Resolving Thumb Actuation Issues on a Kobelco SK150LC

The Kobelco SK150LC, particularly models from the late 1990s, often came with auxiliary hydraulic circuits intended for single-acting attachments such as hydraulic breakers. When a thumb is added—especially a hydraulic thumb—it requires a double-acting hydraulic circuit, meaning fluid must be supplied and returned under pressure to extend and retract the thumb cylinder. If the machine was originally configured for a hammer, it likely lacks the necessary plumbing and valve configuration to operate a thumb properly.

In the case described, the thumb only retracts to the stowed position but cannot extend to grip material. This behavior is consistent with a single-acting setup where fluid is only pressurized in one direction, and the return relies on gravity or spring force.

Key Components and Observations

• The foot pedal is a simple on/off switch with two wires: one for power and one leading to a solenoid.
• The solenoid activates a valve that sends fluid to the thumb cylinder.
• There is no second pedal or directional control, confirming single-acting configuration.
• The thumb cylinder itself is likely double-acting, but the machine’s hydraulic system isn’t configured to support it.

Converting to Double-Acting Hydraulics

To enable full thumb functionality, the following upgrades are typically required:

• Auxiliary control valve with two-way flow: Replace or modify the existing valve to allow pressurized flow in both directions.
• Two-way foot pedal or joystick switch: Install a control that allows the operator to choose extension or retraction.
• Additional hydraulic line and quick coupler: Route a second hose from the valve to the thumb cylinder’s opposite port.
• Electrical relay or pilot control: Depending on the valve type, you may need a pilot-operated spool or solenoid-controlled diverter.

A mechanic familiar with excavator hydraulics can inspect the main control valve to determine whether it includes an auxiliary spool capable of double-acting operation. If not, a retrofit valve block may be required.

Field Case and Remote Challenges

One operator located on a remote island faced difficulty finding a qualified mechanic. In such cases, having a hydraulic schematic is invaluable. With a diagram, remote guidance becomes possible, and parts can be ordered with confidence. A technician in Lincoln, UK, once walked a farmer through a similar conversion using only photos and a schematic, saving thousands in travel and labor.

Common Pitfalls and Solutions

• Solenoid failure: If the solenoid doesn’t fully shift the valve, flow may be restricted. Test with a multimeter and verify audible click when energized.
• Incorrect cylinder type: Some thumbs are single-acting and rely on gravity to retract. Confirm cylinder specs before modifying the system.
• Backpressure issues: Without a return line, fluid may be trapped, causing sluggish or no movement. Install a proper return path to tank.
• Control valve limitations: Older valves may not support pilot control. In such cases, an external diverter valve can be added.

Recommendations for Owners

• Gather detailed photos of the control valve, solenoid, and cylinder ports.
• Identify the valve manufacturer and model number.
• Consult with a hydraulic technician or dealer familiar with Kobelco systems.
• Consider installing a proportional control valve for smoother thumb operation.
• Use high-quality hoses and fittings rated for excavator pressure (typically 3,000 psi).

Final Thoughts

Adding a hydraulic thumb to a Kobelco SK150LC can dramatically improve its versatility, but only if the machine’s hydraulic system is properly configured. Many older units were set up for single-acting tools, and converting to double-acting requires thoughtful upgrades. With the right components and guidance, even remote operators can achieve full thumb functionality—turning a basic excavator into a precision material handler.