CAT 963A Loses Drive Pressure on Right Track

[MikePhua]

The 963A and Its Hydrostatic Drive System
The Caterpillar 963A track loader was introduced in the late 1980s as part of Cat’s push to modernize crawler loaders with hydrostatic drive technology. Powered by a 125-horsepower diesel engine and weighing over 38,000 pounds, the 963A was designed for grading, loading, and site prep in rugged terrain. Caterpillar, founded in 1925, had already dominated the dozer and loader markets, and the 963A helped bridge the gap between versatility and power.
Unlike older mechanical drive loaders, the 963A uses a hydrostatic transmission—meaning hydraulic pressure drives each track independently. This system offers smooth control, variable speed, and precise maneuvering. However, it also introduces complexity, and when one side loses drive pressure, troubleshooting becomes a layered process.
Terminology Notes

• Hydrostatic Transmission: A system using hydraulic pumps and motors to transmit power to the tracks
  
• Drive Motor: A hydraulic motor mounted near each final drive that converts fluid pressure into rotational movement
  
• Charge Pressure: The baseline hydraulic pressure that feeds the system and prevents cavitation
  
• Servo Valve: A control valve that regulates flow to the drive motors based on joystick input
  
• Case Drain: A low-pressure return line that allows internal leakage from hydraulic components to exit safely
  

Symptoms of Pressure Loss on One Track
When the right track of a 963A fails to build drive pressure, operators may notice:

• No response when pushing the joystick forward or backward
  
• The left track functions normally, allowing pivot turns but no straight movement
  
• Audible whining or cavitation sounds from the hydraulic pump
  
• Fluid level remains normal, but movement is inconsistent or absent
  
• No diagnostic codes if the machine lacks electronic monitoring
  

In 2021, a contractor in Texas reported that his 963A would spin left but refused to move forward. After cleaning the machine and checking fluid levels, he discovered a failed servo valve on the right side, which had jammed due to contamination.
Diagnostic Strategy and Inspection Sequence
To isolate the issue:

• Check hydraulic fluid level and condition—milky or foamy fluid may indicate air ingress
  
• Inspect the charge pressure at the pump using a test port—should be within spec (typically 300–600 psi)
  
• Compare drive pressure between left and right motors using gauges
  
• Examine the servo control linkage and electrical connections
  
• Inspect the case drain flow—excessive flow may indicate internal leakage in the motor
  
• Remove and inspect the right drive motor for wear, scoring, or seal failure
  

If the motor is receiving pressure but not turning, internal damage is likely. If no pressure is reaching the motor, the issue may lie in the pump, servo valve, or control circuit.
Common Failure Points and Repair Options
Frequent causes of drive pressure loss include:

• Servo Valve Failure
  Dirt or wear can cause the valve to stick or misdirect flow. Rebuild kits are available, but replacement is often faster.
  
• Drive Motor Internal Leak
  Worn seals or damaged pistons allow pressure to bypass, reducing torque. Rebuilding requires precision machining and clean conditions.
  
• Pump Output Imbalance
  A worn swashplate or broken spring in the hydrostatic pump can reduce flow to one side. Full pump rebuild may be necessary.
  
• Contaminated Fluid
  Metal shavings or water in the hydraulic fluid can damage valves and motors. Flushing the system and replacing filters is essential.
  
• Electrical or Mechanical Linkage Issues
  If the joystick or control rod is misaligned, the servo valve may not receive proper input.
  

In 2023, a fleet manager in Alberta replaced a right-side drive motor after discovering excessive case drain flow. The rebuilt motor restored full drive pressure, and the machine returned to service with improved responsiveness.
Preventative Measures and Long-Term Reliability
To prevent future failures:

• Change hydraulic filters every 500 hours
  
• Sample fluid for contamination every 1,000 hours
  
• Keep the reservoir sealed and clean during service
  
• Avoid aggressive joystick movements under load
  
• Train operators to recognize early signs of pressure loss
  

For machines operating in dusty or wet environments, consider installing magnetic suction screens and desiccant breathers to protect fluid integrity.
Conclusion
Loss of drive pressure on one track of a CAT 963A is a serious issue, but it’s often solvable with methodical diagnostics and targeted repair. Whether it’s a servo valve, drive motor, or pump imbalance, each component plays a role in the hydrostatic system’s performance. With proper maintenance and attention to fluid health, the 963A can continue to deliver reliable power and precision on the jobsite.