Comprehensive Guide to the Komatsu PC120-6 Auxiliary Valve: Function, Troubleshooting, and Maintenance

[MikePhua]

Introduction to the Komatsu PC120-6 Auxiliary Valve
The Komatsu PC120-6 is a widely used medium-sized hydraulic excavator renowned for its robust performance and reliability. A critical component in its hydraulic system is the auxiliary valve, responsible for controlling the flow of hydraulic fluid to auxiliary attachments such as hydraulic breakers, thumbs, and compactors. This valve enables the operator to precisely manage additional tools, expanding the excavator’s functionality beyond standard bucket operations.
Understanding the structure, operation, common issues, and maintenance of the auxiliary valve enhances machine performance, reduces downtime, and extends component life.

Structure and Function of the Auxiliary Valve

• The auxiliary valve is mounted on the spool stack within the main control valve assembly. It regulates hydraulic pressure and directs fluid flow to auxiliary circuits based on operator input.
  
• It consists of multiple spools, relief valves, and control passages, designed to handle high-pressure hydraulic fluid and provide proportional flow for attachments.
  
• Key parts include:
  • Spool(s): Sliding elements that open and close flow paths.
    
  • Relief Valve: Safety device protecting the system from overpressure by allowing fluid to divert when pressure exceeds limits.
    
  • Load Sensing (LS) Ports: Connect to the load-sensing system to adjust pump output according to hydraulic demand.
    
• The auxiliary valve works in conjunction with the main pump and pilot control system to provide smooth, responsive control for all auxiliary functions.
  

Common Issues and Troubleshooting

• Loss of Auxiliary Power or Slow Attachment Movement:
  This may result from spool sticking, worn seals, or internal leakage within the valve assembly. Hydraulic contamination can accelerate wear and cause sediment build-up, impairing spool movement.
  
• Erratic or Jerky Operation:
  Inconsistent hydraulic flow due to valve malfunction or improper relief valve settings can lead to uneven attachment motion.
  
• Incorrect Pressure Adjustments:
  Misadjusted relief valves can cause insufficient or excessive pressure, impacting attachment performance and potentially damaging the hydraulic circuit or attachments.
  
• Hydraulic Oil Contamination:
  Dirty or degraded hydraulic fluid increases wear on valve components and causes operational failures. Regular fluid filtration is essential.
  
• Leakage Inside Valve Body:
  Internal leakage reduces effective pressure at the spool, weakening auxiliary functions.
  

Maintenance and Service Recommendations

• Regular Inspection:
  Periodically check the valve body and fittings for leaks, corrosion, or physical damage. Examine hoses and connections feeding auxiliary circuits.
  
• Hydraulic Fluid Care:
  Maintain clean, manufacturer-recommended hydraulic fluid and replace filters on schedule. Contamination control is crucial to valve longevity.
  
• Spool and Seal Service:
  During valve overhaul or repair, replace worn spools, seals, and internal valve components to restore proper function.
  
• Pressure Adjustment:
  Use service manuals to correctly set main and auxiliary relief pressures and pilot control settings, ensuring optimal hydraulic performance.
  
• Testing:
  Conduct pressure and flow tests with appropriate gauges to diagnose valve or circuit issues accurately.
  

Additional Parameters and Practical Solutions

• If auxiliary functions progressively lose power, suspect gradual valve deterioration or pump issues; confirm via pressure and flow diagnostics.
  
• Keep pilot pressure lines and control linkages clean and undamaged to avoid pilot signal loss causing valve misoperation.
  
• Use OEM or high-quality aftermarket parts when servicing to ensure valve integrity and system compatibility.
  
• When adding or modifying auxiliary attachments, verify valve and hydraulic circuit compatibility and load capacity to prevent overloading.
  

Terminology Explained

• Spool: The movable component within a valve that controls the hydraulic fluid direction.
  
• Relief Valve: A safety valve that opens to release fluid when pressure exceeds a preset limit.
  
• Load Sensing (LS): A system that adapts pump output to the exact hydraulic demand, improving efficiency.
  
• Pilot Control: Low-pressure control signals that operate main valve components.
  
• Internal Leakage: Hydraulic fluid loss within valve components reducing pressure and flow efficiency.
  
• Hydraulic Contamination: Presence of dirt, debris, or water in hydraulic fluid damaging components.
  

Real-World Experiences and Case Studies
A Komatsu PC120-6 owner encountered a gradual power loss in the auxiliary circuit controlling a hydraulic breaker. After extensive pressure testing, the root cause was traced to a sticking spool valve exacerbated by contaminated hydraulic fluid. Cleaning and seal replacement restored full functionality.
In another instance, improper adjustment of the auxiliary relief valve led to sudden jerky motion during attachment operation. Realigning the relief valve pressure settings according to the factory manual eliminated the issue, improving operator control and attachment longevity.
Hydraulic technicians often emphasize that consistent hydraulic fluid maintenance, coupled with vigilant valve inspection and adjustment, dramatically extends valve life and maintains excavator versatility in changing jobsite demands.

Summary
The auxiliary valve on the Komatsu PC120-6 excavator is a vital hydraulic component enabling multi-functional attachment control. Its precise design with spools, relief valves, and integrated load sensing ensures smooth operation of accessories under varying hydraulic loads.
Common issues such as spool sticking, leakage, contamination, and pressure misadjustments can impair auxiliary performance but are preventable with proactive maintenance and proper troubleshooting. Key recommendations include regular hydraulic fluid care, accurate pressure setting, and replacing worn valve components with quality parts.
Experience from field use and repairs highlights the importance of systematic diagnostics and adherence to manufacturer specifications for lasting performance. Operators and service teams equipped with this knowledge can ensure their PC120-6 machines deliver reliable and efficient auxiliary functions, maximizing jobsite productivity and machine versatility.