Diagnosing Hydraulic Failure on the CAT 951C Track Loader

[MikePhua]

The CAT 951C and Its Hydraulic System
The Caterpillar 951C track loader was introduced in the late 1970s as part of Caterpillar’s evolution from cable-operated machines to fully hydraulic systems. With an operating weight of around 30,000 lbs and powered by a turbocharged diesel engine, the 951C was designed for heavy-duty earthmoving, land clearing, and material loading. Its hydraulic system drives the lift arms, bucket tilt, and auxiliary functions, relying on a Vickers 19 GPM cartridge pump mounted at the front-left side of the engine.
Caterpillar, founded in 1925, had by this time become the global leader in tracked loaders, and the 951C was widely adopted across North America and Europe. Its rugged frame and modular design made it a favorite among farmers, contractors, and municipal fleets.
Terminology Notes

• Cavitation: A condition where air enters the hydraulic pump, causing it to whine and lose pressure.
  
• Float Position: A hydraulic control setting that allows the lift arms to move freely, used during maintenance or transport.
  
• Pump Cartridge: A replaceable internal component of the hydraulic pump that handles fluid displacement.
  
• Belly Pan: A protective plate under the machine that can trap hydraulic fluid during leaks.
  

Symptoms and Initial Observations
The operator reported that the hydraulics failed while lifting a load near a tree stump. The pump began to whine, and the bucket stopped responding. After refilling the hydraulic reservoir, the system briefly recovered but failed again under load. Eventually, the machine lost all hydraulic function.
Key symptoms included:

• Whining noise from the pump
  
• Loss of lift and tilt functions
  
• Hydraulic fluid visibly leaking from low-pressure hose couplings
  
• Reservoir repeatedly low despite refills
  

Diagnosis and Field Strategy
The most likely causes were:

• Air intrusion due to low fluid levels, leading to cavitation
  
• Loose or damaged supply lines, especially between the tank and pump
  
• Pump mounting bolts missing, allowing the pump to rotate and disengage from the drive shaft
  
• Internal blockage or collapsed hose, restricting fluid intake
  

Recommended steps:

• Inspect and tighten all hydraulic pump mounting bolts
  
• Check supply lines for cracks, leaks, or disconnections
  
• Remove and inspect the hydraulic filter; cut open the pleats to check for metal debris
  
• Drain and clean the belly pan to detect hidden leaks
  
• Engage float position to safely raise the loader arms for access
  

Operator Anecdotes and Practical Advice
One operator discovered both pump mounting bolts had fallen out, allowing the pump to rotate and disengage from the splined drive shaft. After reinstalling the bolts, the hydraulics returned to normal. Another technician noted that the pump’s intake line is unfiltered and vulnerable to blockage from tank debris or collapsed internal hoses.
A mechanic in the UK explained that the hydraulic return line inside the tank includes a rubber hose that, if damaged, can froth the oil and cause pump starvation. He recommended splitting the tank open if frothing persists despite repairs.
A farmer in Wisconsin shared that his 951C had a similar issue during winter. Ice buildup in the belly pan masked a major leak, and only after thawing and cleaning did the source become visible.
Preventive Measures and Long-Term Solutions

• Torque pump mounting bolts to spec and inspect monthly
  
• Replace worn or cracked hoses, especially those inside the tank
  
• Install a sight gauge on the reservoir to monitor fluid levels easily
  
• Flush the hydraulic system annually to remove contaminants
  
• Keep a spare filter and pump cartridge in the field kit for emergency repairs
  

Conclusion
Hydraulic failure on the CAT 951C often stems from air intrusion, loose pump mounts, or internal line damage. While the symptoms may mimic pump failure, the root cause is frequently mechanical or fluid-related. By inspecting mounting bolts, checking intake lines, and analyzing filter debris, operators can restore function and prevent future breakdowns. In legacy machines like the 951C, every bolt and hose tells a story—and keeping them tight keeps the story going.