Troubleshooting Throttle Control Failure on Komatsu PC120-5

[MikePhua]

Komatsu’s Mid-Size Excavator Legacy
The Komatsu PC120-5 is part of the fifth-generation lineup of hydraulic excavators developed during the late 1980s and early 1990s. Komatsu, founded in 1921 in Japan, became a global leader in construction machinery by emphasizing mechanical reliability and operator-focused design. The PC120 series was engineered for urban excavation, utility trenching, and light demolition, offering a balance between power and maneuverability.
With an operating weight of approximately 12 metric tons and a bucket capacity ranging from 0.4 to 0.6 cubic meters, the PC120-5 was widely adopted across Asia, North America, and Europe. Its diesel engine, typically the Komatsu S4D102E, delivered around 90 horsepower, and its electronically assisted throttle system was considered advanced for its time.
Throttle Control System Overview
The throttle control on the PC120-5 is governed by a combination of components:

• Governor motor
  
• Fuel control dial (throttle potentiometer)
  
• Governor controller (ECU)
  
• MVC solenoid (Motor Valve Control)
  
• Diagnostic LED panel
  

When functioning properly, the operator adjusts the fuel control dial, which sends a signal to the governor controller. The controller then activates the governor motor to adjust fuel delivery, increasing or decreasing engine RPM. The MVC solenoid plays a role in modulating hydraulic pump response based on engine speed.
Terminology and Technical Notes

• Governor Motor: An electric actuator that adjusts the fuel rack or throttle linkage based on input from the controller.
  
• Potentiometer (Fuel Control Dial): A variable resistor that translates dial position into voltage signals.
  
• MVC Solenoid: A valve that modulates hydraulic pump output in response to engine load and speed.
  
• ECU (Electronic Control Unit): The brain of the system, interpreting signals and coordinating motor response.
  
• Diagnostic LEDs: Visual indicators used to display fault codes and system status.
  

Common Symptoms and Diagnostic Steps
In cases where the throttle dial fails to control engine speed, even after replacing the governor motor and dial switch, deeper electrical or control issues are likely. A typical diagnostic sequence includes:

• Inspecting the diagnostic LED panel behind the operator seat. One panel shows three diodes (two red, one green) for engine control status. The other displays alphanumeric fault codes for hydraulic control.
  
• Checking for fault codes such as “18” and “25,” which often relate to MVC solenoid or governor controller issues.
  
• Verifying power input and output at the governor controller. If power enters but no signal exits, the controller may be internally failed.
  
• Examining the circuit board for corrosion, cracked solder joints, or burnt components.
  
• Testing continuity and voltage across the potentiometer terminals.
  

In one documented case, a technician suspected the governor controller was faulty despite the circuit board appearing intact. The Komatsu service tech confirmed that although the board looked fine visually, internal logic failure prevented signal transmission to the motor. Replacing the controller resolved the issue.
Field Anecdotes and Practical Advice
A contractor in Ohio faced a similar issue with his PC120-5. After replacing the governor motor and throttle dial, the machine still wouldn’t respond. Diagnostic LEDs revealed fault codes pointing to the MVC solenoid. Upon inspection, the solenoid was functioning, but the controller wasn’t sending activation signals. Rather than immediately replacing the controller, he tested voltage output and confirmed the absence of signal. After sourcing a refurbished controller, the throttle system returned to normal.
In another case in British Columbia, a forestry operator experienced intermittent throttle response. The issue was traced to a loose ground wire behind the controller housing. Re-securing the connection restored full functionality without replacing any components.
Recommendations for Repair and Prevention

• Always verify fault codes using the onboard diagnostic LEDs before replacing components.
  
• Use a multimeter to confirm voltage at each stage: dial, controller input, controller output, and motor terminals.
  
• Inspect wiring harnesses for corrosion, rodent damage, or loose connectors.
  
• Avoid replacing expensive components like the governor controller without confirming failure through signal testing.
  
• Maintain clean electrical connections and protect control boxes from moisture and vibration.
  

Conclusion
Throttle control failure on the Komatsu PC120-5 can stem from multiple sources, including the governor motor, dial potentiometer, MVC solenoid, or the controller itself. While visual inspection is helpful, electrical testing is essential to pinpoint the fault. With proper diagnostics and attention to wiring integrity, operators can avoid unnecessary part replacements and restore full engine control. The PC120-5 remains a reliable workhorse, and understanding its electronic throttle system is key to keeping it productive in the field.