Komatsu PC200-8E0 Fault Code S-15: Complete Technical Guide

What is Komatsu PC200-8E0 Fault Code S-15?

Fault Code S-15 on the Komatsu PC200-8E0 indicates a starter motor relay circuit abnormality, specifically detecting an electrical malfunction in the starting system relay control circuit. This code is triggered when the Engine Control Module (ECM) detects abnormal voltage or resistance in the circuit that controls the starter relay, preventing proper energization of the starter motor.

This fault is critical for the PC200-8E0 because it directly affects the machine's ability to start reliably. The ECM monitors the starter relay circuit continuously during cranking operations, and any deviation from the expected voltage parameters (typically 12-24V DC depending on system configuration) will generate this diagnostic trouble code. For used excavators, this code often surfaces due to aging electrical components, corroded connections, or deteriorated wiring harnesses that have experienced years of vibration and environmental exposure.

Common Symptoms

When Code S-15 is active on your Komatsu PC200-8E0, operators typically experience:

• No-crank condition – the engine fails to turn over when the start key is engaged, though dashboard lights may illuminate normally
• Intermittent starting issues – the machine starts occasionally but fails unpredictably, especially during cold starts or after prolonged idle periods
• Audible clicking sounds from the starter relay area without starter motor engagement
• Warning indicator illumination on the monitor panel, specifically the malfunction lamp or check engine light
• Battery drain symptoms due to relay remaining energized or cycling improperly

Potential Causes

The most common technical causes for Code S-15 on used PC200-8E0 excavators include:

• Faulty starter motor relay – internal contact degradation or coil failure (extremely common in machines with 5,000+ operating hours)
• Corroded or loose connector pins at the relay socket or ECM harness connections (check X3 connector specifically)
• Damaged wiring harness between the ECM and starter relay, particularly at known rub points near the battery box and frame rails
• Failed neutral safety switch or hydraulic pilot pressure switch sending incorrect signals to the starting circuit
• Low battery voltage (below 11.5V) causing insufficient relay coil energization
• ECM internal relay driver circuit failure (less common but possible in high-hour machines)

How to Troubleshoot and Fix Code S-15

Step 1: Perform Visual Inspection Begin with the battery terminals and main ground connections. On used excavators, corrosion at these points causes 40% of starting system faults. Remove, clean with wire brush, and apply dielectric grease. Inspect the starter relay (located near the battery compartment) for physical damage, overheating discoloration, or loose mounting.

Step 2: Test Relay and Circuit Voltage Using a digital multimeter (DMM), measure voltage at the relay coil terminals with the key in the start position. You should see battery voltage (12V nominal). If no voltage appears, trace back to the ECM output pin (consult PC200-8E0 wiring diagram for exact pin location on connector X3). Check for open circuits or damaged harness insulation at flex points near the cab mount and undercarriage.

Step 3: Check Relay Function and Resistance Remove the starter relay and perform a bench test. Measure coil resistance (typically 70-90 ohms) and apply direct battery voltage to energize. You should hear an audible click and measure continuity across the contact terminals. For used equipment, replace relays showing intermittent operation or resistance outside specifications.

Step 4: Verify Neutral Safety Interlock The neutral safety switch and pilot pressure switches must signal proper conditions before the ECM allows starting. Using Komatsu diagnostic software (KOMTRAX or KDW), monitor switch inputs in real-time. Bypass temporarily (for testing only) to isolate faulty safety switches common in older machines.

Step 5: Advanced ECM Testing If all external components test properly, connect Komatsu-specific diagnostic equipment to read detailed ECM parameters. Check for internal relay driver faults within the ECM. On high-hour used excavators, ECM connector pin corrosion is frequent—remove connector, inspect for green oxidation, clean with electrical contact cleaner, and apply contact lubricant.

Critical for Used Equipment: Always inspect harness wear points where cables contact the frame or move with the cab. Years of vibration create insulation breakdown invisible to casual inspection.

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Disclaimer: This guide provides general diagnostic information for Code S-15 on Komatsu PC200-8E0 excavators. Always consult the official Komatsu service manual for your specific machine serial number, and consider professional diagnosis for complex electrical issues. Improper electrical work can damage the ECM or create safety hazards.