Why Does the Kobelco SK135SRLC Track to the Left in High Speed

[MikePhua]

Background of the Kobelco SK135SRLC
The Kobelco SK135SRLC is a short-radius hydraulic excavator introduced in the early 2000s as part of Kobelco’s push toward urban-friendly, fuel-efficient machines. Kobelco Construction Machinery Co., Ltd., founded in 1930 in Japan, has long been a pioneer in hydraulic excavator development. By the time the SK135SRLC was released, Kobelco had already established itself as a global leader in compact and mid-sized excavators, with sales exceeding 50,000 units annually across North America, Europe, and Asia.
The SK135SRLC was designed for tight job sites, offering a reduced tail swing radius without compromising digging depth or lifting capacity. It features a Tier II-compliant engine, advanced hydraulic circuitry, and a two-speed travel motor system—commonly referred to as “turtle” (low speed) and “rabbit” (high speed) modes. These modes are controlled via pilot-operated travel levers, which engage the travel motors through a series of proportional valves and pressure regulators.
Symptoms of the Travel Issue
A recurring issue reported with some SK135SRLC units is a tendency to veer or track to the left when operating in high-speed mode. In low-speed mode, the machine behaves normally, but when switched to rabbit speed, the right track appears underpowered, resulting in a leftward drift. The problem intensifies as the hydraulic oil warms up, suggesting a thermal sensitivity in the system. Operators have noted that the machine struggles to perform pivot turns or 360-degree rotations in high speed, and in some cases, the engine bogs down under travel load until the levers are released.
Understanding the Hydraulic Travel System
The SK135SRLC uses a dual travel motor setup, each independently controlled by hydraulic flow from the main pump. These motors are axial piston types with variable displacement, allowing for speed modulation. The travel motors receive flow through a travel valve group, which includes:

• Proportional solenoid valves
  
• Pressure-reducing valves
  
• Shuttle valves
  
• Speed selector solenoids
  

In rabbit mode, the system increases pump output and shifts the motors to high displacement. If one side receives less flow or pressure—due to valve malfunction, internal leakage, or electronic control failure—the machine will track unevenly.
Potential Causes of Leftward Tracking
Several root causes can contribute to this behavior:

• Solenoid Failure: The speed selector solenoid for the right travel motor may be malfunctioning, preventing full displacement shift in high speed.
  
• Valve Block Contamination: Debris or varnish buildup in the travel valve block can restrict flow, especially when oil is hot and viscosity drops.
  
• Pump Output Imbalance: The main hydraulic pump may be delivering uneven flow due to wear or internal leakage, especially if the pump compensator is misadjusted.
  
• Travel Motor Wear: The right travel motor could have worn swash plates or piston shoes, reducing torque output under load.
  
• Electrical Control Issues: Faulty wiring or intermittent signal loss to the right motor’s control solenoid can cause erratic behavior.
  

Diagnostic Strategy
To isolate the issue, a structured diagnostic approach is recommended:

1. Pressure Testing: Install test ports on both travel motor lines and compare pressure readings in turtle and rabbit modes. A significant drop on the right side indicates flow restriction or motor inefficiency.
   
2. Solenoid Function Check: Use a multimeter to verify voltage and resistance across the speed selector solenoids. Activate manually to confirm mechanical response.
   
3. Valve Block Inspection: Remove and inspect the travel valve block for contamination, scoring, or stuck spools. Clean and replace O-rings as needed.
   
4. Pump Flow Test: Conduct a flow meter test at rated RPM to ensure both circuits are receiving equal output. Compare against factory specs (typically 2 × 130 L/min for this model).
   
5. Motor Case Drain Analysis: Measure case drain flow from both travel motors. Excessive flow from the right motor suggests internal leakage.
   

Real-World Anecdote
A contractor in Oklahoma reported similar symptoms on a 2001 SK135SRLC used for pipeline trenching. Initially, the machine tracked fine, but after 30 minutes of operation, it began dragging left in rabbit mode. After replacing the travel solenoids and flushing the hydraulic system, the issue persisted. Eventually, a teardown of the right travel motor revealed worn piston shoes and a cracked valve plate—likely caused by years of operation in dusty conditions without regular filter changes. After replacing the motor and recalibrating the pump compensator, the machine returned to normal function.
Preventative Measures
To avoid similar issues, operators and technicians should implement the following:

• Hydraulic Oil Maintenance: Replace oil every 2,000 hours or annually, whichever comes first. Use ISO 46 grade with anti-wear additives.
  
• Filter Replacement: Change return and pilot filters every 500 hours. Inspect suction strainers quarterly.
  
• Electrical Harness Inspection: Check connectors and wiring for corrosion or abrasion, especially near the valve block.
  
• Travel Motor Servicing: Rebuild motors every 5,000 hours or if case drain flow exceeds 15% of rated displacement.
  
• Valve Calibration: Use diagnostic software to verify valve response curves and recalibrate solenoids annually.
  

Industry Perspective
Hydraulic travel asymmetry is not unique to Kobelco. Similar issues have been reported in Caterpillar 314C, Komatsu PC138US, and Hitachi ZX135 models. In 2019, a recall was issued for certain Doosan DX140 units due to travel valve block contamination during assembly. Manufacturers are increasingly integrating onboard diagnostics and CAN-bus systems to detect and log travel anomalies in real time.
Conclusion
The leftward tracking issue in the Kobelco SK135SRLC during high-speed travel is a multifactorial problem rooted in hydraulic imbalance, thermal sensitivity, and component wear. A methodical diagnostic approach—combining pressure testing, electrical checks, and mechanical inspection—can pinpoint the fault. With proper maintenance and timely intervention, the SK135SRLC remains a reliable workhorse for confined job sites and urban excavation.