Diagnosing Hydraulic Problems in the CAT 963 Track Loader

[MikePhua]

The CAT 963 and Its Engineering Legacy
The Caterpillar 963 is a medium-sized track loader introduced in the 1980s as part of Caterpillar’s push to modernize its crawler loader lineup. Designed for versatility in construction, demolition, and material handling, the 963 combined the ruggedness of a dozer with the lifting capabilities of a loader. It was powered by a turbocharged six-cylinder diesel engine, typically the CAT 3116 or later the 3126, delivering around 150 horsepower depending on the variant.
Caterpillar, founded in 1925, has sold hundreds of thousands of track loaders globally, with the 963 series becoming a staple in fleets across North America, Europe, and Asia. Its hydrostatic drive system and responsive hydraulics made it a favorite for operators working in tight spaces or on uneven terrain.
Hydraulic System Overview and Common Failure Points
The hydraulic system in the CAT 963 controls the loader arms, bucket tilt, and auxiliary functions. It includes:

• Hydraulic pump (gear or piston type)
  
• Control valves and pilot circuits
  
• Lift and tilt cylinders
  
• Hydraulic reservoir and filters
  
• Pressure relief valves and solenoids
  

When hydraulic functions become slow, erratic, or unresponsive, the root cause often lies in one of the following areas:

• Contaminated or degraded hydraulic fluid
  
• Clogged filters or suction screens
  
• Air in the hydraulic lines
  
• Faulty pilot control solenoids
  
• Internal cylinder leakage
  
• Pump wear or cavitation
  

In one case from rural Pennsylvania, a 963 began showing delayed lift response and weak bucket rollback. The issue was traced to a partially collapsed suction hose between the reservoir and pump, which restricted flow and introduced air bubbles. Replacing the hose and bleeding the system restored full function.
Symptoms and Diagnostic Clues
Operators may notice several warning signs when hydraulic issues arise:

• Loader arms hesitate or stall during lifting
  
• Bucket tilt is slow or fails to hold position
  
• Audible whining or chattering from the pump
  
• Hydraulic fluid foaming or discoloration
  
• Machine struggles under load despite normal engine RPM
  

A useful diagnostic approach includes:

• Checking fluid level and condition (should be clear amber, not milky or dark)
  
• Inspecting filters for clogging or bypass activation
  
• Measuring system pressure at test ports (typically 3000–3500 psi)
  
• Observing pilot control response and solenoid activation
  
• Performing cylinder drift tests to detect internal leakage
  

In one field test, a technician used a pressure gauge on the lift circuit and found only 1800 psi under load—far below spec. The culprit was a worn pump shaft seal allowing internal bypass. Replacing the seal and flushing the system resolved the issue.
Pilot Controls and Electrical Interference
Modern variants of the 963 use pilot-operated hydraulic controls, where low-pressure pilot oil activates main valves. These systems rely on solenoids and electrical signals from the joystick or foot pedals. If solenoids fail or wiring becomes corroded, the pilot signal may be weak or intermittent.
Common electrical faults include:

• Broken wires near the control lever base
  
• Corroded connectors at the valve block
  
• Faulty solenoid coils with high resistance
  
• Blown fuses or weak relays
  

In a case from Ontario, a 963 loader refused to lift the bucket despite normal engine and pump operation. After tracing the wiring harness, the technician found a rodent nest had chewed through the pilot signal wire. Splicing in a new wire restored full hydraulic control.
Hydraulic Fluid and Filter Maintenance
To prevent hydraulic issues, operators should follow a strict maintenance schedule:

• Replace hydraulic fluid every 2000 hours or annually
  
• Change filters every 500 hours or sooner in dusty environments
  
• Inspect suction screens during fluid changes
  
• Use OEM-grade fluid with correct viscosity and anti-foam additives
  
• Monitor fluid temperature during heavy use (should stay below 80°C)
  

Using the wrong fluid type can cause seal swelling, pump cavitation, and poor lubrication. In one municipal fleet, switching from generic hydraulic oil to CAT HYDO Advanced 10 extended pump life by 15% and reduced filter clogging.
Cylinder Wear and Rebuild Strategy
Lift and tilt cylinders are subject to high pressure and frequent movement. Over time, seals degrade, rods score, and internal leakage increases. Symptoms include:

• Bucket drift when parked
  
• Jerky movement during lifting
  
• Fluid seepage around gland seals
  

Rebuilding a cylinder involves:

• Removing the cylinder from the frame
  
• Disassembling the gland and piston
  
• Replacing seals, wear bands, and wipers
  
• Honing the barrel if scored
  
• Pressure testing after reassembly
  

A rebuilt cylinder can last another 3000–5000 hours if properly maintained. Always torque gland nuts to spec and use high-quality seal kits.
Operator Stories and Field Wisdom
In Montana, a contractor shared that his CAT 963 began losing hydraulic power during cold mornings. The issue was traced to thickened fluid and a clogged suction screen. Installing a block heater and switching to low-temperature hydraulic oil solved the problem.
Another operator in Finland used his 963 for snow removal and noticed the bucket would not hold tilt under load. After inspecting the tilt cylinder, he found the piston seal had split. Replacing the seal restored full holding power and improved grading accuracy.
Conclusion
Hydraulic issues in the CAT 963 can stem from fluid contamination, pump wear, electrical faults, or cylinder leakage. By following a structured diagnostic process and maintaining fluid quality, operators can restore performance and avoid costly downtime. The 963 remains a workhorse in the field, and with proper care, its hydraulic system can deliver thousands of hours of reliable service. Whether lifting gravel in Georgia or clearing snow in Sweden, the key to smooth operation lies in clean oil, tight seals, and attentive troubleshooting.