Komatsu PC120-6 Fault Code E-3: Complete Diagnostic Guide

What is Komatsu PC120-6 Fault Code E-3?

Fault Code E-3 on the Komatsu PC120-6 indicates a malfunction in the engine speed sensor circuit, specifically detecting an abnormal or missing signal from the engine revolution pickup sensor.

This code is generated by the machine's monitor controller when it fails to receive a valid RPM signal from the magnetic pickup sensor mounted on the engine flywheel housing. The sensor monitors crankshaft rotation and transmits critical speed data to the controller for fuel injection timing and hydraulic pump regulation.

On the PC120-6's 6D95L engine, accurate speed sensing is essential for proper fuel delivery and preventing overspeed conditions. When this circuit fails, the machine cannot accurately regulate engine performance.

Common Symptoms

• Engine warning lamp illuminates on the monitor panel
• Erratic or non-functional tachometer readings
• Engine may run rough or exhibit hunting (RPM fluctuation)
• Machine enters limp mode with reduced hydraulic response
• Intermittent stalling, particularly during warm-up periods

Potential Causes

The most frequent causes of E-3 on used PC120-6 units include:

• Sensor failure due to heat cycling and age-related deterioration
• Wiring harness damage where cables route near exhaust manifold (known friction point on this model)
• Connector corrosion at the sensor plug, especially in humid operating environments
• Improper air gap between sensor tip and flywheel ring gear teeth
• Controller malfunction (less common but possible on high-hour machines)

How to Troubleshoot and Fix Code E-3

Step 1: Visual Inspection Locate the engine speed sensor on the flywheel housing (driver's side). Inspect the wiring harness for chafing, melting, or rodent damage. Check the connector for corrosion, bent pins, or moisture intrusion. On used excavators, pay particular attention to previous repair splices.

Step 2: Electrical Testing Using a multimeter, measure sensor resistance between terminals—specification is typically 190-250 ohms at 20°C. Test for continuity between each wire and ground to rule out short circuits. Verify supply voltage reaches the sensor connector (reference machine wiring diagram).

Step 3: Signal Verification With the engine cranking, use an oscilloscope or AC voltage function to confirm the sensor generates a pulsing signal. Expected output is 1-3 VAC during cranking. No signal confirms sensor or air gap failure.

Step 4: Gap Adjustment or Replacement Verify sensor-to-flywheel air gap (typically 0.5-1.5mm). Adjust or replace the sensor as required. Clear fault codes using Komatsu diagnostic software and verify repair.

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Disclaimer: This guide is for informational purposes. Always consult qualified technicians and refer to official Komatsu service manuals before performing repairs.