Investigating Hydraulic Tank Clunking on the Caterpillar 955L Track Loader

[MikePhua]

The 955L and Its Hydraulic System Design
The Caterpillar 955L track loader was introduced in the mid-1970s as part of CAT’s evolution from cable-operated machines to fully hydraulic systems. Built for rugged earthmoving, demolition, and quarry work, the 955L featured a 3304 four-cylinder diesel engine paired with a robust hydraulic system powering lift arms, bucket tilt, and steering clutches. With an operating weight exceeding 30,000 lbs and a breakout force over 20,000 lbs, the 955L became a staple in fleets across North America and Asia.
Its hydraulic system includes a reservoir mounted near the rear of the machine, feeding a gear-type pump via suction lines. Return flow from cylinders and motors re-enters the tank through baffled ports designed to reduce aeration and turbulence. Over time, wear, contamination, and internal component shifts can cause unusual noises—especially clunking sounds—originating from the hydraulic tank.
Identifying the Source of Clunking Sounds
A clunking noise from the hydraulic tank may indicate:

• Loose or broken internal baffles
  
• Detached suction or return lines striking the tank wall
  
• Air ingestion causing cavitation and pressure spikes
  
• Debris or foreign objects inside the reservoir
  
• Excessive fluid movement due to low oil level or foaming
  

Operators often report hearing the sound during startup, shutdown, or sudden hydraulic actuation. In one case from Alberta, a 955L used for gravel loading began producing a metallic clunk when lifting full buckets. After inspection, a broken baffle plate was found floating inside the tank.
Inspection and Diagnostic Procedure
To investigate the issue:

• Drain the hydraulic tank and remove the access cover
  
• Use a flashlight and mirror to inspect internal welds and baffle integrity
  
• Check suction and return lines for secure mounting and proper routing
  
• Look for signs of impact, wear, or metal fragments
  
• Inspect the breather cap and relief valve for proper function
  
• Refill with clean hydraulic fluid and monitor for aeration or foam
  

If the clunking occurs only during specific hydraulic functions, test system pressure and flow at the pump and control valves. Sudden pressure drops or spikes may cause fluid hammer, which can resonate through the tank.
Common Causes and Solutions
Typical culprits include:

• Loose baffle welds: Re-weld or replace the baffle plate
  
• Detached suction line: Re-secure with clamps and inspect for cracks
  
• Air ingestion: Replace suction seals and ensure fluid level is above minimum
  
• Foaming fluid: Switch to anti-foam hydraulic oil and install a tank diffuser
  
• Foreign debris: Flush the tank and install a magnetic drain plug
  

Some operators retrofit internal screens or mesh guards to prevent line movement. Others add rubber dampers to reduce vibration and noise transmission.
Preventive Maintenance and Long-Term Reliability
To avoid future issues:

• Inspect hydraulic tank internals every 1,000 hours
  
• Replace suction filters and strainers every 500 hours
  
• Use high-quality hydraulic oil with anti-wear additives
  
• Monitor fluid temperature and avoid overheating
  
• Install sight gauges and level indicators for quick checks
  

In Shanghai, a contractor added a tank inspection port and LED light to his 955L fleet. This allowed quick visual checks during service intervals and helped identify a loose return elbow before it caused damage.
Field Anecdote and Practical Insight
In Ohio, a demolition crew noticed intermittent clunking from their 955L’s hydraulic tank. After draining the reservoir, they found a broken weld bead and a loose baffle corner. Welding the plate back in place and adding a rubber isolator eliminated the noise. They later added a vibration sensor to the tank wall to monitor future anomalies.
Conclusion
Clunking sounds from the hydraulic tank of a Caterpillar 955L are more than a nuisance—they often signal internal wear or fluid dynamics issues that can compromise system performance. With thorough inspection, targeted repairs, and preventive upgrades, operators can restore quiet operation and protect vital hydraulic components. In demanding environments, every sound tells a story—and the smart operator listens.