Caterpillar 955L Governor Issues: Diagnosis, Causes, and Solutions

[MikePhua]

Overview of the Caterpillar 955L Governor
The Caterpillar 955L, a mid-sized crawler loader, utilizes a mechanical governor system to regulate engine speed under varying load conditions. The governor plays a crucial role in maintaining consistent RPMs to ensure smooth operation, fuel efficiency, and prevention of engine damage. Problems with the governor can result in erratic engine behavior, loss of power, and operational difficulties.
Symptoms of Governor Problems
Common signs that the 955L’s governor may be malfunctioning include:

• Engine RPM hunting or fluctuating unexpectedly
  
• Difficulty maintaining idle speed or sudden engine stalls
  
• Loss of engine power under load
  
• Engine overspeed or inability to reach full RPM
  
• Delayed throttle response or surging during acceleration
  

Recognizing these symptoms early can help prevent more serious engine damage.
Typical Causes of Governor Issues
Several factors can cause governor malfunctions on the 955L:

• Mechanical wear or damage: Governor linkages, springs, and levers can wear out or become bent, affecting precision.
  
• Contamination and corrosion: Dirt, rust, or sludge buildup inside the governor mechanism impairs smooth movement.
  
• Fuel system irregularities: Faulty fuel injection pump or clogged injectors can mimic governor problems by disrupting fuel delivery.
  
• Improper adjustments: Incorrect governor settings or throttle linkage can cause RPM instability.
  
• Vacuum or pressure leaks: On some models, vacuum leaks impact governor function by altering control signals.
  

Diagnostic Procedures
To pinpoint governor problems, technicians generally follow these steps:

• Visually inspect governor linkages and mechanical parts for wear or damage
  
• Check and clean the governor mechanism, removing dirt and corrosion
  
• Measure engine RPM stability with a tachometer during various operating conditions
  
• Inspect fuel injection system components for clogging or malfunction
  
• Adjust governor spring tension and linkage as per manufacturer specifications
  
• Test for vacuum leaks or pressure inconsistencies affecting governor operation
  

Proper diagnostics require mechanical knowledge and sometimes specialized tools.
Adjusting and Repairing the Governor
When governor issues are confirmed, repair steps include:

• Replacing worn or bent linkage components and springs
  
• Cleaning or overhauling the governor assembly
  
• Resetting the governor speed stop and throttle adjustments
  
• Repairing or servicing fuel injection components to restore proper fuel flow
  
• Sealing vacuum leaks and tightening connections
  

Experienced mechanics emphasize the importance of following Caterpillar’s adjustment procedures closely, as improper settings can worsen the problem.
Case Example: Surging Engine on a 955L
A contractor reported the engine on their 955L surging uncontrollably during loading operations. After ruling out fuel system issues, the mechanic found the governor spring had stretched and the linkage bushings were worn, causing erratic throttle response. After replacing the spring and bushings and adjusting the governor stop, the engine returned to smooth operation. This underscores the importance of inspecting mechanical parts during troubleshooting.
Preventative Maintenance Tips
To minimize governor-related problems, operators and technicians should:

• Conduct regular visual inspections of the governor linkage and components
  
• Keep the governor mechanism clean and lubricated
  
• Adhere to scheduled fuel system maintenance, including filter replacement and injector cleaning
  
• Avoid rough handling of throttle controls to prevent linkage wear
  
• Monitor engine RPM behavior and address abnormalities promptly
  

Governor Terminology

• Governor spring: Controls the tension and speed range of the governor
  
• Linkage: Mechanical rods and levers connecting the governor to the throttle and fuel injection system
  
• Throttle stop: A mechanical limiter that sets maximum engine RPM
  
• Fuel injection pump: Delivers precise fuel quantities to the engine based on governor signals
  
• RPM hunting: Rapid oscillations in engine speed caused by unstable governor control
  

Summary of Troubleshooting Steps

• Inspect linkage and springs for wear or damage
  
• Clean governor components and remove buildup
  
• Check fuel system integrity and injector function
  
• Adjust governor settings following official manuals
  
• Test for and repair vacuum or pressure leaks
  
• Replace faulty mechanical parts as needed
  

Conclusion
Governor issues on the Caterpillar 955L can significantly affect machine performance and reliability. A systematic approach combining mechanical inspection, cleaning, fuel system checks, and precise adjustment usually resolves the majority of problems. Regular maintenance and early intervention are essential to ensure stable engine operation and maximize the lifespan of this vital control system.