CAT 315L Weak Travel on the Right Track

[MikePhua]

Background of the CAT 315L
The CAT 315L belongs to Caterpillar’s early 300-series hydraulic excavators, introduced in the late 1990s when CAT was transitioning from mechanically controlled systems to more refined hydraulic and pilot control designs. The 315L was positioned as a compact-to-mid-size excavator, typically weighing around 15–16 tons, powered by a CAT 3116 diesel engine producing roughly 100–110 horsepower depending on configuration. During its production run, the 315L became popular with contractors, municipalities, and rental fleets due to its balance of power, reliability, and relatively low operating cost. Thousands of units were sold globally, and many remain in service today, especially in secondary markets, because of their simple structure and strong parts support.
Understanding the Travel System
On the CAT 315L, each track is driven independently by its own hydraulic travel motor and final drive. Oil flow from the main pump is distributed through the travel control valve to the left and right motors. When one track becomes noticeably weaker than the other, the issue is almost always related to unequal hydraulic flow, pressure loss, or mechanical resistance on that side. Because the system is symmetrical by design, any imbalance points directly to a fault rather than normal wear.
Typical Symptoms of a Weak Right Track
Operators often report several consistent behaviors:

• The machine turns easily to one side but struggles to turn the opposite way
  
• The right track stalls or slows under load while the left track pulls strongly
  
• Straight-line travel requires constant joystick correction
  
• Power feels normal for digging and swinging, but weak only in travel
  

These symptoms usually worsen when climbing slopes or pushing against resistance, where travel torque demand can exceed several thousand newton-meters at the final drive.
Hydraulic Causes of Uneven Travel
The most common cause of a weak single track is hydraulic pressure loss on that circuit. Key hydraulic-related possibilities include:

• Internal leakage in the right travel motor
  
• Worn or sticking travel control valve spool
  
• Faulty crossover relief valve bleeding pressure
  
• Damaged pilot control signal reducing valve stroke
  

Even a small internal leak can dramatically reduce available torque. For example, a drop of 10–15 percent in motor pressure can translate into a much larger reduction in track pulling force.
Travel Motor Wear and Internal Leakage
Travel motors on machines of this age often suffer from piston or barrel wear. When internal clearances increase, oil bypasses internally instead of producing torque. This condition rarely causes external oil leaks, which can mislead owners into overlooking the motor. A classic sign is a track that moves fine in the air but becomes weak when the machine is on the ground.
Final Drive and Mechanical Resistance
While hydraulics are the primary suspect, mechanical issues must not be ignored. The final drive includes planetary gears, bearings, and seals that must rotate freely. Problems that can overload the system include:

• Worn or failing final drive bearings
  
• Contaminated or low final drive oil
  
• Excessive track tension increasing rolling resistance
  
• Mud, debris, or damage in the undercarriage
  

A final drive that is mechanically tight can consume hydraulic power, making the track feel weak even if pressure is adequate.
Crossover Relief Valves and Pressure Balance
Each travel circuit is protected by relief valves designed to limit maximum pressure and absorb shock loads. If the relief valve on the right side is stuck slightly open or incorrectly set, oil will escape back to tank before full pressure is achieved. This is a common but often overlooked cause, especially on older machines where valve springs fatigue over time.
Pilot Controls and Operator Input
On the CAT 315L, travel commands are transmitted through pilot pressure. A weak or restricted pilot signal to the right travel spool can prevent full valve opening. Causes include:

• Pilot hose restrictions
  
• Contaminated pilot oil
  
• Worn joystick or pilot valve components
  

In some cases, the issue feels mechanical but is actually caused by reduced pilot pressure, which may only be a few bar lower than normal yet enough to limit travel speed.
Hydraulic Oil Condition and System Health
Hydraulic oil quality plays a major role in travel performance. Machines that have exceeded recommended oil change intervals often show uneven function first in high-load circuits like travel. Metal particles, varnish, or water contamination accelerate wear in motors and valves, compounding power loss over time.
Structured Troubleshooting Strategy
A logical diagnostic process helps avoid unnecessary repairs:

• Compare left and right track performance under identical conditions
  
• Lift the machine and test travel speed in the air versus on the ground
  
• Check final drive oil level and condition on the weak side
  
• Measure travel motor pressure if gauges are available
  
• Inspect pilot pressure consistency to both travel spools
  

This approach helps distinguish between hydraulic loss and mechanical drag.
Repair Options and Cost Considerations
Repair choices depend on the root cause:

• Travel motor reseal or rebuild if internal leakage is confirmed
  
• Relief valve cleaning or replacement if pressure is bleeding off
  
• Final drive service if mechanical resistance is found
  
• Pilot system service for signal-related issues
  

In many real-world cases, rebuilding a travel motor restores full performance at a fraction of the cost of replacing the entire final drive assembly.
Real-World Experience from the Field
A contractor operating a mid-sized CAT excavator reported a similar issue where one track struggled during trench backfill but performed normally in light travel. After months of operation, the problem worsened until the machine could barely pivot. A travel motor teardown revealed worn piston shoes and scoring inside the barrel. After rebuilding, the machine regained full travel power and remained reliable for years. This type of story is common among owners who delay diagnosis, assuming the problem is “just age.”
Why the CAT 315L Is Still Worth Fixing
Despite being an older platform, the CAT 315L remains desirable due to its strong structure, reliable engine, and straightforward hydraulic layout. With proper attention to travel system issues, these machines can continue productive work well beyond 20,000 operating hours. Addressing a weak track early prevents secondary damage to pumps, valves, and undercarriage components.
Key Technical Terms Explained
Travel motor
A hydraulic motor that drives the track through the final drive
Final drive
A gear reduction system that multiplies motor torque to move the track
Pilot pressure
Low-pressure hydraulic signal used to control main valves
Crossover relief valve
A valve that limits maximum pressure and absorbs shock loads in travel circuits
Internal leakage
Oil bypassing within a hydraulic component without external leakage
Track tension
The preload applied to the track chain, affecting rolling resistance
Long-Term Maintenance Recommendations
To prevent future travel imbalance:

• Maintain correct track tension
  
• Change hydraulic oil and filters on schedule
  
• Monitor final drive oil regularly
  
• Address minor travel differences early
  

With disciplined maintenance, the CAT 315L can remain a dependable machine, and a weak right track can often be resolved before it turns into a major failure.