Diagnosing and Recovering from Hydraulic Pump Failure on a CAT 953

[MikePhua]

Sudden Hydraulic Loss and the Telltale Whine
A 1984 Caterpillar 953 track loader experienced a sudden loss of hydraulic function following a loud whine that lasted approximately ten seconds. No fluid was found on the ground, but upon inspection, the hydraulic reservoir was nearly full and emitted steam when opened. The fluid had thinned dramatically, resembling water in consistency. These symptoms strongly suggest internal pump failure, likely due to overheating and cavitation.
Terminology Clarification

• Cavitation: The formation of vapor bubbles in a hydraulic fluid due to low pressure, which collapse violently and damage internal surfaces.
  
• Vane Pump: A type of hydraulic pump using sliding vanes to move fluid; known for smooth flow but sensitive to contamination and wear.
  
• Cartridge Kit: A replaceable internal assembly for vane pumps, including the rotor, vanes, and cam ring.
  
• Pressure Relief Valve: A safety component that limits system pressure to prevent damage; if stuck, it can cause excessive heat and noise.
  

Initial Diagnosis and Common Failure Points
The whine followed by hydraulic shutdown points to a vane pump failure, possibly a cracked cam ring or seized rotor. The presence of steam and thinned fluid indicates the pump was running dry or under extreme pressure. A stuck pressure relief valve may have contributed to the overheating. In vane pumps, once the cam ring cracks, fluid bypasses internally, and pressure drops rapidly.
Rebuild vs. Replace
Fortunately, vane pumps are modular. Rather than replacing the entire unit, technicians can install a cartridge kit, restoring function in a few hours. Key steps include:

• Drain and inspect the hydraulic fluid for contamination or discoloration.
  
• Remove the pump and disassemble carefully, noting vane orientation.
  
• Replace the rotor, vanes, and cam ring with a matched kit.
  
• Clean all mating surfaces and reinstall with fresh seals.
  
• Prime the pump before startup to prevent dry running.
  

One technician emphasized the importance of not allowing the pump to run without oil even for a second, as vane tips can seize instantly under dry conditions.
The Mystery of Temporary Recovery
Months later, the same machine was restarted and the hydraulics worked normally. This suggests the failure may have been thermal or pressure-related rather than mechanical. Possible explanations include:

• The pressure relief valve reset after cooling.
  
• Air trapped in the system vented during shutdown.
  
• The fluid regained viscosity after sitting.
  

However, this recovery should not be trusted. If the pump overheated once, internal damage may be progressing silently. Continued use without inspection risks catastrophic failure.
Preventive Measures and Long-Term Solutions

• Replace hydraulic filters and inspect them for metal shavings or seal fragments.
  
• Monitor fluid temperature during operation; install a gauge if needed.
  
• Use high-quality hydraulic oil with anti-foam and anti-wear additives.
  
• Service pressure relief valves annually to prevent sticking.
  
• Consider installing a fluid cooler if operating in hot climates or under heavy load.
  

Conclusion
Hydraulic pump failure on a CAT 953 often begins with subtle signs—noise, heat, and fluid thinning—before complete shutdown. Vane pumps can be rebuilt with cartridge kits, but underlying causes like pressure relief valve malfunction must be addressed. Temporary recovery after shutdown may mask deeper issues. Proactive maintenance and careful diagnostics are essential to avoid repeat failures and costly downtime.