Resolving Thumb Hydraulic Relief Issues on the Hitachi ZX160 Excavator

[MikePhua]

The Hitachi ZX160 and Its Auxiliary Hydraulic System
The Hitachi ZX160, introduced in the early 2000s, is a mid-size hydraulic excavator built for versatility in general construction, utility trenching, and forestry applications. With an operating weight around 36,000 pounds and a 4-cylinder Isuzu engine producing approximately 120 horsepower, the ZX160 offers a balance of power and precision. One of its key features is the auxiliary hydraulic circuit, which enables the use of attachments like thumbs, grapples, and compactors.
The auxiliary circuit is controlled via pilot-operated valves and includes relief valves to protect the system from overpressure. These relief valves are critical for regulating hydraulic force during thumb operation—both in squeezing and opening motions.
Symptoms of Thumb Relief Valve Malfunction
A common issue with the ZX160 involves the thumb losing pressure in one direction—either weak squeeze or slow opening. In one case, the operator noticed that the thumb had no gripping force, prompting a line swap to test the cylinder. After switching hoses, the squeeze returned but the opening became weak, confirming the issue was hydraulic and not mechanical.
Additional symptoms included:

• Audible fluid bypassing sounds when activating the thumb
  
• Directional weakness depending on hose orientation
  
• No visible leaks or damage to the cylinder
  

These signs pointed to a malfunctioning relief valve in the auxiliary circuit.
Locating and Testing the Relief Valves
On the ZX160, the auxiliary hydraulic relief valves are mounted directly on the main control valve block. They sit above and below the four-bolt flange connections where the auxiliary plumbing enters the valve body. These valves regulate pressure for each direction of flow—one for extending the thumb, the other for retracting.
To diagnose the issue:

• Follow the hydraulic lines from the thumb cylinder back to the control valve
  
• Identify the relief valves associated with the auxiliary circuit
  
• Swap the relief valves to see if the problem changes direction
  
• If the issue reverses, the faulty valve has been identified
  

In one successful repair, the operator disassembled the suspect relief valve and found a small piece of debris—non-metallic and likely a fragment of seal or hose lining—blocking the internal passage. After cleaning and reassembling, the thumb operated normally for over eight hours of continuous use.
Monitoring Hydraulic Pressure
For deeper diagnostics, the ZX160’s onboard monitor can be used to check pump delivery pressure. While basic models only show fuel and temperature, more advanced setups allow pressure readings:

• Press and hold the “Set” button while turning the key on
  
• After the bulb check, release the button
  
• Use the “Select” button to scroll to “Pump 2 Delivery Pressure”
  
• At half throttle, extend the thumb fully and record the pressure
  

The relief setting for the thumb’s barrel-end circuit should be approximately 14–16 MPa (2,000–2,300 psi). If pressure is significantly lower, the relief valve may be stuck open or misadjusted.
Preventive Measures and Long-Term Care
To avoid future relief valve issues:

• Flush hydraulic fluid annually and replace filters
  
• Inspect hoses for internal delamination or shedding
  
• Use high-quality fluid with proper viscosity for seasonal conditions
  
• Avoid sudden directional changes under full load
  
• Periodically test relief valve settings using a pressure gauge
  

Conclusion
Thumb operation issues on the Hitachi ZX160 often trace back to auxiliary relief valve malfunction. By understanding the layout of the hydraulic system and using methodical diagnostics—such as valve swapping and pressure monitoring—operators can resolve directional weakness without costly component replacement. With proper maintenance and attention to hydraulic cleanliness, the ZX160 remains a reliable platform for demanding attachment work.