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The Kobelco SK120 and Its Hydraulic Legacy
The Kobelco SK120 hydraulic excavator was introduced in the late 1990s as part of Kobelco’s push into electronically managed, pilot-controlled machines. Kobelco Construction Machinery, a division of Kobe Steel founded in 1930, became known for its advanced hydraulic systems and fuel-efficient engines. The SK120, with its 28,000-pound operating weight and 90–100 horsepower engine, was widely adopted in Asia, North America, and Australia. By the early 2000s, thousands of SK120 units had been sold globally, often praised for their smooth pilot controls and responsive travel motors.
Understanding the Travel System Architecture
The SK120 uses a dual-track hydraulic travel system powered by two independent travel motors. These motors receive pressurized fluid from the main pump, which is controlled by pilot pressure signals. The travel levers are cable-actuated, sending mechanical input to pilot valves that regulate flow to the motors.
Key terminology:
In one diagnostic case, the SK120 exhibited intermittent tracking behavior:
Diagnostic Strategy and Pressure Testing
A technician in Missouri spent days tracing a tracking issue on an SK120. After replacing hoses and checking travel motors, the fault was traced to a weak pilot signal caused by a partially collapsed hose near the pump. Once replaced, both tracks operated normally.
In another case, a forestry crew in Oregon experienced similar symptoms during cold weather. The pilot pressure dropped due to thickened fluid, and the pump failed to respond. Switching to a lower-viscosity hydraulic oil resolved the issue.
Conclusion
Tracking failure in the Kobelco SK120 is often linked to pilot pressure irregularities and pump control faults. By methodically testing pilot circuits, observing pressure behavior, and inspecting mechanical linkages, technicians can isolate and resolve the issue. The SK120 remains a reliable excavator, and with proper diagnostics, even complex hydraulic problems can be solved efficiently.
The Kobelco SK120 hydraulic excavator was introduced in the late 1990s as part of Kobelco’s push into electronically managed, pilot-controlled machines. Kobelco Construction Machinery, a division of Kobe Steel founded in 1930, became known for its advanced hydraulic systems and fuel-efficient engines. The SK120, with its 28,000-pound operating weight and 90–100 horsepower engine, was widely adopted in Asia, North America, and Australia. By the early 2000s, thousands of SK120 units had been sold globally, often praised for their smooth pilot controls and responsive travel motors.
Understanding the Travel System Architecture
The SK120 uses a dual-track hydraulic travel system powered by two independent travel motors. These motors receive pressurized fluid from the main pump, which is controlled by pilot pressure signals. The travel levers are cable-actuated, sending mechanical input to pilot valves that regulate flow to the motors.
Key terminology:
- Pilot Pressure: Low-pressure hydraulic signal used to control high-pressure functions.
- Negative Control Pump: A pump whose displacement is reduced by pilot pressure, common in older Kobelco models.
- Swash Plate: An internal pump component that adjusts flow rate based on angle.
- Brake Spool: A valve that releases hydraulic brakes on the travel motors when pressure is applied.
In one diagnostic case, the SK120 exhibited intermittent tracking behavior:
- Left travel worked occasionally, with pressure increase at the pump.
- Right travel required holding the pedal down for several seconds before engaging.
- Measured pressure at A1 and A2 ports was only 200 psi when travel valves were activated.
- No pressure was detected at the pump outlet or travel motor inlet during failure.
Diagnostic Strategy and Pressure Testing
- Measure Pilot Pressure at Neutral
With all levers in neutral and engine at full throttle, pilot pressure should read around 500 psi at both pumps. This confirms baseline signal integrity.
- Engage Travel Levers and Observe Pressure Drop
A healthy system will show a pressure drop when a travel lever is engaged, indicating the pump is responding to demand. In this case, the left lever showed no drop, while the right dropped below 200 psi—suggesting weak or blocked pilot signal.
- Check Swash Plate Response
If the pump does not advance the swash plate, flow to the travel motors remains low. This can be caused by:- Faulty pilot valve
- Blocked pilot line
- Internal pump regulator failure
- Faulty pilot valve
- Inspect Brake Spool and Springs
If the brake spool fails to release, the travel motor remains locked. In this case, the spool and springs were confirmed functional.
- Deadhead and Tee Test Pilot Lines
Use both methods to isolate upstream and downstream faults. A deadhead test checks maximum pressure; a tee test monitors flow under load.
- Clean or Replace Pilot Valves
Contaminants or wear can reduce signal strength. Rebuild kits are available for most Kobelco pilot valves.
- Verify Pump Control Linkage
Ensure mechanical linkages to the swash plate actuator are intact and properly adjusted.
- Flush Hydraulic System
Contaminated fluid can block pilot passages. Use manufacturer-recommended flushing procedures and filters.
A technician in Missouri spent days tracing a tracking issue on an SK120. After replacing hoses and checking travel motors, the fault was traced to a weak pilot signal caused by a partially collapsed hose near the pump. Once replaced, both tracks operated normally.
In another case, a forestry crew in Oregon experienced similar symptoms during cold weather. The pilot pressure dropped due to thickened fluid, and the pump failed to respond. Switching to a lower-viscosity hydraulic oil resolved the issue.
Conclusion
Tracking failure in the Kobelco SK120 is often linked to pilot pressure irregularities and pump control faults. By methodically testing pilot circuits, observing pressure behavior, and inspecting mechanical linkages, technicians can isolate and resolve the issue. The SK120 remains a reliable excavator, and with proper diagnostics, even complex hydraulic problems can be solved efficiently.
