Hitachi EX120-2 Hydraulic Lockout and Pilot Pressure Failure

[MikePhua]

Quick answer
A Hitachi EX120-2 excavator that won’t move or lift likely suffers from pilot pressure loss due to a stuck pilot relief valve or blocked pilot circuit. Electrical faults in the pump control solenoids may also contribute, but restoring pilot flow is the first priority.
EX120-2 background and hydraulic control system
The Hitachi EX120-2 is a mid-sized hydraulic excavator introduced in the early 1990s, widely used in construction and utility work. It features a closed-center hydraulic system with electronically modulated pump displacement. The machine relies on pilot pressure—low-pressure hydraulic oil—to actuate control valves and send signals to the main pump and spool ends.
Key components include:

• Pilot pump: Supplies low-pressure oil to joysticks and control valves
  
• Main pump: Delivers high-pressure oil to cylinders and motors
  
• Pump control solenoids: Modulate displacement based on demand
  
• PVC (Pump Valve Controller): Sends electrical signals to solenoids
  

Without pilot pressure, the joysticks cannot send commands, and the main control valve spools remain inactive.
Initial symptoms and troubleshooting steps
The machine initially showed jerky movement in the bucket and tracks. After replacing the DP sensor and angle sensor, three stuck spools were freed. However, the machine then lost all movement—tracks, boom, and bucket were unresponsive.
Key observations:

• Pilot pressure present at the filter inlet, but not reaching control levers or spool ends
  
• Pilot filter became hot, suggesting flow restriction or internal bypass
  
• No engine load during travel attempts, indicating no hydraulic engagement
  
• Pump solenoids not receiving proper negative pulse from PVC
  

These symptoms point to a pilot circuit blockage or relief valve malfunction.
Pilot relief valve and circuit inspection
The pilot relief valve regulates pilot pressure and protects the system from overload. If stuck open or clogged, it can divert pilot oil back to tank, starving the control valves.
Recommended steps:

• Locate the pilot relief valve, typically near the pilot pump or control valve block
  
• Disassemble and inspect for contamination or stuck components
  
• Clean or replace the valve as needed
  
• Check pilot lines for kinks, blockages, or collapsed hoses
  

One technician noted that without pilot oil reaching the joysticks, pump displacement control is irrelevant—the spools won’t move, and the machine remains inert.
Electrical control and pump solenoids
Later inspection revealed that the pump displacement solenoids were not functioning due to missing negative pulses from the PVC. While this affects pump output, it’s secondary to pilot pressure loss.
Recommendations:

• Test solenoid resistance and voltage supply
  
• Inspect PVC wiring harness for damage or corrosion
  
• Verify safety lever and interlock switch operation
  
• Check for broken push rods or frozen linkages under the cab
  

In one case, a frozen push rod on a CAT excavator disabled hydraulics until a bent protection plate was removed and the rod replaced.
Avoiding premature component replacement
A common mistake is replacing sensors and solenoids without confirming pilot pressure. As one technician warned, “If you start buying every part that’s broken or worn or you think is not right, you won’t have the money to buy what’s needed to get up and running.”
Focus first on restoring pilot flow, then address electrical modulation and pump control.
Conclusion
A Hitachi EX120-2 that won’t move is likely suffering from pilot pressure failure due to a stuck relief valve or blocked pilot circuit. Electrical faults in the pump controller may compound the issue, but pilot flow must be restored before spool movement and hydraulic engagement can occur. Careful diagnosis and staged repair will prevent wasted expense and get the machine back to work.