Komatsu PC200LC-6 Throttle Controller Diagnosis and Electronic System Restoration

[MikePhua]

The PC200LC-6 and Komatsu’s Hydraulic Excavator Milestone
The Komatsu PC200LC-6 is a benchmark in the evolution of mid-size hydraulic excavators. Introduced in the 1990s, it combined mechanical robustness with early electronic control systems, offering improved fuel efficiency, smoother throttle response, and enhanced diagnostic capabilities. With an operating weight around 45,000 pounds and powered by a Komatsu S6D102E diesel engine producing approximately 140 horsepower, the PC200LC-6 was built for versatility in construction, mining, and infrastructure development.
Komatsu, founded in 1921 in Japan, has sold millions of excavators worldwide. The PC200 series remains one of its most successful product lines, with the -6 generation marking a transition toward electronically managed engines and integrated monitoring systems.
Terminology Notes

• Throttle Controller: An electronic module that regulates engine speed based on operator input and machine load.
  
• Servo Motor: A reversible electric motor connected to the throttle actuator arm, controlled by the throttle controller.
  
• Feedback Potentiometer: A sensor within the servo that reports throttle position back to the controller for closed-loop regulation.
  
• Machine Computer: The onboard processor managing non-engine functions such as speed sensors, display, and auxiliary systems.
  

Symptoms of Throttle Controller Failure
Operators may encounter:

• Engine speed stuck at idle or fails to respond to throttle input
  
• Display panel shows no digital output or remains blank
  
• Throttle servo motor does not actuate or moves erratically
  
• Diagnostic codes related to speed sensor or controller communication
  
• Machine operates normally hydraulically but lacks engine RPM control
  

A technician in Alberta reported that a PC200LC-6 refused to throttle up despite a functioning servo motor. After bench testing the controller and inspecting the wiring harness, they discovered a corroded jumper connector near the key switch, which had interrupted power to the throttle computer.
Root Causes and Diagnostic Strategy
Throttle control issues typically result from:

• Loss of Power Supply to Controller
  • Blown fuse, faulty relay, or corroded connector interrupts voltage
    
  • Diagnosed by tracing power from fuse #2 through relay to connector C3
    
• Faulty Servo Motor or Feedback Potentiometer
  • Worn gears or sensor drift causes erratic throttle movement
    
  • Confirmed by bench testing motor and measuring potentiometer resistance
    
• Corroded Jumper Connectors or Harness Pins
  • Moisture ingress near the key switch area leads to signal loss
    
  • Detected by visual inspection and continuity testing
    
• Machine Computer Communication Fault
  • Missing speed sensor input prevents throttle controller from adjusting RPM
    
  • Diagnosed via scan tool and connector C2 signal verification
    
• Display Panel Malfunction
  

• No feedback or error codes shown due to internal failure
  
• Confirmed by removing panel and inspecting diode and circuit board integrity
  

Inspection steps:

• Remove panel behind mirror to access throttle and machine computers
  
• Check voltage at connector C3 and verify fuse #2 integrity
  
• Inspect four relays near engine computer and test for switching function
  
• Locate jumper connector M20 near key switch and test for corrosion
  
• Bench test throttle controller with known-good power and signal inputs
  

Repair Procedure and Component Recommendations
To restore throttle control:

• Clean and reseat all connectors, especially M20 and C3
  
• Replace corroded jumper wires and apply dielectric grease
  
• Bench test throttle controller and verify servo motor operation
  
• Replace faulty potentiometer or servo motor if resistance is inconsistent
  
• Update or replace display panel if no output is detected
  
• Verify speed sensor signal to machine computer and repair harness if needed
  

Recommended components:

• Komatsu PC200LC-6 throttle controller module
  
• Servo motor with integrated feedback potentiometer
  
• Jumper connector M20 with four-pin harness
  
• Display panel with diode and circuit board set
  
• Diagnostic interface compatible with Komatsu CAN protocol
  

A field mechanic in Oregon rebuilt a PC200LC-6 throttle system after repeated idle lockouts. By replacing the servo motor and cleaning the jumper connector, the excavator regained full RPM control and completed a season of trenching without further faults.
Preventive Maintenance and Long-Term Solutions
To prevent future throttle issues:

• Inspect and clean connectors quarterly, especially near the cab and key switch
  
• Replace fuses and relays every 1,000 hours or during seasonal service
  
• Test servo motor and potentiometer annually
  
• Monitor display panel output and log fault codes
  
• Use sealed connectors and protective sleeves in high-moisture environments
  

Some fleet managers retrofit older PC200LC-6 units with upgraded throttle controllers and waterproof harnesses. Others install auxiliary RPM sensors to provide redundant feedback to the machine computer.
Recommendations for Operators and Technicians

• Maintain a diagnostic log with fault codes, voltage readings, and connector inspections
  
• Train operators to report throttle lag or idle lockouts immediately
  
• Stock spare servo motors, controllers, and jumper connectors for field repairs
  
• Partner with Komatsu dealers for updated schematics and retrofit kits
  
• Consider full electronic system inspection during engine overhaul or seasonal downtime
  

For machines operating in excavation, demolition, or utility trenching, throttle responsiveness is essential for productivity and fuel efficiency.
Conclusion
Throttle controller issues on the Komatsu PC200LC-6 are often rooted in electrical integrity and sensor feedback. With methodical diagnostics, targeted repairs, and preventive care, the electronic throttle system can be restored to full function. In the rhythm of hydraulic excavation, engine speed is the pulse—and when the controller responds and the servo moves true, the machine digs with precision.