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The Kobelco SK120 Mark III excavator can suffer from simultaneous electrical and hydraulic issues, especially in aging units with compromised wiring and controller components. Diagnosing these problems requires a methodical approach to both the gauge cluster and pump control systems.
Machine Background and Production History
The SK120 series was introduced by Kobelco Construction Machinery in the late 1990s as a compact yet powerful excavator for utility, forestry, and light demolition work. The Mark III variant featured improved hydraulic responsiveness and an integrated electronic control system. Kobelco, a subsidiary of Kobe Steel, has built a reputation for smooth hydraulic performance and durable undercarriage design. The SK120 sold widely in North America, Southeast Asia, and Australia, with thousands of units still in operation today.
Terminology Note
In one SK120 Mark III, the gauge cluster would power on briefly with a solid black screen, then shut off. No RPM, fuel, or temperature readings were visible. The machine still operated, but without any feedback or alerts. Key findings included:
The machine also exhibited erratic travel behavior. The left track would stop under load, swing motion would trigger travel engagement, and downhill braking was inconsistent. Observations included:
The Kobelco SK120 Mark III can develop complex electrical and hydraulic faults as it ages, especially when wiring is compromised and controllers fail. By methodically separating electronic and hydraulic diagnostics, operators can restore functionality and avoid unnecessary part replacement. The machine’s modular design allows for targeted repairs, but success depends on understanding how pump control, pilot pressure, and electrical feedback interact under load.
Machine Background and Production History
The SK120 series was introduced by Kobelco Construction Machinery in the late 1990s as a compact yet powerful excavator for utility, forestry, and light demolition work. The Mark III variant featured improved hydraulic responsiveness and an integrated electronic control system. Kobelco, a subsidiary of Kobe Steel, has built a reputation for smooth hydraulic performance and durable undercarriage design. The SK120 sold widely in North America, Southeast Asia, and Australia, with thousands of units still in operation today.
Terminology Note
- Gauge Cluster: The instrument panel displaying RPM, fuel level, temperature, and system alerts.
- Pump Governor Solenoid: An electronic valve that adjusts the swash plate angle of the hydraulic pump to regulate flow.
- Swivel Joint: A rotating hydraulic coupling that allows fluid transfer between upper and lower structures.
- H/S/FC/D Switch: A mode selector that changes pump flow distribution for different operating conditions.
- Pilot Pressure: Low-pressure hydraulic signal used to control high-pressure functions like travel and swing.
In one SK120 Mark III, the gauge cluster would power on briefly with a solid black screen, then shut off. No RPM, fuel, or temperature readings were visible. The machine still operated, but without any feedback or alerts. Key findings included:
- Controller failure: The gauge cluster has a dedicated controller mounted under the right-hand panel. If this unit is water-damaged or internally shorted, the display will fail completely.
- Power and ground confirmed: Voltage checks showed 24V supply and solid ground, ruling out wiring faults.
- No test procedure available: The only way to confirm controller failure is to swap with a known working unit. Refurbishment attempts often fail due to lack of parts or internal corrosion.
The machine also exhibited erratic travel behavior. The left track would stop under load, swing motion would trigger travel engagement, and downhill braking was inconsistent. Observations included:
- Pump control solenoids disconnected: Wires to both pump governor solenoids were cut, disabling electronic swash plate control.
- Mode selector impact: In “D” mode, pump 1 handles boom and swing, while pump 2 drives the tracks. This split mode can mask pump issues but slows all functions.
- Swivel joint suspected: Delayed travel response and loss of swing brake suggest internal leakage or pilot pressure loss at the swivel.
- Hydraulic leak incident: A low fluid level caused pump chatter and loss of left track and swing, confirming sensitivity to fluid volume.
- Inspect and reconnect pump governor solenoid wires. Use the schematic to trace control signals from the controller and mode switch.
- Test pilot pressure at the travel control valve and swivel joint. Use adapters to measure response time and pressure drop.
- Replace the gauge cluster controller if no display functions return after power verification.
- Run the machine in “D” mode to isolate pump functions and identify which pump is underperforming.
- Check brake valve operation during downhill travel and swing-to-travel transitions.
- Seal the controller compartment against moisture intrusion.
- Replace hydraulic fluid and filters every 500 hours to prevent cavitation and pump wear.
- Use dielectric grease on solenoid connectors to prevent corrosion.
- Keep a log of travel and swing anomalies to correlate with fluid levels and operating temperature.
- Train operators to recognize mode selector impacts on pump behavior.
The Kobelco SK120 Mark III can develop complex electrical and hydraulic faults as it ages, especially when wiring is compromised and controllers fail. By methodically separating electronic and hydraulic diagnostics, operators can restore functionality and avoid unnecessary part replacement. The machine’s modular design allows for targeted repairs, but success depends on understanding how pump control, pilot pressure, and electrical feedback interact under load.
