What is Sumitomo SH360HD-6 Fault Code 73?

Fault Code 73 on the Sumitomo SH360HD-6 excavator indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal temperature reading detected by the Engine Control Module (ECM). This diagnostic trouble code (DTC) specifically signals that the coolant temperature sensor is either sending values outside the normal operating range, has experienced a circuit interruption, or the wiring harness has developed a short or open circuit condition.

This code is critical for the SH360HD-6 because the ECM relies on accurate coolant temperature data to control fuel injection timing, engine protection protocols, and hydraulic oil temperature management. When Code 73 activates, the engine may enter a derate mode to prevent overheating damage, significantly reducing your machine's productivity and potentially masking actual overheating conditions that could cause catastrophic engine failure.

Common Symptoms

• Check Engine light or warning lamp illuminated on the instrument cluster
• Engine enters reduced power mode (derate) limiting hydraulic functions and travel speed
• Erratic temperature gauge readings or gauge stuck at cold/hot positions regardless of actual engine temperature
• Cooling fan running continuously at high speed even when engine is cold
• Difficulty starting in cold weather conditions or extended cranking times

Potential Causes

The most common causes of Code 73 on used SH360HD-6 excavators include:

• Coolant temperature sensor failure due to age-related degradation or internal element breakdown
• Damaged wiring harness between the sensor and ECM, particularly at flex points near the engine mount or along the frame rail where vibration causes wire chafing
• Corroded or loose connector pins at the sensor plug—extremely common on machines operating in wet or coastal environments
• ECM internal fault (less common) affecting the temperature sensor input circuit
• Coolant contamination causing sensor element fouling or coating
• Previous repair attempts using incorrect sensor specifications or aftermarket parts with wrong resistance values

How to Troubleshoot and Fix Code 73

Step 1: Visual Inspection Begin by locating the coolant temperature sensor on the engine block (typically near the thermostat housing). Inspect the wiring harness from sensor to ECM for obvious damage, rubbing points against frame members, melted insulation, or rodent damage. Check the connector for corrosion, pushed-back pins, or moisture intrusion—common on used excavators with 5,000+ operating hours.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure sensor resistance. At room temperature (68°F/20°C), resistance should typically read between 2,000-3,000 ohms (consult your service manual for exact specifications). Test the harness continuity from the ECM connector to the sensor plug—any reading above 5 ohms indicates wire damage. Check for short to ground by measuring resistance between each sensor wire and chassis ground (should read infinite/OL).

Step 3: Sensor and Harness Replacement If resistance values are out of specification or harness shows continuity issues, replace the coolant temperature sensor with a genuine Sumitomo or OEM-equivalent part. For used excavators, always replace the connector pigtail simultaneously, as corrosion often damages connector internals invisibly. Route new harness sections away from heat sources and secure with proper vibration-resistant clamps. Clear the code using diagnostic software and perform a test cycle monitoring live data to confirm proper temperature readings (should rise gradually from ambient to 180-200°F operating range).

Disclaimer: This guide provides general troubleshooting information. Always consult the official Sumitomo service manual for your specific serial number range and consider professional diagnostic assistance for complex electrical issues or if you lack proper testing equipment.