Diagnosing Uneven Track Power in Hydraulic Excavators

[MikePhua]

The Rise of the Cat 312 and Its Hydraulic Architecture
The Caterpillar 312 hydraulic excavator was introduced in the mid-1990s as part of Cat’s push into the 12–14 ton class, offering a balance of reach, power, and transportability. With a swing radius suited for urban and utility work, the 312 became a staple in rental fleets and owner-operator businesses. Its hydraulic system, built around pilot-assisted controls and dual travel motors, was designed for smooth operation and reliable tracking.
Caterpillar’s hydraulic architecture in this era relied heavily on rotary manifolds, pilot valves, and cross-port reliefs to manage flow between functions. While robust, these systems are sensitive to seal degradation, spool wear, and hose collapse—especially after 10,000 hours of service.
Common Causes of One Weak Track
When one track on an excavator becomes noticeably weaker—either in speed, torque, or responsiveness—it signals an imbalance in hydraulic delivery. This can manifest as sluggish movement, difficulty climbing inclines, or inconsistent tracking during turns.
Typical culprits include:

• Hardened or leaking O-rings in the rotary manifold
  
• Collapsed internal lining of a hydraulic hose
  
• Worn travel motor components
  
• Malfunctioning cross-port relief valves
  
• Stroke limitation in the main control valve spool
  
• Pressure loss due to pump wear or internal leakage
  

In one documented case, a John Deere 690B exhibited similar symptoms due to a collapsed hose interior. The outer hose appeared intact, but the inner lining had delaminated, restricting flow to the travel motor.
Terminology Clarification

• Rotary Manifold (Swivel Joint): A rotating hydraulic coupling that allows fluid to pass between the upper and lower structures of the excavator.
  
• Cross-Port Relief Valve: A valve that limits pressure spikes between two opposing hydraulic ports, often used in travel circuits.
  
• Pilot Control: A low-pressure hydraulic system that actuates main valves via joystick input.
  
• Travel Motor: A hydraulic motor mounted in each track drive, converting fluid pressure into rotational movement.
  

Diagnostic Strategy and Pressure Testing
To isolate the fault, technicians should begin with pressure testing at the travel motor ports. This involves:

• Installing gauges on both forward and reverse lines for each track
  
• Comparing pressure readings under load and during idle
  
• Observing pressure drop when joystick is released
  
• Swapping cross-port relief valves to see if the issue changes direction
  
• Lifting each track off the ground and timing full-speed rotation cycles
  

If the weak track changes sides when the house is rotated 180 degrees, the issue may lie in the rotary manifold seals. If the weakness persists regardless of orientation, the fault is likely downstream—either in the motor, hose, or valve bank.
Component Replacement and Cautionary Tales
Replacing a travel motor may resolve the issue, but it can also expose hidden damage elsewhere. In one case, a Cat 350 had a weak track due to motor failure. After replacement, the same side seized up within 24 hours—revealing broken final drive gear teeth that had gone unnoticed. The repair escalated from $6,000 to over $20,000.
Recommendations:

• Inspect final drives for metal debris before motor replacement
  
• Use OEM or high-quality remanufactured motors with warranty
  
• Replace hoses showing signs of external wear or internal collapse
  
• Test relief valve settings against factory specs
  

Swivel Joint and Seal Integrity
The rotary manifold is a common failure point, especially in older machines. Seals inside the swivel can harden over time, leading to internal bypass. This reduces pressure to one side of the travel circuit, causing weak tracking.
To confirm:

• Monitor pressure drop across the swivel during travel
  
• Check for fluid seepage or contamination around the manifold
  
• Replace seals with factory kits and verify torque specs during reassembly
  

A technician in British Columbia reported that replacing the swivel seals on a PC200 restored full travel power after months of intermittent weakness.
Preventive Maintenance and Long-Term Solutions
To avoid one-track weakness:

• Replace hydraulic fluid every 1,000 hours and test for contamination
  
• Inspect hoses annually for internal collapse using borescope or flow testing
  
• Cycle travel motors monthly during off-season storage
  
• Keep rotary manifold seals fresh with scheduled rebuilds every 5,000 hours
  
• Train operators to avoid sharp turns under full load, which stresses relief valves
  

For machines operating in cold climates, use low-viscosity hydraulic fluid and warm up systems before full operation. Cold seals are more prone to cracking and bypass.
Conclusion
A weak track on an excavator is rarely caused by a single component. It’s often the result of cumulative wear across hoses, valves, motors, and seals. By approaching the issue systematically—starting with pressure testing and isolating variables—technicians can restore balanced travel and prevent costly misdiagnoses. In hydraulic systems, precision matters, and even a small leak or restriction can throw the entire machine off track.