Diagnosing Sluggish Hydraulics on the Hitachi EX100-2 Excavator

[MikePhua]

The Hitachi EX100-2 and Its Hydraulic Architecture
The Hitachi EX100-2, released in the mid-1990s, was part of Hitachi’s second-generation excavator lineup, designed to offer improved electronic control and hydraulic efficiency over its predecessor, the EX100. With an operating weight of roughly 10 metric tons and powered by a 4-cylinder Isuzu diesel engine, the EX100-2 became a popular choice for contractors seeking a balance between size, fuel economy, and digging power.
Its hydraulic system is governed by a dual-pump setup with electronically modulated control valves. The machine uses a Pressure Control Valve (PCV) and an Electronic Control Unit (ECU) to regulate pump output based on operator input and load demand. When functioning properly, the system delivers smooth, responsive movement across all functions—boom, arm, bucket, and travel.
Symptoms of Hydraulic Slowness and Initial Observations
A common issue reported by operators is a sudden drop in hydraulic speed across all functions, including travel. In some cases, movement briefly improves when multiple functions are activated simultaneously or when controls are released and re-engaged. This behavior suggests that the pumps are not stroking fully or that the control signals are being interrupted.
Initial steps often include:

• Checking fuel delivery to rule out engine load issues
  
• Replacing hydraulic filters and pilot filters
  
• Inspecting solenoid wiring and connectors for corrosion or damage
  

In one case, unplugging the Differential Pressure (DP) sensor temporarily restored hydraulic speed, indicating that the sensor was limiting pump stroke electronically.
Understanding the Role of the DP Sensor and Solenoids
The DP sensor monitors pressure differential across the hydraulic system and signals the ECU to adjust pump stroke accordingly. When functioning correctly, it helps optimize fuel efficiency and prevent overloading. However, a faulty DP sensor can falsely signal low demand, causing the pumps to remain in low-output mode.
The EX100-2 uses two solenoids mounted on the hydraulic pump to control stroke. These solenoids are prone to failure due to heat, vibration, and age. Swapping solenoids or their wiring can help isolate the issue. If unplugging the DP sensor improves performance, it suggests that the pumps are capable of full stroke but are being electronically restricted.
Pilot Pressure and System Diagnostics
Pilot pressure is critical for valve actuation. The EX100-2’s pilot system should deliver approximately 711 psi (49 bar) under normal conditions. In one diagnostic case, pilot pressure measured nearly 1,200 psi—far above spec. Excessive pilot pressure can cause sluggish or erratic valve response, especially if seals or spools are worn.
To test pilot pressure:

• Connect a hydraulic gauge to the pilot line near the control valve
  
• Run the engine at operating RPM
  
• Record pressure during idle and function activation
  
• Compare readings to factory specifications
  

If pilot pressure is too high, inspect the pilot relief valve and consider replacing it. High pilot pressure may also indicate a blocked return line or malfunctioning accumulator.
Electronic Diagnostics and Communication Challenges
The EX100-2 supports diagnostic communication via a 4-pin connector located beneath the right armrest. Tools like Hitachi’s Dr.ZX or MPDr can read fault codes from the PCV and ECU. However, compatibility issues may arise if the diagnostic tool does not recognize the EX100-2 model or lacks the correct adapter.
To manually check for fault codes:

• Short terminals #2 and #4 in the diagnostic connector
  
• Turn the key to the “on” position
  
• Observe the blink pattern on the PCV LED
  
• Decode single or double-digit flashes using the service manual
  

If no codes are present, the PCV light will remain off. Fault codes can help identify issues with sensors, solenoids, or internal valve logic.
Field Anecdotes and Practical Solutions
One operator in Maine reported that replacing both pump solenoids improved performance slightly, but the boom remained too weak to lift a full bucket. After checking pilot pressure and confirming excessive readings, he ordered a new pilot relief valve and DP sensor. Although the Dr.ZX tool failed to communicate with the machine, manual code checks revealed no faults.
Another technician in Ireland noted similar symptoms on an EX100-3 and traced the issue to a damaged wire leading to the rear solenoid. After repairing the harness and resetting the ECU, full hydraulic speed was restored.
Recommendations for Restoration and Preventive Care
To restore hydraulic performance on the Hitachi EX100-2:

• Replace faulty DP sensor and pump solenoids with OEM-grade parts
  
• Verify pilot pressure and adjust relief valve as needed
  
• Inspect wiring harnesses for age-related damage
  
• Clean all connectors and apply dielectric grease
  
• Use manual fault code checks if diagnostic tools fail
  
• Flush hydraulic fluid and replace filters every 1,000 hours
  

For long-term reliability:

• Monitor pump stroke behavior during operation
  
• Keep diagnostic tools updated and compatible
  
• Document all repairs and pressure readings for future reference
  
• Train operators to recognize early signs of hydraulic restriction
  

Conclusion
Sluggish hydraulics on the Hitachi EX100-2 often stem from electronic miscommunication, solenoid failure, or pilot pressure anomalies. By methodically testing sensors, solenoids, and pressure levels, operators can pinpoint the root cause and restore full function. With proper diagnostics and attention to hydraulic behavior, the EX100-2 remains a capable and dependable excavator—even decades after its release.