Impact of One Weak Travel Motor on the Other: Excavator Mobility Explained

[MikePhua]

Introduction: When One Tread Slows, the Whole Machine Suffers
If you notice one side of your excavator tracks dragging or operating sluggishly, it’s tempting to assume the fault lies solely with that travel motor. But excavator drive systems are interdependent—and one weak motor can create symptoms or stress on the other side or the overall system. Here’s how and why.
Hydraulic Pressure Relationships and Case Drain Flow
Each travel motor receives hydraulic flow via the machine’s central swivel manifold. If one motor develops internal wear or excessive leakage—often measured via elevated case drain flow—it can impair system pressure. Excessive internal leakage reduces available pressure to both motors, potentially causing both to run weakly—even if one is mechanically sound. The case drain flow benchmark is usually 2–10 % of input fluid; higher flow indicates worn components that impair motor torque.
Swivel Joint Leaks Masquerading as Motor Failure
Seals within the swivel joint (the rotating connection carrying hydraulic fluid to both sides) often begin to leak after around 5,000 service hours. A leak here reduces flow to one or both travel circuits. What seems like a weak motor may actually stem from a leaking (and restricting) swivel joint—so replacing the "failing" motor won't solve the real issue.
Charge Pump Strength and Speed Mode Control
A failing charge pump delivering insufficient pilot pressure may prevent full pressure into the travel motor’s internal brake. This mimics a weak motor—especially under load. Similarly, if a speed‑control valve is stuck (for example, locked in high‑speed mode), one travel side may underperform, putting stress on the other and causing steering pull or differential speed.
Mechanical Friction and Track Tension Effects
Too-tight tracks or worn undercarriage components increase mechanical friction. One motor working harder than the other may overheat or lose power prematurely, especially if one side has stiffer track tension, leading to uneven wear and torque imbalance.
Scenarios: One Side Weak, Both Affected

• Uneven travel speed: Weak side stalls under load; the machine pulls toward stronger side.
  
• Reduced maximum speed / climbing capacity: Overall torque drops.
  
• Unbalanced load strain: The healthy motor works harder, overheating or increasing wear risk.
  

This illustrates that system imbalance often stems outside the motor itself.
Checklist: Diagnosing the Root Cause

• Check case drain flow: with the motor stalled, cap the drain port and measure the outflow. Excessive flow indicates internal wear—not external drag.
  
• Inspect swivel joint seals for leaks or contamination limiting flow.
  
• Test charge pump pressure: low pilot pressure can mimic short motor stroke or internal brake engaged.
  
• Verify track tension: check alignment, idler wear, and track condition.
  
• Confirm speed control logic: ensure no one motor is locked in high-speed (low torque) mode.
  

Only after these steps should you consider motor teardown or replacement.
Real‑World Case: Diagnosing a Slow Right Side
A rental company reported an excavator with a sluggish right track and pull to the left. Inspection found the swivel joint seal leaking, reducing flow to both sides. Replacing the seals and purging contamination restored both motors' hydraulic pressure—and mobility returned. The right motor was mechanically fine but starved hydraulically.
Conclusion: Holistic Troubleshooting Saves Time and Cost
A weak or failing travel motor rarely operates in isolation. Often, system leaks, low pressure, or uneven mechanical load cause multiple symptoms that mislead diagnosis. Effective resolution comes from testing case drain levels, inspecting swivel joints, verifying charge pump and speed control systems, and checking mechanical drag or track tension—before replacing hardware. By diagnosing correctly, you avoid unnecessary motor rebuilds and restore full mobility efficiently.
Glossary of Terms

• Travel Motor: A hydraulic motor that drives an excavator track via the final‑drive.
  
• Final Drive: Gearbox assembly that multiplies torque from the travel motor to drive the track.
  
• Case Drain Flow: Hydraulic leak flow that exits the motor’s internal case; excess suggests wear.
  
• Swivel Joint: Rotating hydraulic manifold under the cab that supplies both travel circuits.
  
• Charge Pump: A small hydraulic pump providing reservoir pressure to operate travel motor relief/brake valves.