Solving Hydraulic Slowdown in Hitachi EX120-2 and EX200-3 Excavators

[MikePhua]

Hitachi Excavator Lineage and the EX Series
Hitachi Construction Machinery, founded in 1970, has built a global reputation for producing reliable hydraulic excavators. The EX120-2 and EX200-3 models, introduced in the 1990s, were part of a generation that emphasized mechanical durability and simplified electronics. These machines became popular in Southeast Asia, North America, and Africa, with tens of thousands sold worldwide. Their appeal lies in robust steel construction, straightforward hydraulic architecture, and compatibility with a wide range of attachments.
Despite their strengths, aging EX-series excavators often develop hydraulic performance issues—particularly sluggish response, weak travel, and overheating. These symptoms can be frustratingly intermittent and difficult to diagnose without a structured approach.
Terminology Annotation

• PVC Controller: The Powertrain Vehicle Controller, an onboard computer that manages engine speed, hydraulic pump output, and sensor feedback.
  
• DP Sensor: Differential Pressure sensor that monitors pilot pressure and adjusts pump displacement accordingly.
  
• Solenoid Valve: An electrically actuated valve that controls hydraulic flow based on signals from the PVC.
  
• Pilot Circuit: A low-pressure hydraulic system that sends control signals to main valves and actuators.
  

Common Symptoms and Initial Observations
Operators report that the engine idles and revs normally, but hydraulic functions—including boom, bucket, and travel—become slow or unresponsive. In some cases, the machine works fine for 30–40 minutes before deteriorating. The engine shows no signs of load, and hydraulic fluid temperatures rise significantly.
Initial troubleshooting often reveals:

• Cracked or oil-soaked pump solenoids
  
• Voltage discrepancies between solenoid connectors
  
• PVC controller replacement history
  
• Fuse F7 repeatedly blowing prior to controller swap
  

One operator measured 24V at the front solenoid and only 11.5V at the rear, suggesting a wiring or controller fault. Replacing both solenoids did not resolve the issue, pointing to deeper electrical or sensor-related problems.
Sensor Failures and Electrical Gremlins
The EX-series machines are known for sensor sensitivity. A single failed sensor can cause the PVC to misinterpret system demands, resulting in reduced pump output or erratic behavior. Common culprits include:

• DP sensor malfunction causing incorrect pump displacement
  
• Angle sensor misreading boom position
  
• RPM sensor failing to sync engine speed with hydraulic demand
  
• Pressure sensor delivering false feedback to the PVC
  

In one case, a faulty DP sensor caused the pump to run at maximum displacement continuously, overheating the hydraulic fluid and melting plastic components. Replacing the sensor and resealing the solenoid valve box resolved the issue.
Wiring Harness and Connector Issues
Loose or corroded connectors at the PVC can mimic sensor failure. Several technicians reported that jiggling the harness temporarily restored function, indicating poor contact. However, this “sweet spot” became harder to find over time, suggesting internal wire fatigue.
Recommendations include:

• Inspecting all connectors for corrosion, bent pins, and loose crimps
  
• Using dielectric grease to protect contacts
  
• Replacing the harness if intermittent faults persist
  

Hydraulic Tank Screen and Relief Valve Checks
In one instance, a Hitachi EX150 exhibited similar symptoms due to a clogged screen inside the hydraulic tank. Removing and cleaning the screen restored flow and eliminated sluggishness. Another operator replaced the main relief valve, hoping to solve the issue, but found no improvement—highlighting the importance of targeted diagnostics.
Dr. EX Diagnostic Tool and Software Limitations
Some users attempted to diagnose the problem using the Dr. EX scan tool, which reads sensor data and system pressures. While helpful, the tool often failed to detect intermittent faults or misconfigured aftermarket sensors. One technician found success only after using OEM software and cables sourced from John Deere, which shares engine architecture with Hitachi.
Recommended Diagnostic and Repair Strategy
To resolve hydraulic slowdown:

• Replace both pump solenoids with OEM-grade units
  
• Verify voltage at solenoid connectors with engine off and key on
  
• Inspect and reseal the solenoid valve box to prevent pilot oil bypass
  
• Replace the DP sensor with a model specific to Hitachi EX120-2 or EX200-3
  
• Clean the hydraulic tank screen and flush fluid if overheating occurs
  
• Check all wiring harnesses and PVC connectors for continuity and corrosion
  
• Use OEM diagnostic software if available
  

Preventive Maintenance and Long-Term Reliability
To avoid future issues:

• Replace hydraulic fluid every 1,000 hours or annually
  
• Inspect and clean electrical connectors quarterly
  
• Keep spare DP sensors and solenoids on hand for field replacement
  
• Monitor hydraulic temperatures during operation and shut down if overheating
  
• Document all sensor replacements and wiring repairs for future reference
  

Conclusion
Hydraulic slowdown in Hitachi EX-series excavators is often caused by a combination of sensor failure, solenoid degradation, and electrical instability. While the symptoms may appear mechanical, the root cause is frequently electronic. With a structured diagnostic approach and attention to wiring integrity, operators can restore full hydraulic performance and extend the life of these dependable machines. The EX120-2 and EX200-3 remain valuable assets when properly maintained, even decades after their release.