Komatsu PC850-8R Fault Code S-5: Complete Diagnostic Guide

What is Komatsu PC850-8R Fault Code S-5?

Fault Code S-5 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 irregular or missing pulses from the engine speed sensor, which monitors crankshaft rotation and timing.

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 sensor readings, the machine cannot optimize combustion efficiency or maintain proper hydraulic system performance. In used excavators, this sensor circuit is particularly vulnerable to wear, vibration damage, and connector corrosion.

Common Symptoms

When Code S-5 is active, operators typically experience:

• Engine cranks but fails to start, or starts with difficulty and runs roughly
• Sudden engine shutdown during operation without warning
• Erratic engine RPM fluctuations or inability to reach full throttle
• Hydraulic system performance degradation due to inconsistent engine speed
• Warning lamp illumination on the monitor panel with possible derate mode activation

Potential Causes

The most common technical causes for Code S-5 on used PC850-8R excavators include:

• Engine speed sensor failure due to internal component degradation or heat exposure
• Damaged or corroded wiring harness at known rub points near the flywheel housing
• Loose or corroded electrical connectors at the sensor or ECM interface
• Broken or missing sensor mounting hardware causing air gap issues
• Contaminated sensor face from oil leaks, metal debris, or dirt accumulation
• Flywheel ring gear damage affecting magnetic pulse generation
• ECM internal faults or software corruption (less common)

How to Troubleshoot and Fix Code S-5

Step 1: Visual Inspection Begin by inspecting the engine speed sensor located on the flywheel housing. Check for physical damage, loose mounting bolts, and proper air gap (typically 0.5-1.5mm). On used excavators, examine the wiring harness for abrasion points where it contacts the engine block or frame rails.

Step 2: Electrical Testing Using a digital multimeter, measure sensor resistance (typically 190-250 ohms at 20°C). Check voltage output while cranking—you should see AC voltage pulses (0.5-5V). Inspect all connector pins for corrosion, bent terminals, or moisture intrusion, which are common in older machines.

Step 3: Signal Verification Connect Komatsu diagnostic software (KOMTRAX Plus) to monitor live sensor data. Verify the ECM receives consistent pulses during cranking. If signal is intermittent, check harness continuity and ground circuits. Inspect the flywheel ring gear for missing teeth or damage.

Step 4: Component Replacement Replace the sensor if readings are out of specification. For used excavators, always replace the connector pigtail simultaneously to prevent future corrosion issues. Clear codes and perform a complete operational test under load.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual and consider professional technician assistance for complex electrical diagnostics and repairs.