic codes related to pump control or solenoid voltage
  
• Audible change in pump tone when solenoid is disconnected
  

In one case from a contractor in Alberta, an EX120-2 began losing swing torque intermittently. After checking the swing motor and valve block, technicians traced the issue to a faulty displacement solenoid on the second pump. Replacing the solenoid restored full performance.
Testing and Troubleshooting Procedure
To diagnose solenoid issues:

• Use a multimeter to measure resistance across the solenoid terminals. A reading outside the 10–15 ohm range may indicate internal failure.
  
• Check voltage supply during operation. The solenoid should receive 24V when active.
  
• Inspect wiring harness for corrosion, abrasion, or loose connectors.
  
• Remove the solenoid and inspect the spool for debris or sticking.
  
• Swap solenoids between pumps to isolate the fault.
  

If the solenoid is functional but the pump remains unresponsive, the issue may lie in the ECU, pressure sensors, or hydraulic feedback loop.
Replacement and Installation Tips
When replacing a displacement solenoid:

• Use OEM or high-quality aftermarket parts rated for hydraulic environments
  
• Clean the mounting port thoroughly and replace the O-ring seal
  
• Torque the solenoid to manufacturer specs to prevent leaks
  
• Recalibrate the ECU if required to recognize the new component
  
• Test pump response under load and monitor for overheating or cavitation
  

Some operators retrofit inline pressure gauges to monitor pump output and detect anomalies early.
Preventive Measures and Long-Term Reliability
To extend solenoid life:

• Inspect electrical connectors monthly
  
• Protect wiring harnesses from heat and abrasion
  
• Replace hydraulic filters every 500 hours to prevent contamination
  
• Avoid prolonged idling with hydraulic functions engaged
  
• Monitor fluid temperature and use synthetic oil in extreme climates
  

In Shanghai, a fleet manager implemented a solenoid inspection protocol every 1,000 hours. Over the next year, hydraulic downtime dropped by 30%, and solenoid-related failures were eliminated entirely.
Conclusion
Pump displacement solenoids on the Hitachi EX120-2 play a critical role in hydraulic performance and fuel efficiency. Understanding their function, diagnosing faults accurately, and maintaining the surrounding systems ensures smooth operation and extended machine life. Whether trenching, lifting, or grading, a responsive hydraulic system begins with healthy solenoids and informed operators.