Komatsu PC750-7 Fault Code S-2: Complete Diagnostic Guide

What is Komatsu PC750-7 Fault Code S-2?

Fault Code S-2 on the Komatsu PC750-7 indicates a malfunction in the engine speed sensor circuit or an abnormal signal from the crankshaft position sensor system. This code is triggered when the Electronic Control Module (ECM) detects irregular or missing pulses from the speed sensor, which is critical for monitoring engine RPM and timing fuel injection precisely.

The engine speed sensor provides real-time data to the ECM about crankshaft rotation. Without accurate readings, the PC750-7's SAA6D140E-3 engine cannot optimize fuel delivery, timing, or power output. This fault directly impacts machine performance, potentially causing rough operation or complete shutdown to prevent engine damage.

Common Symptoms

When Code S-2 is active, operators typically experience:

• Engine cranks but fails to start or starts with extreme difficulty
• Sudden engine stalling during operation, especially under load
• Erratic engine RPM fluctuations or inability to maintain consistent speed
• Loss of power and reduced hydraulic performance across all functions
• Warning lights illuminated on the monitor panel with audible alarms

Potential Causes

The most common technical reasons for S-2 on used PC750-7 excavators include:

• Failed or contaminated engine speed sensor due to heat exposure or oil contamination
• Damaged sensor wiring harness from rubbing against engine components or frame rails (common wear point near the flywheel housing)
• Corroded or loose electrical connectors at the sensor or ECM interface
• Broken or missing reluctor ring teeth on the crankshaft or flywheel
• ECM internal faults or corrupted software (less common but possible in high-hour machines)
• Inadequate sensor air gap caused by mounting bracket wear or improper installation

How to Troubleshoot and Fix Code S-2

Step 1: Visual Inspection Begin by locating the engine speed sensor on the flywheel housing (driver's side of engine). Inspect the wiring harness for obvious damage, chafing, or oil contamination. Check connector pins for corrosion, bent terminals, or moisture intrusion—common issues in used excavators operating in harsh environments.

Step 2: Electrical Testing Using a digital multimeter, measure sensor resistance (typically 190-250 ohms for Komatsu sensors, but verify with service manual). Test supply voltage at the connector (should read approximately 5V with key on). Check for continuity between sensor and ECM pins, noting any open circuits.

Step 3: Sensor Air Gap Verification Measure the air gap between sensor tip and reluctor ring using feeler gauges (specification typically 0.5-1.5mm). Excessive gap indicates worn mounting or sensor positioning issues common in high-hour machines.

Step 4: Advanced Diagnostics Connect Komatsu KOMTRAX or compatible diagnostic software to monitor live sensor data. Observe signal pattern during cranking—irregular waveforms indicate sensor failure or reluctor damage. For used excavators, inspect the flywheel reluctor ring for missing teeth or debris accumulation.

Step 5: Component Replacement If testing confirms sensor failure, replace with genuine Komatsu parts (part number varies by serial number). Always apply dielectric grease to connectors and secure harnesses away from heat sources and moving components.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific serial number and consider professional diagnostic services for complex electrical issues.