Diagnosing Hydraulic Power Loss in the Hitachi EX100-2: A Deep Dive into Pressure, Valves, and Field Experience

[MikePhua]

Introduction
The Hitachi EX100-2 excavator, a staple in mid-size earthmoving operations, is known for its reliability and hydraulic responsiveness. However, like many aging machines, it can present complex issues that challenge even seasoned mechanics. One recurring problem is a loss of hydraulic digging power despite fast actuator movement—an issue that demands a nuanced understanding of hydraulic systems, control valves, and diagnostic procedures. This article explores the technical anatomy of the EX100-2’s hydraulic system, common failure points, and real-world troubleshooting strategies, enriched with terminology, field anecdotes, and historical context.
Understanding the Symptoms
Operators encountering hydraulic power loss often report:

• Fast boom and stick movement without sufficient digging force
  
• Inability to exceed 4200 psi at the pump outlet test port
  
• No improvement after adjusting relief valves or replacing components
  

These symptoms suggest that while flow is present, pressure is not building adequately under load—a classic sign of internal bypassing or valve malfunction.
Terminology Explained

• Main Relief Valve: Regulates maximum system pressure to prevent overload.
  
• Surge Relief Valve: Protects the system from pressure spikes during rapid directional changes.
  
• Unloading Valve: Controls pressure buildup by diverting flow until demand is sensed.
  
• Swash Plate Control Piston: Adjusts pump displacement based on load demand.
  
• DP Sensor (Differential Pressure Sensor): Monitors pressure differences to inform control logic.
  
• Angle Sensor: Measures swash plate position to regulate pump output.
  

Common Diagnostic Steps and Observations

• Relief Valve Adjustment and Replacement
  • Tightening the main relief valve showed no pressure change.
    
  • Replacing the valve and O-rings yielded no improvement.
    
  • Surge relief valve adjusted to spec (5300–5350 psi) also had no effect.
    
• Unloading Valve Inspection
  • Valve spool moved freely when tested with a magnet.
    
  • Valve replaced with a new unit—still no pressure gain.
    
  • Suggests possible internal leakage or control signal failure.
    
• Sensor and Solenoid Checks
  

• Angle sensor voltage adjusted from 4.745V to 4.412V.
  
• Solenoids swapped with no change in performance.
  
• No diagnostic blink codes from the PVC controller, indicating no electrical faults.
  

Field Anecdote: Wisconsin Excavator Blues
A contractor in Blue Mounds, Wisconsin purchased a used EX100-2 that ran smoothly during inspection but failed to dig effectively on-site. Despite fast hydraulics, the machine couldn’t generate enough force to penetrate compacted soil. After replacing relief valves and inspecting the unloading valve, the issue persisted. The mechanic suspected a deeper problem—possibly in the pump’s internal control piston bore, which could bleed off pressure before it reached the actuators.
Historical Context: Load Sensing and Hydraulic Evolution
The EX100-2 belongs to a generation of excavators that began integrating load-sensing hydraulics—a system that adjusts pump output based on demand. While efficient, these systems are sensitive to wear and contamination. In older machines, control pistons and valve spools can wear out, causing internal leakage that mimics valve failure. This complexity marked a shift from purely mechanical systems to electro-hydraulic hybrids, requiring more sophisticated diagnostics.
Related Case: The Silent Leak in Pennsylvania
An equipment mechanic in Pennsylvania encountered a similar issue with an EX-series machine. After exhausting valve and sensor diagnostics, they disassembled the pump and discovered excessive wear in the swash plate control piston bore. The leak was internal and undetectable through external pressure tests. Replacing the pump case resolved the issue, restoring full digging power.
Best Practices for Troubleshooting Hydraulic Power Loss

• Always verify pressure readings under load, not just at idle
  
• Inspect and clean valve spools for sticking or scoring
  
• Replace O-rings with manufacturer-specified materials
  
• Check sensor voltages against service manual specifications
  
• Use diagnostic blink codes to rule out electrical faults
  
• Consider internal pump wear if all external components test normal
  

Conclusion
Hydraulic power loss in the Hitachi EX100-2 is a multifaceted issue that can stem from valve malfunction, sensor miscalibration, or internal pump wear. While relief valve replacement and sensor adjustment are logical first steps, persistent problems may require deeper mechanical inspection. The EX100-2’s legacy as a durable workhorse remains intact—but like any veteran machine, it demands respect, patience, and a methodical approach to keep it digging strong.