Sumitomo SH350HD-5 Fault Code 118: Complete Diagnostic Guide

What is Sumitomo SH350HD-5 Fault Code 118?

Fault Code 118 on the Sumitomo SH350HD-5 excavator indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal temperature reading detected by the Engine Control Module (ECM). This code typically triggers when the ECM receives voltage signals outside the expected range from the coolant temperature sensor, suggesting either sensor failure, wiring issues, or genuine engine overheating conditions.

The coolant temperature sensor plays a critical role in the SH350HD-5's Isuzu engine management system. It provides real-time data that the ECM uses to adjust fuel injection timing, idle speed, and cooling fan operation. On used excavators, this sensor's accuracy directly impacts fuel efficiency, emissions control, and prevents catastrophic engine damage from overheating. Ignoring Code 118 can lead to poor engine performance, increased fuel consumption, or complete engine shutdown in protective mode.

Common Symptoms

When Code 118 is active on your SH350HD-5, operators typically experience:

• Check Engine light illuminated on the instrument panel with possible overheat warning indicator
• Hard starting conditions, especially during cold starts, or prolonged cranking times
• Cooling fan running continuously at high speed, even when engine temperature appears normal
• Engine derating or power loss as the ECM enters protective mode limiting RPM and hydraulic functions
• Black smoke from exhaust due to incorrect fuel mixture calculations based on faulty temperature data

Potential Causes

The most common technical reasons for Code 118 on used SH350HD-5 excavators include:

• Coolant temperature sensor failure due to age-related deterioration or internal resistance drift (common after 5,000+ operating hours)
• Damaged or corroded wiring harness at connector points near the thermostat housing where heat and vibration cause insulation breakdown
• Shorted or open circuit in sensor wiring, particularly at harness rub points along the engine block
• Corroded electrical connectors at the sensor or ECM due to coolant leaks or moisture intrusion
• Low coolant levels causing genuine overheating and triggering legitimate high-temperature readings
• Failed ECM (less common) with internal circuit board issues affecting temperature input channels

How to Troubleshoot and Fix Code 118

Step 1: Visual Inspection and Preliminary Checks

Begin by checking actual coolant levels and inspecting for visible leaks around the radiator, hoses, and water pump. On used excavators, examine the sensor wiring harness from the coolant temperature sensor (typically located near the thermostat housing) to the ECM for signs of chafing, burns, or rodent damage. Inspect the sensor connector for corrosion, bent pins, or coolant contamination—a common issue on older machines.

Step 2: Sensor Resistance Testing

Disconnect the coolant temperature sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At room temperature (20°C/68°F), resistance should typically read between 2,000-3,000 ohms; at operating temperature (80°C/176°F), it should drop to approximately 300-400 ohms. Compare readings against Sumitomo's specifications. If readings are infinite (open circuit) or near zero (short circuit), replace the sensor.

Step 3: Circuit Voltage and Wiring Verification

With the sensor disconnected, check for 5-volt reference voltage at the harness connector using your multimeter. The ECM should supply approximately 5 volts to the sensor. Also verify ground continuity on the sensor ground wire. If voltage is absent or significantly different, trace the wiring for breaks or shorts. On used SH350HD-5 models, pay special attention to harness routing near exhaust manifolds where heat degradation is prevalent.

Step 4: Advanced Diagnostics

Use Sumitomo diagnostic software or a compatible OEM scan tool to monitor live coolant temperature data while the engine warms up. The reading should increase gradually and stabilize around 82-88°C. Erratic readings or values stuck at extreme temperatures (-40°C or 140°C+) confirm sensor or wiring issues. Before replacing components on used equipment, clean all connectors with electrical contact cleaner and apply dielectric grease to prevent future corrosion.

Disclaimer: This guide provides general diagnostic procedures for Code 118. Always consult the official Sumitomo service manual for your specific serial number and consider professional assistance from certified Sumitomo technicians for complex electrical diagnostics or ECM-related issues.