Sumitomo SH240-6 Fault Code 73: Expert Diagnostic Guide

What is Sumitomo SH240-6 Fault Code 73?

Fault Code 73 on the Sumitomo SH240-6 excavator indicates a malfunction in the Engine Water Temperature Sensor circuit. This code is triggered when the Engine Control Module (ECM) detects an abnormal voltage signal from the coolant temperature sensor, typically caused by an open circuit, short circuit, or sensor reading outside the expected range (usually 0.5-4.5 volts).

The engine water temperature sensor is critical for proper engine management on this machine. It directly affects fuel injection timing, engine warm-up cycles, and cooling fan operation. When this sensor fails or provides incorrect readings, the ECM cannot properly regulate engine temperature, potentially leading to overheating, poor fuel economy, or engine protection mode activation.

Common Symptoms

• Check Engine Light illuminated on the dashboard with stored diagnostic code 73
• Engine derating or power reduction as the ECM enters a protective mode
• Cooling fan running continuously at high speed, even when the engine is cold
• Difficult cold starting or rough idle during warm-up periods
• Inaccurate temperature gauge readings on the instrument cluster, often showing cold or maximum readings

Potential Causes

The most common technical causes for Code 73 on used SH240-6 excavators include:

• Faulty coolant temperature sensor due to internal element degradation (common after 5,000+ operating hours)
• Damaged or corroded wiring harness between the sensor and ECM, particularly at connector pins near the engine block where heat and vibration cause wear
• Poor electrical connections at the sensor connector due to corrosion, moisture intrusion, or loose terminals
• Shorted sensor ground wire caused by harness rubbing against the engine block or frame
• ECM internal fault (rare, but possible in high-hour machines with electrical system stress)

How to Troubleshoot and Fix Code 73

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block near the thermostat housing. Check for physical damage, coolant leaks around the sensor base, and corrosion on the electrical connector. On used excavators, examine the wiring harness for abrasion points where it contacts metal surfaces—a common failure point on the SH240-6.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At room temperature (20°C/68°F), resistance should read approximately 2,000-3,000 ohms. Test the harness continuity from the sensor connector back to the ECM, checking for opens or shorts to ground. Measure supply voltage at the harness connector with the ignition on—it should read approximately 5 volts.

Step 3: Component Replacement and Verification If sensor resistance is out of specification or harness testing reveals damage, replace the faulty component using OEM or quality aftermarket parts. Clean all connector terminals with electrical contact cleaner and apply dielectric grease before reconnecting. Clear the fault code using diagnostic software (Sumitomo-specific or compatible scanner), then run the engine through a complete heat cycle to verify the repair.

Critical Note for Used Equipment: Before replacing the sensor, always inspect connector pins for "backed-out" terminals and check for coolant contamination inside the connector—both common issues on older machines that can cause false fault codes.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Sumitomo service manual for your specific machine serial number and consider professional assistance for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.