Sumitomo SH350HD-6 Fault Code 73: Complete Diagnostic Guide

What is Sumitomo SH350HD-6 Fault Code 73?

Fault Code 73 on the Sumitomo SH350HD-6 excavator indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading detected by the ECM (Engine Control Module).

This diagnostic trouble code specifically monitors the coolant temperature sensor circuit integrity and the engine's thermal management system. The ECM relies on accurate coolant temperature data to control fuel injection timing, idle speed adjustments, and fan engagement. On the SH350HD-6's Isuzu engine platform, this sensor plays a critical role in preventing thermal damage and maintaining optimal combustion efficiency. When this code triggers, the ECM may default to a preset temperature value, potentially causing poor fuel economy, hard starting, or inadequate engine protection.

Common Symptoms

When Code 73 is active on your SH350HD-6, operators typically experience:

• Check engine light or malfunction indicator illuminated on the instrument panel
• Cooling fan running continuously at high speed, regardless of actual engine temperature
• Difficulty starting when the engine is cold, or extended cranking times
• Poor fuel efficiency and black smoke from the exhaust due to improper fuel mapping
• Engine temperature gauge reading incorrectly (stuck at cold, hot, or fluctuating erratically)

Potential Causes

The most common technical failures triggering Code 73 on used SH350HD-6 excavators include:

• Faulty coolant temperature sensor (CTS) due to internal resistance drift or element failure
• Damaged wiring harness between the sensor and ECM, particularly at flex points near the engine mount or firewall
• Corroded or loose connector pins at the sensor plug, especially on machines with coolant leak history
• Shorted or open circuit in the sensor signal wire caused by harness chafing against engine components
• ECM internal fault (rare) affecting the temperature input circuit
• Low coolant level causing intermittent sensor exposure and erratic readings

How to Troubleshoot and Fix Code 73

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor location on the engine block (typically near the thermostat housing). Check for coolant leaks, corrosion on the connector, and damaged wiring. On used excavators, examine the wiring harness routing for abrasion points where it contacts the engine or frame.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. 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 sensor connector to the ECM. Check for short circuits to ground by measuring resistance between each signal wire and chassis ground (should read infinite resistance).

Step 3: Circuit Voltage Verification With the ignition on and sensor disconnected, measure reference voltage at the harness connector (should be approximately 5 volts). If voltage is absent or incorrect, trace the circuit back toward the ECM, checking for damaged wires or poor connections at intermediate connectors.

Step 4: Component Replacement If testing confirms sensor failure, replace the coolant temperature sensor with an OEM or equivalent part. Before installation, apply dielectric grease to connector terminals to prevent future corrosion. Clear the fault code using diagnostic software compatible with Sumitomo systems (such as dealer-level tools or aftermarket scan tools) and verify proper operation through a heat cycle test.

Disclaimer: This guide provides general diagnostic information. Complex electrical issues or ECM programming may require factory diagnostic equipment and trained technicians. Always consult the manufacturer's service manual and consider professional assistance for critical repairs on used heavy equipment.