Komatsu PC850-8R Fault Code S-6: Complete Diagnostic Guide

What is Komatsu PC850-8R Fault Code S-6?

Fault Code S-6 on the Komatsu PC850-8R 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 an irregular or missing signal from the sensor that monitors engine rotational speed and crankshaft position.

The engine speed sensor is critical for the PC850-8R's performance because it provides real-time data to the ECM for fuel injection timing, engine speed control, and overall power management. Without accurate readings, the machine cannot optimize combustion efficiency or maintain proper hydraulic system performance. In Komatsu's SAA6D170E-5 engine used in this model, this sensor directly affects the common rail fuel injection system and turbocharger control.

Common Symptoms

When Code S-6 is active, operators typically experience:

• Engine fails to start or cranks without firing due to loss of crankshaft position data
• Sudden engine shutdown during operation, often without warning
• Erratic engine RPM or inability to maintain consistent idle speed
• Loss of power and reduced hydraulic response during digging or lifting operations
• Warning lamp illumination on the monitor panel with accompanying fault code display

Potential Causes

The most common technical causes for Code S-6 on used PC850-8R excavators include:

• Failed or contaminated engine speed sensor due to heat exposure, oil contamination, or internal component degradation
• Damaged sensor wiring harness particularly at known rub points near the flywheel housing or along the engine block
• Corroded or loose connector pins at the sensor plug, especially on machines with high operating hours in wet or corrosive environments
• Excessive air gap between the sensor tip and the crankshaft reluctor ring caused by mounting bracket wear or sensor backing out
• ECM internal fault or corrupted software, though less common than sensor-related issues
• Broken or damaged reluctor ring teeth on the crankshaft, typically from previous mechanical damage

How to Troubleshoot and Fix Code S-6

Step 1: Visual Inspection Begin by locating the engine speed sensor on the flywheel housing (driver's side of the engine). Inspect the wiring harness for obvious damage, abrasion, or oil contamination. On used excavators, check for harness routing issues where cables may contact hot exhaust components or moving parts.

Step 2: Connector and Air Gap Check Disconnect the sensor connector and inspect for corrosion, bent pins, or moisture intrusion. Clean with electrical contact cleaner if needed. Verify the sensor mounting is tight and check the air gap specification (typically 0.5-1.5mm for Komatsu sensors) using feeler gauges between the sensor tip and reluctor ring.

Step 3: Electrical Testing Using a digital multimeter, measure sensor resistance across the two sensor terminals (specification typically 190-250 ohms at 20°C). Check for continuity in the wiring harness back to the ECM connector pins. Test for short circuits to ground on both signal wires.

Step 4: Signal Testing and Component Replacement If available, use Komatsu diagnostic software (KOMTRAX Plus) to monitor live sensor signal voltage while cranking (should show AC voltage pulses). If readings are abnormal or absent despite good resistance values, replace the sensor. For used machines with 8,000+ hours, consider replacing both the sensor and connector as preventive maintenance.

Step 5: Post-Repair Verification After repairs, clear the fault code using diagnostic software, start the engine, and verify stable operation. Monitor for code recurrence during a full operational cycle including loaded hydraulic work.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and consider professional diagnosis for complex electrical issues or if you lack proper diagnostic equipment.