Hydraulic Lag in the Hitachi EX12-2 and How to Resolve It

[MikePhua]

The EX12-2 and Its Compact Excavator Lineage
The Hitachi EX12-2, released in the early 1990s, was part of Hitachi’s compact excavator series designed for urban utility work, landscaping, and small-scale trenching. With an operating weight of roughly 2,800 kg and a Yanmar diesel engine producing around 20 horsepower, the EX12-2 was engineered for maneuverability and fuel efficiency. Its hydraulic system featured a variable displacement pump and proportional control valves, allowing precise movement in tight spaces. Though no longer in production, the EX12-2 remains in service across Europe and Asia due to its reliability and ease of transport.
Symptoms of Hydraulic and Track Lag
A recurring issue in aging EX12-2 units is hydraulic slowdown after warm-up. Operators report that after 15–20 minutes of operation, both the boom and track movements become sluggish. If the machine is shut down and restarted, performance temporarily returns to normal before degrading again. This pattern suggests a thermal or electronic fault rather than mechanical wear.
Displacement Solenoids and Angle Sensor Faults
The EX12-2 uses displacement solenoids to regulate pump output based on demand. These solenoids adjust the swashplate angle inside the hydraulic pump, controlling flow rate. If the solenoids fail or receive erratic signals, the pump may default to low displacement, reducing hydraulic power. Similarly, the angle sensor—which monitors swashplate position—can misreport data to the controller, causing incorrect flow commands.
Common failure modes include:

• Heat-induced resistance changes in solenoid coils
  
• Loose or corroded connectors at the sensor harness
  
• Internal wear in the sensor potentiometer
  

These components are sensitive to temperature and vibration, which explains why the issue appears after warm-up and resets briefly after shutdown.
Diagnostic Strategy and Field Testing
To isolate the fault:

• Use a multimeter to check solenoid resistance cold and hot. A healthy coil should read between 10–15 ohms.
  
• Inspect the angle sensor for smooth voltage change during manual swashplate movement.
  
• Check for voltage drop at the solenoid connector under load.
  
• Monitor hydraulic pressure at the pump outlet during operation. A drop below 2,500 psi under load indicates displacement failure.
  

If diagnostic tools are unavailable, a practical test involves bypassing the solenoid with a manual override or applying direct 12V power to confirm response.
Repair Recommendations and Component Access
If the solenoid is faulty:

• Replace with OEM or aftermarket unit rated for continuous duty
  
• Use dielectric grease on connectors to prevent future corrosion
  
• Secure wiring harnesses with vibration-resistant clamps
  

If the angle sensor is faulty:

• Replace with a compatible potentiometer rated for hydraulic environments
  
• Calibrate sensor output to match controller input range (typically 0.5–4.5V)
  

In one documented repair, replacing both solenoids and cleaning the sensor harness restored full hydraulic performance, even under extended operation.
Preventive Measures for Aging Units

• Flush hydraulic fluid every 1,000 hours to remove varnish and debris
  
• Replace inline filters and inspect return screens for metal particles
  
• Shield electronic components from engine heat using reflective tape or heat sinks
  
• Install a cabin voltmeter to monitor system voltage during operation
  

Conclusion
Hydraulic lag in the Hitachi EX12-2 is often caused by thermal degradation of displacement solenoids or faulty angle sensor feedback. These components are critical to pump regulation and are prone to failure in older machines. With targeted diagnostics and careful replacement, operators can restore full responsiveness and extend the service life of this compact excavator. The EX12-2 remains a testament to Hitachi’s engineering, and with proper care, it continues to perform in demanding environments.