Why Is the Boom Not Lifting and the Arm Not Retracting on a Hitachi EX55UR

[MikePhua]

The Hitachi EX55UR and Its Compact Excavator Legacy
The Hitachi EX55UR is a zero-tail swing compact excavator designed for urban construction, utility trenching, and landscaping. Introduced in the late 1990s, it was part of Hitachi’s UR (Ultra Radius) series, which prioritized maneuverability in tight spaces without sacrificing hydraulic power. With an operating weight of approximately 5.5 metric tons and powered by a Yanmar 4TNV88 engine, the EX55UR offered a blend of fuel efficiency and responsive hydraulics.
Hitachi Construction Machinery, a division of Hitachi Ltd., has long been a leader in hydraulic excavator design. By the early 2000s, the EX series had sold tens of thousands of units globally, with the EX55UR gaining popularity in Europe and Asia for its compact footprint and reliability in confined job sites.
Symptoms of Hydraulic Dysfunction
When the boom fails to lift and the arm refuses to retract, the issue typically lies within the hydraulic circuit. These two functions are powered by separate cylinders but often share common control valves, pilot pressure sources, and pump output. The fact that both functions are affected simultaneously suggests a systemic fault rather than isolated cylinder failure.
Common symptoms include:

• No movement when joystick is actuated
  
• Engine does not bog down under load (indicating no hydraulic demand)
  
• Other functions like swing or travel may still operate normally
  
• No unusual noise or vibration from the pump
  

This points toward a control valve issue, pilot pressure loss, or a blockage in the main hydraulic lines feeding the boom and arm circuits.
Understanding Pilot Pressure and Control Valve Operation
Pilot pressure is a low-pressure hydraulic signal used to actuate the main control valves. In the EX55UR, the pilot system typically operates at 400–600 psi and is generated by a dedicated pilot pump or tapped from the main pump via a flow divider.
If pilot pressure is lost due to:

• A failed pilot pump
  
• Clogged pilot filter
  
• Broken pilot line or fitting
  
• Faulty joystick valve
  

Then the control valves for boom and arm will not respond, even if the main pump is functioning correctly.
In one documented case, a technician found a cracked pilot line under the cab floor of an EX55UR. The leak was small but enough to drop pilot pressure below the threshold needed to shift the spool valves. After replacing the line and bleeding the system, full function was restored.
Control Valve Spool Sticking and Contamination
The main control valve block contains multiple spool valves that direct flow to specific actuators. If the boom and arm spools are stuck due to contamination, varnish buildup, or internal scoring, they may fail to shift even when pilot pressure is present.
Recommended steps include:

• Tapping the valve body lightly while actuating the joystick (to free stuck spools)
  
• Checking for magnetic debris on pilot screens
  
• Flushing the hydraulic system and replacing filters
  
• Inspecting joystick valve for internal leakage
  

In older machines, especially those with irregular fluid changes, varnish can form on spool surfaces, causing them to bind. A mechanic in Malta once resolved a similar issue by soaking the valve block in a cleaning solution and cycling the spools manually before reinstalling.
Hydraulic Pump Output and Load Sensing
The EX55UR uses a variable displacement axial piston pump with load-sensing capability. If the pump fails to respond to demand signals, it may not generate sufficient flow for boom and arm functions.
Possible causes include:

• Broken load-sensing line from control valve to pump
  
• Faulty pressure compensator
  
• Internal pump wear or piston scoring
  

To diagnose, install a pressure gauge at the pump outlet and monitor pressure during joystick actuation. If pressure remains low or static, the pump may not be stroking properly. In such cases, rebuilding or replacing the pump may be necessary.
Electrical Interlocks and Safety Solenoids
Some EX55UR models include electrical interlocks that prevent hydraulic function unless certain conditions are met—such as seat occupancy, safety lever position, or travel mode disengagement.
Check for:

• Faulty seat switch or bypass
  
• Safety lever not fully engaged
  
• Travel mode solenoid stuck in active position
  

A simple test is to bypass the safety circuit temporarily and observe whether boom and arm functions return. If so, the issue lies in the electrical control system rather than hydraulics.
Recommendations for Troubleshooting
To isolate the fault, follow this sequence:

1. Check pilot pressure at the joystick valve
   
2. Inspect pilot lines and fittings for leaks or damage
   
3. Tap control valve spools while actuating functions
   
4. Test pump output pressure under load
   
5. Verify electrical interlocks and solenoid function
   
6. Flush hydraulic fluid and replace filters
   

If no resolution is found, consider removing the control valve block and inspecting spool movement manually. In some cases, a partial teardown reveals internal corrosion or broken springs that are not visible externally.
Conclusion
The Hitachi EX55UR is a compact powerhouse, but like all hydraulic machines, it relies on clean fluid, responsive valves, and intact pilot circuits to function properly. When the boom and arm fail simultaneously, the issue is rarely mechanical—it’s almost always hydraulic or electrical. With methodical diagnostics and attention to pilot pressure and valve behavior, even stubborn faults can be resolved. In the world of compact excavators, precision troubleshooting is the difference between downtime and dirt moved.