Hitachi EX120-3 Hydraulic Lag and PVC System Faults

[MikePhua]

Hydraulic hesitation and loss of power in the Hitachi EX120-3 excavator often stem from faults in the pump control system, including the angle sensor, DP sensor, and PVC computer. These issues become more pronounced as hydraulic oil heats up, leading to sluggish multi-function response and intermittent control delays.
Hitachi EX120-3 Overview
The EX120-3 is a mid-size hydraulic excavator produced by Hitachi Construction Machinery in the late 1990s. It features a 4-cylinder Isuzu diesel engine, electronically regulated hydraulic pumps, and a pilot-controlled joystick system. Hitachi’s EX series gained global popularity for its smooth operation and modular electronics, with tens of thousands of units sold across Asia, Europe, and North America. The EX120-3 introduced a PVC (Pump Valve Controller) system that electronically adjusts pump stroke based on operator input and machine load.
Terminology Note

• PVC (Pump Valve Controller): The onboard computer that regulates pump output and hydraulic response.
  
• Angle Sensor (A/S): A sensor mounted on the pump that reports swash plate position to the PVC.
  
• DP Sensor: Differential pressure sensor that monitors pilot signal strength and system demand.
  
• Pilot Valve: A low-pressure control valve that directs hydraulic oil to actuate main valves.
  
• DR ZX: Hitachi’s diagnostic tool used to interface with the PVC and read fault codes.
  

Symptoms and Field Diagnosis
Operators report that the EX120-3 loses hydraulic speed and power when attempting to use multiple functions simultaneously. Stick and bucket movements become sluggish, and tracking fails under load. The issue worsens as hydraulic oil heats up, suggesting a thermal sensitivity in the control system.
Initial repairs included:

• Adding shims to the pilot valve to improve spool response
  
• Replacing the main pressure relief valve
  
• Inspecting pilot filter hoses for clunking and vibration
  

These steps improved cold-start performance and reduced pilot line noise, but did not resolve the hot-oil slowdown.
Shade Tree Test for Angle Sensor
To test the angle sensor without diagnostic tools:

• Turn the key OFF and unplug the angle sensor.
  
• Start the engine at low idle.
  
• If the pump goes to full stroke and loads the engine heavily, the electrical control is likely functioning.
  
• Turn the key OFF before reconnecting the sensor.
  

This test helps confirm whether the PVC is receiving and responding to stroke signals.
Advanced Diagnostics and Solutions

• Check all 1-amp fuses in the PVC circuit. Do not replace with higher amperage fuses, as this risks damaging the controller.
  
• Use DR ZX to scan for fault codes. If the tool fails to communicate, the PVC may be faulty.
  
• Replace the DP sensor if pilot pressure readings are erratic or delayed.
  
• Replace the PVC computer if communication fails and all wiring checks out. A non-responsive PVC can cause dead zones and delayed function activation.
  
• Inspect wiring harnesses for corrosion, especially in humid or mountainous environments.
  

In Hawaii, a contractor found that replacing both the DP sensor and PVC restored full hydraulic performance. The DR ZX tool had failed to connect, which was a key indicator of PVC failure. After replacement, the machine operated smoothly even under multi-function load.
Preventive Maintenance Tips

• Replace pilot filters every 500 hours to prevent clogging and pressure lag.
  
• Monitor hydraulic oil temperature and viscosity—use factory-recommended fluids.
  
• Keep electrical connectors sealed and dry to prevent oxidation.
  
• Log control delays and correlate with temperature and load to identify patterns.
  
• Use diagnostic tools regularly to catch sensor faults before they escalate.
  

Conclusion
Hydraulic lag in the Hitachi EX120-3 is often a symptom of failing electronic control components, especially the angle sensor, DP sensor, and PVC computer. While mechanical adjustments may offer temporary relief, full resolution requires electrical diagnostics and targeted replacement. With proper inspection and system understanding, operators can restore full hydraulic responsiveness and extend the life of this reliable excavator.