Restoring Throttle Motor Wiring on a Komatsu PC120-5 Excavator

[MikePhua]

Komatsu PC120-5 Background and Throttle Control System
The Komatsu PC120-5 excavator was part of Komatsu’s fifth-generation hydraulic excavator series, launched in the late 1980s and widely used through the 1990s. With an operating weight around 12 metric tons and powered by a Komatsu S4D102E diesel engine, the PC120-5 combined mechanical reliability with early electronic control systems. One of its key innovations was the use of an electronic throttle motor, a stepper-type actuator that adjusted engine RPM based on operator input and load demand.
Terminology Note

• Stepper Motor: A motor that moves in discrete steps, used for precise control of throttle position.
  
• Governor Controller: The electronic module that interprets throttle signals and drives the stepper motor.
  
• Phase Coil: Internal winding in the motor; typically labeled Phase A and Phase B.
  
• Diagnostic Lamp Array: A set of LEDs on the controller used to display fault codes and system status.
  

Common Wiring Failures and Symptoms
Over time, wiring harnesses on the PC120-5 can degrade due to vibration, heat, and previous owner modifications. A frequent issue involves the square plug on the throttle motor being damaged or missing entirely. This plug typically carries four wires—each corresponding to a motor phase or control signal. When miswired, the motor may pulse erratically, fail to respond, or cause the engine to idle incorrectly.
In one case, a machine in New Zealand had mismatched wire colors between the harness and the motor. Attempts to match colors directly (e.g., red to red, black to black) failed, and the motor pulsed without achieving stable RPM control.
Correct Wiring Strategy and Pinout Guidance
When the original plug is missing, the safest approach is to rebuild the wiring from scratch, using known pinouts and verifying coil pairs with an ohmmeter. The throttle motor typically uses two coil pairs:

• Phase A: Black and Green
  
• Phase B: Red and Yellow
  

To identify coil pairs:

• Use an ohmmeter to measure resistance between wires.
  
• Coil pairs will show continuity (typically 10–50 ohms), while unrelated wires will show open circuit.
  
• Label wires accordingly and connect to the controller based on known pinout diagrams.
  

A common pinout for Komatsu throttle motors (viewed from the female connector with latch on top):

• Pin 1: Black
  
• Pin 2: Green
  
• Pin 3: Red
  
• Pin 4: White or Yellow
  

Note: Wire colors may vary by region and serial number. Machines built for non-US markets often have different harness standards.
Controller Diagnostics and Fault Isolation
The governor controller on the PC120-5 includes a diagnostic lamp array:

• Center green light: Power and system OK
  
• Outer red lights: Fault indicators
  

If the throttle motor pulses or fails to respond, check the controller for fault codes. A flashing red light may indicate open coil, short circuit, or signal mismatch. In one case, a technician used a PC400 diagnostic video to locate the controller and interpret the lamp sequence, even though the PC120-5 layout differed slightly.
Recommendations for Repair and Verification

• Rebuild the wiring using shielded cable and waterproof connectors.
  
• Confirm coil pairs with an ohmmeter before connecting.
  
• Use dielectric grease on all terminals to prevent corrosion.
  
• Secure wires with loom and clamps to prevent vibration damage.
  
• Test throttle response through full RPM range before closing panels.
  

Field Insight and Practical Advice
In Vietnam, a technician rebuilt the throttle motor wiring using a custom harness and restored full throttle control in under two hours. In the UK, a PC120-5 owner discovered that his machine had been retrofitted with non-OEM wiring, which caused erratic throttle behavior. Replacing the harness with a factory-style layout resolved the issue.
Conclusion
Throttle motor wiring on the Komatsu PC120-5 is critical to engine performance and operator control. When original plugs are missing or damaged, rebuilding the harness using verified coil pairs and proper pinouts is the most reliable solution. With careful diagnostics and attention to wire integrity, the PC120-5 can be restored to smooth, responsive throttle operation—preserving the utility of one of Komatsu’s most enduring mid-size excavators.