Hydraulic Mode Failure and Control Logic in the Furukawa 738LS Excavator

[MikePhua]

The 738LS and Furukawa’s Excavator Legacy
The Furukawa 738LS excavator, manufactured in the late 1990s, represents a transitional phase in Japanese heavy equipment design—where mechanical robustness began to merge with electronic control systems. Furukawa, originally founded in the late 19th century as a mining and industrial company, expanded into construction machinery in the postwar decades. By the 1990s, its excavators were known for their durable undercarriages, responsive hydraulics, and straightforward serviceability.
The 738LS was designed for mid-range excavation tasks, with an operating weight around 13 metric tons and a diesel engine producing approximately 90–100 hp. It featured a hydraulic system capable of multiple digging modes, including a “slow dig” function intended for precision trenching and controlled bucket movement. This mode was activated via the operator panel and signaled by an indicator light, but its execution depended on a coordinated response between electronic inputs and hydraulic valve behavior.
Terminology Annotation
- Hydraulic Mode Selector: An electronic switch or dial that allows the operator to choose between digging speeds or operational profiles.
- Solenoid Valve: An electrically actuated valve that directs hydraulic fluid based on control signals, often used to toggle between flow rates or circuit paths.
- Pilot Pressure: Low-pressure hydraulic signal used to control main valves, often regulated by electronic commands.
- Control Logic Fault: A mismatch between operator input and system response due to electrical or software malfunction.
Symptoms of Mode Activation Failure
Operators may encounter a situation where the “slow dig” indicator light illuminates correctly on the control panel, but the excavator continues to operate at standard digging speed. This suggests that the electrical signal is reaching the dashboard but not triggering the hydraulic system to reduce flow or pressure.
Typical symptoms include:

• Mode light activates but no change in bucket or arm speed
  
• No fault codes displayed on diagnostic interface
  
• Hydraulic response remains aggressive despite mode selection
  
• No audible change in pump tone or valve shift
  
• System behaves normally in other modes
  

This points to a breakdown in the communication between the mode selector and the hydraulic control valves—likely involving a solenoid failure, wiring fault, or logic board malfunction.
A Story from the Balkans
In Banja Luka, Bosnia and Herzegovina, a private contractor operating a 1997 Furukawa 738LS noticed that the slow dig mode failed to engage during trenching for a water line. The light on the dashboard activated, but the bucket moved at full speed. After inspecting the hydraulic control block, he discovered that the solenoid responsible for reducing pilot pressure had failed internally. Replacing the solenoid restored proper function, and the machine resumed precise digging operations.
Diagnostic Strategy and Component Testing
To isolate the fault:

• Confirm that the mode selector switch sends voltage to the solenoid
  
• Test the solenoid coil for resistance and continuity
  
• Inspect wiring harness for corrosion, abrasion, or loose connectors
  
• Check pilot pressure at the control valve during mode activation
  
• Listen for solenoid actuation when switching modes
  
• Verify that the hydraulic pump adjusts displacement in response to mode change
  

If the solenoid receives voltage but does not actuate, it may be internally seized or burned out. If no voltage is present, the fault lies upstream—possibly in the control panel or logic board.
Recommendations for Repair and Optimization
To restore proper mode function:

• Replace faulty solenoid valves with OEM-rated components
  
• Clean all electrical connectors and apply dielectric grease
  
• Secure wiring harnesses to prevent vibration damage
  
• Test pilot pressure regulators and replace worn seals
  
• Update control logic firmware if available from Furukawa service channels
  
• Document voltage readings and component replacements for future reference
  

For machines operating in dusty or humid environments, consider installing protective boots over solenoids and sealing connectors with heat-shrink tubing.
Preventative Maintenance and System Reliability
To maintain hydraulic mode integrity:

• Inspect mode selector and solenoid wiring quarterly
  
• Replace hydraulic filters every 500 hours
  
• Monitor pilot pressure during operation and log anomalies
  
• Keep control panel dry and free of condensation
  
• Train operators to report mode inconsistencies immediately
  
• Maintain a service binder with schematics and troubleshooting notes
  

For legacy machines like the 738LS, sourcing original diagrams and service manuals is critical. If unavailable, reverse-engineering the wiring and valve layout may be necessary.
Conclusion
The Furukawa 738LS excavator’s slow dig mode failure highlights the importance of synchronized electrical and hydraulic control systems. While the indicator light may suggest proper activation, true mode engagement depends on solenoid function, pilot pressure regulation, and clean signal pathways. By applying structured diagnostics and preventative care, operators can restore precision digging capabilities and extend the life of this durable mid-size excavator. Furukawa’s legacy in mechanical design remains evident, but its electronic systems require equal attention to ensure consistent field performance.