Komatsu PC138USLC-2 Fault Code S-2: Complete Diagnostic Guide

What is Komatsu PC138USLC-2 Fault Code S-2?

Fault Code S-2 on the Komatsu PC138USLC-2 indicates a starter system malfunction, specifically a failure in the starting circuit or starter motor relay system. This code is triggered when the Engine Control Module (ECM) detects an abnormal signal from the starting circuit, preventing proper engine cranking or indicating a relay malfunction.

This fault is critical for the PC138USLC-2 because it directly affects the machine's ability to start reliably. The starter system integrates with the machine's safety interlock circuits, including the travel lock lever and hydraulic neutral position switches. When S-2 appears, the excavator may fail to crank entirely or exhibit intermittent starting behavior, leading to significant downtime on job sites.

Common Symptoms

When Code S-2 is active on your Komatsu PC138USLC-2, operators typically experience:

• No engine cranking when the key is turned, despite electrical power to the dash panel
• Clicking sounds from the starter relay area without actual starter motor engagement
• Intermittent starting where the machine starts on some attempts but not others
• Starter motor continues running after engine starts (stuck relay condition)
• Warning lights illuminated on the instrument cluster without diagnostic display showing specific fault details

Potential Causes

The most common technical reasons for S-2 fault codes on used PC138USLC-2 excavators include:

• Faulty starter relay (located in the main relay box under the operator's cab)
• Corroded or loose connections at the starter motor terminals or relay harness connections
• Worn starter motor solenoid contacts causing high resistance in the circuit
• Damaged wiring harness between the ignition switch and starter relay, particularly at known rub points near the swing bearing
• Failed neutral safety switches in the hydraulic pilot circuit preventing starter engagement
• Low battery voltage (below 22V on 24V systems) causing insufficient relay actuation
• ECM power supply issues affecting the starting signal output circuit

How to Troubleshoot and Fix Code S-2

Step 1: Perform Visual Inspection

Begin by inspecting all starter circuit connections for corrosion, loose terminals, or damaged wiring. On used excavators, pay special attention to the main harness routing near the swing bearing where cables frequently experience chafing. Check battery terminals for tightness and corrosion—clean with a wire brush and apply dielectric grease.

Step 2: Test Battery and Starter Relay

Using a digital multimeter, verify battery voltage under load (should maintain above 22V during cranking attempt). Locate the starter relay in the relay box and listen for an audible click when the key is turned. Test relay coil resistance (typically 80-120 ohms) and check for 24V signal at the relay control terminal during start attempt.

Step 3: Check Safety Interlock Circuits

Verify that all safety interlock switches (travel lock, hydraulic neutral position) are functioning correctly. Use the multimeter to confirm continuity through these circuits. Bypass switches individually (for diagnostic purposes only) to isolate faulty components.

Step 4: Inspect Starter Motor

If relay functions properly, remove and bench-test the starter motor. Check solenoid operation and measure current draw (should be 150-250 amps during cranking). Inspect the pinion gear for wear and ensure proper engagement with the flywheel ring gear.

Step 5: Verify ECM Output Signal

Connect Komatsu diagnostic software (KOMTRAX or compatible scan tool) to verify the ECM is sending proper start signal. Check for related fault codes that might indicate ECM power supply problems or internal failures.

Critical for Used Equipment: Before replacing expensive components like the starter motor or ECM, thoroughly inspect all harness connectors for "green corrosion" inside terminals—a common issue on older machines. Clean connectors with electrical contact cleaner and apply dielectric grease to prevent future corrosion.

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Disclaimer: This guide provides general troubleshooting procedures for informational purposes. Always consult the official Komatsu service manual for your specific machine serial number and seek professional assistance from certified Komatsu technicians for complex electrical diagnostics or if you're uncertain about any procedure.