CAT 315 Hydraulic Pump Failure Diagnosis Pressure Loss and Component Rebuild Strategy

[MikePhua]

The CAT 315 and Its Excavator Lineage
The Caterpillar 315 hydraulic excavator was introduced in the mid-1990s as part of CAT’s 300-series, designed to serve as a mid-size machine for trenching, site prep, and utility work. With an operating weight around 15 metric tons and powered by a turbocharged diesel engine producing roughly 100 horsepower, the 315 offered a balance of reach, lifting capacity, and fuel efficiency. Its hydraulic system was built around a variable displacement axial piston pump, capable of delivering precise flow and pressure to multiple circuits simultaneously.
Caterpillar’s 300-series excavators became a global standard, with the 315 selling in large volumes across North America, Europe, and Asia. Later variants such as the 315C and 315D introduced electronic controls and emissions upgrades, but the core hydraulic architecture remained largely consistent.
Terminology Annotation:

• Axial Piston Pump: A hydraulic pump where pistons move parallel to the drive shaft, used for high-pressure and variable flow applications.
  
• Variable Displacement: A pump design that adjusts output flow based on system demand, improving efficiency.
  
• Hydraulic Circuit: A network of valves, hoses, and actuators powered by pressurized fluid.
  

In one sewer installation project in Ontario, a CAT 315 was used to trench through clay and gravel, praised for its smooth hydraulic response and low fuel consumption.
Symptoms of Main Pump Failure
When the main hydraulic pump begins to fail, the machine may exhibit:

• Slow or weak boom and stick movement
  
• Loss of travel power or inability to climb grades
  
• Audible whining or cavitation noise from the pump housing
  
• Hydraulic fluid overheating or foaming
  
• Inconsistent pressure readings at test ports
  

These symptoms often point to internal wear, contamination, or control valve malfunction. In older machines, pump degradation can be gradual, masked by operator compensation until performance drops below usable levels.
Terminology Annotation:

• Cavitation: The formation of vapor bubbles in fluid due to pressure drop, causing noise and damage.
  
• Test Port: A designated location for measuring hydraulic pressure using a gauge.
  
• Control Valve: A component that directs fluid flow to specific actuators based on operator input.
  

In one grading job in Texas, a 315 lost swing power mid-operation. The root cause was traced to a worn swash plate inside the pump, which had lost its ability to maintain piston stroke under load.
Diagnostic Procedure and Pressure Testing
To confirm main pump failure, technicians should perform a structured diagnostic:

• Check hydraulic fluid level and condition (look for discoloration, metal particles, or emulsification)
  
• Inspect suction and return filters for clogging or bypass activation
  
• Measure system pressure at multiple test ports during idle and full load
  
• Compare readings to factory specifications (typically 4,500 psi peak for the 315)
  
• Use infrared thermometer to check for localized overheating
  
• Inspect pilot pressure circuit for proper control signal delivery
  

Recommendations:

• Use a calibrated pressure gauge with quick-connect fittings
  
• Record readings under multiple functions (boom lift, travel, swing)
  
• Flush system before replacing pump to prevent contamination
  
• Replace filters and fluid after pump installation
  

Terminology Annotation:

• Swash Plate: A tilted plate inside an axial piston pump that controls piston stroke length and fluid displacement.
  
• Pilot Pressure: A low-pressure signal used to actuate main hydraulic valves.
  
• Bypass Activation: A condition where fluid is rerouted due to filter clogging, often indicated by warning lights or reduced flow.
  

In one municipal fleet in Wisconsin, a CAT 315 was restored to full function after replacing the pump and recalibrating the pilot control valve, improving cycle times and reducing fuel use.
Pump Removal and Rebuild Strategy
Replacing or rebuilding the main pump involves:

• Draining hydraulic fluid and disconnecting battery
  
• Removing access panels and disconnecting hoses
  
• Unbolting the pump from the engine bell housing
  
• Inspecting drive coupler and shaft for wear or misalignment
  
• Installing new pump with torque specs and alignment checks
  
• Priming system and bleeding air from high points
  

Rebuild options include:

• Replacing piston shoes, swash plate, and valve plate
  
• Honing cylinder barrel and inspecting for scoring
  
• Installing new seals and pressure compensator springs
  
• Bench testing for flow and pressure before reinstallation
  

Terminology Annotation:

• Drive Coupler: A flexible connector between engine and pump shaft, transmitting rotational force.
  
• Valve Plate: A machined surface that controls fluid entry and exit in a piston pump.
  
• Pressure Compensator: A mechanism that adjusts pump output based on system demand.
  

In one restoration project in Alberta, a rebuilt pump was installed with upgraded seals rated for synthetic fluid, extending service intervals and improving cold-start performance.
Preventative Maintenance and Fluid Care
To extend pump life and avoid future failures:

• Change hydraulic fluid every 1,000 hours or annually
  
• Replace suction and return filters every 500 hours
  
• Use fluid with anti-wear, oxidation resistance, and seal compatibility
  
• Monitor for early signs of pressure loss or actuator hesitation
  
• Keep a maintenance log with pressure readings and fluid samples
  

In one excavation firm in Florida, implementing a fluid sampling program reduced hydraulic failures by 60% and allowed predictive maintenance scheduling.
Conclusion
Main pump failure in a CAT 315 is a critical issue that affects every function of the machine. With structured diagnostics, proper rebuild techniques, and disciplined fluid care, the excavator can return to full performance. In hydraulic systems, pressure is power—and the pump is the heart that keeps it flowing.