Komatsu PC35MR-2 Fault Code S-9: Complete Diagnostic Guide

What is Komatsu PC35MR-2 Fault Code S-9?

Fault Code S-9 on the Komatsu PC35MR-2 mini excavator indicates a malfunction in the revolution sensor circuit, which monitors engine RPM and crankshaft position. This diagnostic trouble code (DTC) specifically relates to the revolution sensor mounted on the engine block that sends critical speed and timing signals to the Engine Control Module (ECM).

The revolution sensor plays a vital role in fuel injection timing, engine performance optimization, and emissions control on this compact excavator. When the ECM detects an abnormal signal—whether due to intermittent connection, out-of-range voltage, or complete signal loss—it triggers S-9 and may initiate protective engine derating to prevent potential damage. For used PC35MR-2 machines, this sensor circuit is particularly vulnerable to vibration-related failures and connector corrosion.

Common Symptoms

When Code S-9 becomes active, operators typically experience:

• Erratic engine behavior including sudden RPM fluctuations or inconsistent idle
• Hard starting or extended cranking before the engine fires
• Loss of power or unexpected engine derating during operation, especially under load
• Engine stalling at idle or during transitions from low to high throttle
• Diagnostic warning light illuminated on the instrument panel with possible audible alarm

Potential Causes

The most common technical failures triggering S-9 on used PC35MR-2 excavators include:

• Revolution sensor failure due to internal component breakdown or heat damage (common after 3,000+ operating hours)
• Damaged or corroded wiring harness connections at the sensor plug, particularly where the harness passes near the engine mount (known wear point)
• Broken or frayed sensor wiring caused by vibration or rubbing against the engine block over time
• Contaminated or loose sensor mounting, allowing excessive air gap between sensor and flywheel gear
• ECM-side circuit issues including corroded pins in the ECM connector or internal ECM faults (less common)
• Poor ground connections at the engine block ground strap, affecting sensor signal quality

How to Troubleshoot and Fix Code S-9

Step 1: Visual Inspection Begin by locating the revolution sensor on the driver's side of the engine near the flywheel housing. Thoroughly inspect the sensor body for physical damage, cracks, or oil contamination. Check the wiring harness from sensor to ECM for obvious damage, particularly at flex points and where it routes near hot engine components. On used machines, pay special attention to connector corrosion—disconnect the sensor plug and inspect pins for green corrosion or pushed-back terminals.

Step 2: Electrical Testing Using a digital multimeter, measure sensor resistance at the sensor connector (disconnected from harness). Typical specifications range from 200-400 ohms, but consult your service manual for exact values. Test harness continuity from the sensor connector back to the ECM connector with the harness disconnected at both ends—you should read near 0 ohms. Check for shorts to ground by measuring resistance between each sensor wire and chassis ground (should read infinite resistance/open circuit).

Step 3: Sensor Air Gap and Mounting Verify the air gap between the sensor tip and flywheel ring gear teeth. The specification is typically 0.5-1.5mm (0.020"-0.060"). Use feeler gauges to confirm proper clearance. Check that the sensor mounting bolt is torqued properly—a loose sensor creates inconsistent signals. Clean any metal debris or contamination from the sensor tip.

Step 4: Signal Testing and Replacement With an oscilloscope or AC voltage function on your multimeter, crank the engine and verify the sensor produces an AC voltage signal (typically 1-5 volts AC depending on cranking speed). No signal indicates sensor failure. Before replacing the sensor on a used excavator, clean all harness connectors with electrical contact cleaner and apply dielectric grease. If the sensor tests faulty, replace it with a genuine Komatsu part (Part #6754-21-1310 or equivalent) and clear codes using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu PC35MR-2 service manual for exact specifications and procedures. If you're uncomfortable performing electrical diagnostics, consult a certified Komatsu technician to prevent further damage to your machine.