Diagnosing Tracking Imbalance in the Doosan DS55 Mini Excavator

[MikePhua]

The DS55 and Doosan’s Compact Equipment Lineage
The Doosan DS55 mini excavator is part of the company’s compact zero-tail-swing series designed for urban excavation, utility trenching, and landscaping. Doosan, a South Korean manufacturer with roots dating back to 1896, entered the compact equipment market aggressively in the early 2000s, acquiring Daewoo Heavy Industries and later Bobcat. The DS55, with its nimble footprint and hydraulic pilot controls, was built to compete with machines like the Kubota U55 and the John Deere 50G.
Equipped with a two-speed travel motor system and a center-mounted swing valve, the DS55 offers decent breakout force and maneuverability. However, like many compact excavators, it can develop tracking inconsistencies over time—especially after component replacements or hydraulic wear.
Terminology annotation:

• Travel motor: A hydraulic motor mounted on each track that drives the machine forward or backward.
  
• Swash plate: An angled plate inside a hydraulic pump or motor that controls fluid displacement and direction.
  
• Center swing valve: A hydraulic valve that manages flow to the swing motor and sometimes influences travel circuits.
  
• Stop screw: A mechanical limiter that restricts the range of motion of internal hydraulic components.
  

Symptoms of Uneven Tracking
In one field case, a DS55 with serial number 50054 exhibited persistent leftward drift while traveling straight. The operator had to constantly correct the controls, and in reverse, the machine veered sharply in a leftward arc. This behavior persisted even after replacing the left-side travel motor, suggesting the issue was not isolated to the motor itself.
Such symptoms typically point to one of the following:

• Hydraulic imbalance between left and right circuits
  
• Internal leakage or bypassing in the center swing valve
  
• Misadjusted swash plate angle or stop screw
  
• Residual contamination in the hydraulic lines
  

Component Replacement and Persistent Drift
Replacing a failed travel motor is a common fix for tracking issues. However, if the new motor is not matched precisely in displacement or internal calibration to the original, imbalance can occur. Even OEM motors can vary slightly in performance due to wear or manufacturing tolerances.
In this case, the left motor was replaced, but the drift remained. The technician then resealed the center swing valve, suspecting internal leakage. Unfortunately, this made no difference, indicating the root cause lay deeper in the hydraulic control logic or mechanical calibration.
Swash Plate Adjustment and Hydraulic Tuning
The swash plate inside each travel motor determines the volume and direction of hydraulic flow. If one plate is misaligned or restricted by a stop screw, the motor may produce less torque or speed than its counterpart. Adjusting swash plates is a delicate process, typically reserved for factory calibration or advanced field service.
To investigate:

• Measure flow rates to each motor under load using a hydraulic flow meter
  
• Compare pressure readings at both motors during forward and reverse travel
  
• Inspect stop screws for signs of tampering or misalignment
  
• Check for debris or scoring on the swash plate surface
  

One technician in Georgia shared that after replacing a travel motor on a similar Doosan model, he had to recalibrate the swash plate angle using a factory jig to eliminate drift. Without proper tools, this adjustment can be risky and may void warranty coverage.
Valve Block and Circuit Symmetry
The center swing valve, while primarily responsible for boom rotation, may influence travel behavior if its internal seals or spool alignment affect shared hydraulic paths. Resealing the valve is a good first step, but if the spool is worn or the housing is warped, flow irregularities can persist.
Operators should:

• Inspect valve spool for scoring or uneven wear
  
• Verify seal integrity and correct installation
  
• Check for cross-port leakage using pressure gauges
  
• Confirm that valve actuation does not interfere with travel circuits
  

In some cases, replacing the entire valve block may be more effective than resealing, especially if the unit has seen extensive use or contamination.
Additional Considerations and Field Solutions
Other factors that can contribute to tracking imbalance include:

• Uneven track tension or sprocket wear
  
• Contaminated hydraulic fluid affecting valve response
  
• Electrical control signal interference (in electronically modulated systems)
  
• Frame misalignment due to impact or structural fatigue
  

To resolve persistent drift:

• Perform a full hydraulic flush and filter replacement
  
• Calibrate travel motor response using diagnostic software (if available)
  
• Replace both travel motors as a matched set to ensure symmetry
  
• Consult Doosan technical bulletins for known issues or updates
  

Conclusion
Tracking imbalance in the Doosan DS55 mini excavator is a multifaceted issue that may persist even after replacing major components. By examining swash plate alignment, valve block integrity, and hydraulic flow symmetry, technicians can isolate the root cause and restore balanced travel. The DS55 remains a capable machine in tight quarters, but like all compact excavators, it demands precision in hydraulic maintenance and component matching. With careful diagnosis and methodical repair, even stubborn drift can be corrected—bringing the machine back on track, literally and figuratively.