Troubleshooting a Caterpillar 955H That Won’t Move and Has Hydraulic Failure

[MikePhua]

Introduction to the Caterpillar 955H Hydraulic and Mobility Systems
The Caterpillar 955H is a classic track loader widely used in construction and mining for heavy-duty digging, loading, and material handling. Its mobility and operational functions rely heavily on a complex hydraulic system that powers the tracks, loader arms, and attachments. When the machine fails to move and its hydraulic functions stop working, diagnosing the root cause quickly is essential to minimize downtime.
Key Components Involved in Movement and Hydraulics

• Hydraulic Pump(s): Supplies pressurized fluid to the drive motors and hydraulic cylinders.
  
• Hydraulic Fluid Reservoir: Stores hydraulic oil required for the system.
  
• Hydraulic Control Valves: Direct hydraulic fluid to different circuits, including travel and implement functions.
  
• Final Drives and Track Motors: Convert hydraulic pressure into mechanical movement for the tracks.
  
• Transmission: Interfaces between engine power and tracks.
  
• Engine and Electrical System: Must function properly to power pumps and controls.
  

Terminology Clarified

• Hydraulic Lockout: A safety feature that disables hydraulic functions to prevent accidental movement during maintenance.
  
• Travel Motor: Hydraulic motor that drives the track sprockets to propel the machine.
  
• Load Sensing System: Hydraulic system that adjusts flow and pressure based on operational demands.
  
• Charge Pump: Smaller pump supplying lubrication and system pressure independent of the main pump.
  

Common Symptoms and Initial Checks

• Machine engine runs normally but there is no track movement
  
• Hydraulic implements (e.g., loader arms) do not respond
  
• No pressure build-up felt at travel control levers
  
• No hydraulic noises such as pump whine or motor engagement sounds
  
• Warning lights or alarms may be present
  

Step-by-Step Troubleshooting Guide

1. Check Hydraulic Fluid Level and Condition
   • Verify reservoir level is within specifications
     
   • Look for signs of contamination, foaming, or overheating
     
2. Inspect Hydraulic Filters
   • Clogged filters can restrict flow causing loss of hydraulic functions
     
   • Replace filters if dirty or according to maintenance schedule
     
3. Test Engine RPM and Power Output
   • Ensure engine is producing sufficient power for hydraulic pumps
     
   • Low RPM or engine problems can cause insufficient hydraulic pressure
     
4. Verify Hydraulic Pump Operation
   • Listen for pump noise and check for abnormal vibrations
     
   • Use pressure gauges at test ports to check system pressure
     
   • If pressure is zero or very low, pump failure or drive coupling issues may exist
     
5. Check Hydraulic Control Valves and Linkages
   • Confirm valve spools move freely and are not stuck
     
   • Inspect mechanical linkages or electronic controls for faults
     
6. Inspect Final Drives and Travel Motors
   • Look for leaks, damage, or mechanical failures preventing movement
     
   • Test for hydraulic fluid flow to travel motors
     
7. Examine Electrical and Safety Interlocks
   • Verify neutral safety switches and hydraulic lockouts are disengaged
     
   • Check wiring, fuses, and relays related to hydraulic system
     

Case Study: Failed Charge Pump Causing No Movement
A 955H suddenly stopped moving on site while engine ran fine. Pressure testing revealed no system pressure. Technicians traced the issue to a failed charge pump that was not supplying pressure for the main pump’s lubrication and operation. Replacing the charge pump restored hydraulic pressure and movement.
Case Study: Hydraulic Control Valve Seizure
Another 955H suffered total hydraulic loss due to internal valve spool seizure from contaminated fluid. After flushing the system and replacing the valve body, full hydraulic functionality returned. This emphasized the importance of maintaining clean hydraulic fluid and filters.
Maintenance Tips to Avoid Hydraulic and Mobility Failures

• Change hydraulic fluid and filters at recommended intervals
  
• Regularly inspect hoses and fittings for leaks or damage
  
• Monitor engine performance to ensure proper hydraulic pump speed
  
• Keep hydraulic reservoir clean and prevent contamination ingress
  
• Test system pressures periodically to detect early signs of pump or valve wear
  

Anecdote: The Unexpected Small Leak That Ground a Big Machine
An operator once spent hours troubleshooting a 955H that would not move or operate hydraulics. The cause was eventually traced to a tiny pinhole leak in a hydraulic hose under the machine that caused complete pressure loss. The story underscores how minor leaks can cripple major equipment and the need for thorough inspections.
Conclusion
A Caterpillar 955H that won’t move and has no hydraulic function usually indicates issues with hydraulic pressure generation or control. Systematic checks of fluid levels, pump operation, control valves, and final drives will help isolate the cause. Regular maintenance and attention to hydraulic system health are vital to prevent sudden loss of machine functionality and costly downtime.