Sumitomo SH220LC-6 Fault Code 685: Complete Diagnostic Guide

What is Sumitomo SH220LC-6 Fault Code 685?

Fault Code 685 on the Sumitomo SH220LC-6 excavator indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading detected by the Engine Control Module (ECM). This code typically signals either a sensor failure, wiring issue, or actual engine overheating condition that requires immediate attention.

The coolant temperature sensor plays a critical role in the SH220LC-6's Isuzu engine management system. It provides real-time data to the ECM for fuel injection timing, idle speed control, and overheat protection. When this sensor circuit fails or provides inconsistent readings, the ECM cannot properly regulate engine performance, potentially leading to reduced efficiency, increased emissions, or catastrophic engine damage if an actual overheat condition goes undetected.

Common Symptoms

When Code 685 is active on your SH220LC-6, you may experience:

• Engine warning light illuminated on the instrument cluster, often accompanied by a temperature warning icon
• Engine derate mode where power output is automatically reduced to protect the engine
• Erratic temperature gauge readings, showing abnormally high, low, or fluctuating coolant temperatures
• Hard starting or rough idle, especially when the engine is cold
• Cooling fans running continuously at maximum speed regardless of actual engine temperature

Potential Causes

For used Sumitomo SH220LC-6 excavators, the most common causes of Code 685 include:

• Coolant temperature sensor failure due to age, thermal stress, or internal electrical degradation
• Damaged wiring harness near the sensor connector, particularly where the harness runs along the engine block (a known wear point on this model)
• Corroded or loose electrical connectors at the sensor or ECM, common in machines with high operating hours in dusty or wet conditions
• Faulty ECM with internal circuit board damage (less common but possible in older units)
• Actual low coolant level or air pockets in the cooling system causing genuine temperature irregularities
• Grounding issues where the sensor ground circuit has deteriorated due to corrosion

How to Troubleshoot and Fix Code 685

Step 1: Visual Inspection Begin with a thorough physical examination. Check the coolant temperature sensor (typically located on the engine cylinder head near the thermostat housing) and its wiring harness. Look for obvious damage like frayed wires, melted insulation, or corroded connectors. On used excavators, pay special attention to harness routing where it contacts hot engine surfaces or vibrates against metal brackets—these are common failure points on the SH220LC-6.

Step 2: Electrical Testing Using a digital multimeter, test the sensor resistance. Disconnect the sensor connector and measure resistance across the sensor terminals. At room temperature (20°C/68°F), resistance should typically read between 2,000-3,000 ohms. Compare your reading against Sumitomo specifications. Next, check for voltage supply at the harness connector with the ignition on—you should see approximately 5 volts reference voltage from the ECM.

Step 3: Circuit Continuity and Ground Testing Test the wiring between the sensor and ECM for continuity and resistance (should be less than 5 ohms). Check the ground circuit specifically, as corrosion in ground connections is extremely common in older machines. Inspect the ECM connector pins for corrosion or bent terminals.

Step 4: Coolant System Verification Before replacing components, verify the cooling system is properly filled and bled of air. Low coolant or air pockets can cause legitimate temperature spikes that trigger Code 685.

Step 5: Component Replacement If testing confirms sensor failure, replace the coolant temperature sensor with a genuine Sumitomo or OEM-equivalent part. Always apply anti-seize compound to the threads and use a new sealing washer. Clear the fault code using diagnostic software and monitor for recurrence.

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Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Sumitomo service manual for your specific machine serial number. If you're uncomfortable performing electrical diagnostics or if the problem persists after basic troubleshooting, consult a certified Sumitomo dealer or qualified heavy equipment technician.