Komatsu PC55MR-2 Fault Code S-5: Complete Diagnostic Guide

What is Komatsu PC55MR-2 Fault Code S-5?

Fault Code S-5 on the Komatsu PC55MR-2 indicates a malfunction in the engine speed sensor circuit or an engine overspeed condition. This code is triggered when the Electronic Control Module (ECM) detects abnormal signals from the engine speed sensor or when engine RPM exceeds the programmed safety threshold.

The engine speed sensor is critical for proper fuel injection timing, governor control, and overall engine performance on this compact excavator. When S-5 appears, the ECM cannot accurately monitor engine speed, which can lead to improper fuel delivery, loss of power control, and potential engine protection shutdowns. This is particularly important on the PC55MR-2's SAA4D95LE engine, where precise speed monitoring ensures optimal hydraulic system performance and prevents catastrophic engine damage.

Common Symptoms

When fault code S-5 is active, operators typically experience:

• Engine fails to start or starts then immediately shuts down as a protective measure
• Intermittent engine stalling during operation, especially under load
• Erratic engine RPM or inability to maintain steady idle speed
• Loss of hydraulic power due to inconsistent engine speed affecting the hydraulic pump
• Warning lamp illumination on the instrument panel with possible audible alarm

Potential Causes

The most common technical causes for S-5 on used PC55MR-2 excavators include:

• Failed engine speed sensor (magnetic pickup sensor) due to age-related deterioration or heat exposure
• Damaged sensor wiring harness particularly at flex points near the engine mount where vibration causes wire chafing
• Corroded or loose connector pins at the sensor or ECM connection points—extremely common on machines with 3,000+ operating hours
• Worn flywheel ring gear teeth causing irregular sensor signal gaps (more prevalent in high-hour used machines)
• ECM internal fault affecting the speed sensor input circuit
• Poor ground connections between the sensor circuit and engine block due to corrosion buildup

How to Troubleshoot and Fix Code S-5

Step 1: Visual Inspection Begin by locating the engine speed sensor mounted on the flywheel housing. Inspect the sensor for physical damage, oil contamination, or excessive debris buildup. Check the wiring harness from sensor to ECM for obvious abrasion, particularly where it passes through metal brackets or touches hot engine components. On used excavators, examine all connector terminals for corrosion (green/white deposits) and ensure connections are tight and waterproof boots are intact.

Step 2: Sensor Gap and Flywheel Inspection Verify the air gap between the speed sensor tip and flywheel teeth using a feeler gauge—specification is typically 0.5-1.5mm. If adjusted incorrectly or the sensor has backed out (common vibration issue), reset to proper clearance. Inspect the flywheel ring gear teeth for damage, cracks, or missing teeth that would create signal interruptions.

Step 3: Electrical Testing Using a digital multimeter, check sensor resistance across the two terminals—typical specification is 150-300 ohms (verify in your service manual). Test the wiring harness continuity from sensor connector to ECM pins, checking for open circuits or shorts to ground. Measure voltage output while cranking the engine; you should see an AC voltage signal of approximately 0.5-5V that increases with engine speed.

Step 4: ECM and Ground Circuit Verification Check all ground connections between the ECM, engine block, and frame. Clean ground points and apply dielectric grease. If available, use Komatsu diagnostic software (KOMTRAX or KDW) to monitor real-time sensor data and verify ECM functionality. For used machines with intermittent codes, perform a "wiggle test" on harnesses while monitoring live data to identify loose connections.

Step 5: Component Replacement If diagnostics confirm sensor failure, replace with a genuine Komatsu speed sensor (part number should be verified through dealer for PC55MR-2 compatibility). When installing, ensure proper torque specifications and correct air gap. For harness damage on older machines, consider replacing the entire sensor-to-ECM harness rather than splicing repairs, as this prevents future intermittent faults.

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Professional Disclaimer: This guide provides general diagnostic procedures for informational purposes. Fault code diagnosis can be complex and may require specialized tools and training. Always consult the official Komatsu service manual for your specific machine serial number and consider professional diagnostic support for persistent issues. Improper repairs can result in equipment damage or safety hazards.