Sumitomo SH300-6 Fault Code 687: Expert Diagnostic Guide

What is Sumitomo SH300-6 Fault Code 687?

Fault Code 687 on the Sumitomo SH300-6 excavator indicates a malfunction in the Engine Speed Sensor Circuit, specifically detecting an abnormal signal or voltage irregularity from the crankshaft position sensor.

This code triggers when the Electronic Control Module (ECM) receives erratic, missing, or out-of-range signals from the engine speed sensor during operation. The engine speed sensor is critical for proper fuel injection timing, engine RPM monitoring, and overall engine management system performance. On used SH300-6 models, this fault directly impacts combustion efficiency and can lead to significant power loss or operational shutdown if left unresolved.

Common Symptoms

When Code 687 is active on your Sumitomo SH300-6, operators typically experience:

• Check Engine Warning Light illuminated on the instrument panel with possible audible alarm
• Engine RPM Fluctuation or inconsistent idle speed, particularly noticeable during warm-up
• Loss of Engine Power or sudden derating to protective mode during operation
• Hard Starting Conditions or extended cranking time, especially when the engine is cold
• Intermittent Engine Stalling during low-speed hydraulic work or when transitioning between idle and load

Potential Causes

The most common technical causes for Code 687 on used SH300-6 excavators include:

• Damaged Engine Speed Sensor: Physical deterioration from heat exposure, oil contamination, or internal component failure
• Wiring Harness Issues: Chafing, abrasion, or breaks in the sensor wiring near the engine block mounting points—a known wear point on older SH300-6 models
• Corroded Electrical Connectors: Moisture intrusion at the sensor connector causing intermittent signal loss
• ECM Internal Fault: Rare but possible voltage regulator issues within the control module itself
• Incorrect Air Gap: Sensor-to-flywheel spacing outside manufacturer specifications (typically 0.5-1.5mm)
• Magnetic Debris Accumulation: Metal shavings on the sensor tip interfering with magnetic field detection

How to Troubleshoot and Fix Code 687

Step 1: Visual Inspection Begin by locating the crankshaft position sensor on the engine bell housing (typically driver's side, near the flywheel). Inspect the wiring harness for visible damage, particularly where it routes past the engine mount and exhaust components. On used excavators, check for harness rub marks, melted insulation, or oil-soaked wires.

Step 2: Connector and Sensor Examination Disconnect the sensor electrical connector and inspect both male and female terminals for corrosion, bent pins, or moisture. Clean with electrical contact cleaner if contamination is present. Remove the sensor and check for metal debris on the magnetic tip—clean carefully with a lint-free cloth.

Step 3: Electrical Testing Using a digital multimeter, measure sensor resistance between the two sensor terminals (typical specification: 200-400 ohms at room temperature—consult your service manual for exact values). Test wiring continuity from the sensor connector back to the ECM. Check for short circuits to ground on both signal wires.

Step 4: Air Gap Verification Reinstall the sensor and verify proper air gap using feeler gauges. The sensor face should be 0.8-1.2mm from the flywheel teeth. Improper spacing causes weak signal generation.

Step 5: Dynamic Testing With a Sumitomo diagnostic scanner (or compatible OEM software), monitor live engine speed sensor data during cranking. The signal should show consistent pulse patterns. Erratic readings confirm sensor replacement is necessary.

Step 6: ECM Evaluation If all sensor and wiring tests pass, the ECM may require inspection. Check ECM connector terminals for corrosion and verify proper ground connections at the chassis mounting points—critical on older machines.

For used excavators, always prioritize inspecting wear-prone areas before parts replacement. Harness routing corrections and connector refurbishment often resolve this code without expensive component changes.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Sumitomo service manual for your specific serial number and software version. If unfamiliar with electrical diagnostics or high-pressure fuel systems, seek assistance from a certified Sumitomo technician to prevent injury or further equipment damage.