Diagnosing Slow Tracking on a Kobelco SK115SRDZ

[MikePhua]

Kobelco’s Compact Excavator Lineage
Kobelco Construction Machinery, a division of Kobe Steel founded in 1930, has long been recognized for its innovation in hydraulic excavators. The SK115SRDZ is part of Kobelco’s SR series, designed for reduced tail swing and urban maneuverability. With an operating weight around 11.5 metric tons and a 70–80 horsepower engine, the SK115SRDZ balances compact dimensions with full-size digging power. It features a two-speed travel system, pilot-controlled hydraulics, and a robust undercarriage suited for tight job sites and utility work.
Kobelco’s SRDZ designation typically refers to models equipped with zero tail swing and enhanced hydraulic dampening. These machines are popular in North America and Asia, with thousands sold since the early 2000s. Their reputation for smooth operation and fuel efficiency makes them a staple in rental fleets and owner-operator businesses.
Symptoms of Slow Tracking on Both Sides
When an SK115SRDZ begins tracking slowly on both sides, especially struggling to climb even slight grades, the issue is likely systemic rather than isolated to one travel motor. Operators may notice that switching from low to high travel speed yields only a marginal improvement, and other hydraulic functions—such as boom, arm, and bucket—continue to operate normally.
Terminology annotation:
- Tracking: The movement of the excavator via its crawler tracks, controlled by hydraulic motors. - Travel Speed Selector: A switch or lever that toggles between low and high-speed travel modes. - Gradeability: The machine’s ability to climb slopes, influenced by torque, traction, and hydraulic output.
This pattern suggests a reduction in hydraulic flow or pressure affecting both travel circuits simultaneously. Unlike a single motor failure, which would cause asymmetric tracking, bilateral slowness points to upstream issues in the hydraulic system.
Hydraulic Flow and Travel Motor Dependencies
The SK115SRDZ uses a variable displacement hydraulic pump to supply fluid to all functions, including travel. Travel motors are typically fed through a dedicated circuit with flow priority during movement. If the pump is underperforming or the travel circuit is restricted, both motors will suffer.
Potential causes include:

• Weak or worn hydraulic pump unable to maintain flow under load
  
• Clogged main hydraulic filter reducing system pressure
  
• Faulty travel speed solenoid or selector valve
  
• Contaminated pilot lines affecting control signal strength
  
• Internal leakage in the control valve block reducing effective pressure
  

Terminology annotation:
- Variable Displacement Pump: A hydraulic pump that adjusts output flow based on demand, improving efficiency. - Pilot Line: A low-pressure hydraulic line that transmits control signals to actuators and valves. - Solenoid Valve: An electrically actuated valve used to control hydraulic flow direction or speed.
One overlooked issue is the travel speed selector itself. If the solenoid controlling high-speed travel is stuck or receiving weak voltage, the machine may remain in low-speed mode despite operator input. Testing voltage at the solenoid during activation can confirm functionality.
Testing and Diagnostic Recommendations
To isolate the problem, technicians should:

• Check hydraulic fluid level and condition for contamination or aeration
  
• Inspect and replace the main hydraulic filter if clogged
  
• Measure system pressure at the travel circuit using a test port
  
• Verify travel speed solenoid voltage and resistance
  
• Inspect pilot pressure at the travel control valve
  

If pressure readings are below specification, the pump may be worn or the relief valve may be stuck open. Kobelco’s service manual provides target pressures for travel circuits—typically around 4,000 psi under load. A deviation of more than 10% can cause noticeable performance loss.
Field Anecdotes and Practical Fixes
One operator in New York reported that his SK115SRDZ tracked slowly despite normal function elsewhere. After replacing the hydraulic filter and cleaning the travel speed solenoid connector, the issue resolved. The solenoid had corroded internally, causing intermittent signal loss.
Another technician shared that a similar issue on a Komatsu PC120 turned out to be a partially collapsed pilot line, restricting control pressure to the travel valve. Replacing the line restored full travel speed and gradeability.
Preventative Measures and Long-Term Solutions
To maintain optimal travel performance:

• Replace hydraulic filters every 500 operating hours
  
• Inspect solenoid connectors for corrosion and secure grounding
  
• Use OEM hydraulic fluid to maintain viscosity and additive compatibility
  
• Flush pilot lines during major service intervals
  
• Monitor travel motor case drain flow to detect internal leakage
  

Installing a pressure gauge kit with quick-connect fittings allows operators to monitor system health without disassembly. Some fleets retrofit travel speed indicators to confirm mode engagement during operation.
Conclusion
Slow tracking on both sides of a Kobelco SK115SRDZ is rarely a coincidence—it’s a signal of hydraulic imbalance, control failure, or pump degradation. By methodically testing pressure, inspecting filters, and verifying solenoid function, operators can restore full mobility and avoid costly downtime. In compact excavators, movement is everything—and when the tracks hesitate, the diagnosis must be precise.