Sumitomo SH350HD-6 Fault Code 687: Technical Guide

What is Sumitomo SH350HD-6 Fault Code 687?

Fault Code 687 on the Sumitomo SH350HD-6 indicates a malfunction in the engine coolant temperature (ECT) sensor circuit, specifically detecting an abnormal voltage signal or sensor reading outside the acceptable range. This diagnostic trouble code (DTC) is generated when the Electronic Control Module (ECM) receives voltage readings from the coolant temperature sensor that fall outside the predetermined parameters—typically indicating either an open circuit, short circuit, or sensor degradation.

This code is critical for the SH350HD-6 because the ECM relies on accurate coolant temperature data to manage fuel injection timing, engine warm-up cycles, and prevent overheating conditions. When this sensor fails, the engine may operate in a default "limp mode," reducing machine productivity and potentially causing long-term engine damage if ignored.

Common Symptoms

When Code 687 is active on your Sumitomo excavator, operators typically experience:

• Check Engine Light or warning lamp illuminated on the instrument panel
• Engine running in cold-start mode continuously, causing rough idling and excessive fuel consumption
• Reduced engine power or activation of derate mode limiting hydraulic performance
• Cooling fan running at maximum speed constantly, regardless of actual engine temperature
• Difficulty starting in cold weather conditions or prolonged warm-up periods

Potential Causes

The most common technical reasons for Code 687 on used SH350HD-6 excavators include:

• Coolant temperature sensor failure due to internal resistance drift or element corrosion (common in machines with 5,000+ hours)
• Wiring harness damage at known rub points near the engine block or along the valve cover where vibration causes insulation wear
• Corroded or loose connector pins at the sensor plug, especially on machines operated in wet or marine environments
• ECM internal fault affecting the sensor input circuit (rare but possible in high-hour machines)
• Contaminated coolant causing sensor element coating and inaccurate readings

How to Troubleshoot and Fix Code 687

Step 1: Visual Inspection Begin by locating the coolant temperature sensor on the engine block (typically near the thermostat housing). Inspect the wiring harness for visible damage, chafing, or burn marks. On used excavators, check connector terminals for corrosion or "green powder" buildup—clean with electrical contact cleaner if present.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across the sensor terminals. At room temperature (20°C/68°F), resistance should typically read between 2,000-3,000 ohms (consult Sumitomo service manual for exact specifications). Test the harness for continuity to the ECM and check for shorts to ground.

Step 3: Live Data Analysis Connect Sumitomo diagnostic software or a compatible scan tool to monitor live coolant temperature readings. Compare the sensor reading to actual engine temperature using an infrared thermometer. A discrepancy of more than 10°C indicates sensor replacement is necessary.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine or OEM-equivalent coolant temperature sensor. For used machines, always apply dielectric grease to connector pins during reassembly to prevent future corrosion. Clear the fault code and perform a test cycle.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Sumitomo service manual for your specific machine serial number and seek professional assistance from certified technicians for complex electrical diagnostics or repairs.