Sumitomo SH350HD-5 Fault Code 108: Complete Diagnostic Guide

What is Sumitomo SH350HD-5 Fault Code 108?

Fault Code 108 on the Sumitomo SH350HD-5 excavator indicates a malfunction in the Engine Coolant Temperature (ECT) Sensor circuit, specifically detecting an abnormally high voltage signal or open circuit condition. This code is generated when the Engine Control Module (ECM) receives a voltage reading above the manufacturer's specified threshold (typically above 4.8 volts), suggesting the sensor circuit is broken, disconnected, or experiencing a wiring fault.

The ECT sensor plays a critical role in engine management by monitoring coolant temperature and sending voltage signals to the ECM. This data directly influences fuel injection timing, engine warm-up protocols, and cooling fan activation. On used excavators like the SH350HD-5, this sensor circuit is vulnerable to corrosion, harness wear, and connector degradation—issues that become more prevalent as machines accumulate operating hours in harsh construction environments.

Common Symptoms

When Code 108 is active, operators typically experience:

• Check Engine Light illuminated on the instrument cluster with possible fault code display
• Hard starting conditions, especially in cold weather, as the ECM defaults to predetermined temperature values
• Excessive white or black exhaust smoke during warm-up due to improper fuel mixture calculations
• Engine cooling fan running continuously at maximum speed regardless of actual coolant temperature
• Reduced engine performance or activation of derate mode to protect the engine from potential overheating

Potential Causes

The most common technical reasons for Code 108 on used SH350HD-5 excavators include:

• Failed ECT sensor with internal open circuit (common after 8,000+ operating hours)
• Damaged or corroded sensor connector at the thermostat housing—a known rub point on this model
• Broken or chafed wiring harness between the ECT sensor and ECM, particularly where the harness routes near the engine mount bracket
• Corroded or loose ground connections affecting the sensor's reference voltage circuit
• ECM internal circuit failure (rare, but possible in high-hour machines with previous electrical issues)

How to Troubleshoot and Fix Code 108

Step 1: Visual Inspection Begin by locating the ECT sensor on the engine block near the thermostat housing. Inspect the sensor connector for corrosion, bent pins, or moisture intrusion. On used excavators, check for harness chafing against metal surfaces or heat shields—a common wear point on the SH350HD-5.

Step 2: Electrical Testing Disconnect the ECT sensor connector. Using a digital multimeter, measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms (consult your service manual for exact specifications). If the reading shows infinite resistance (open circuit), replace the sensor.

Step 3: Wiring Harness Verification With the sensor disconnected, measure voltage at the harness connector. You should see approximately 5 volts on the signal wire with ignition on. If voltage is absent or erratic, trace the wiring harness back to the ECM, checking for breaks or shorts. Pay special attention to connector pins for green corrosion or terminal spread.

Step 4: Ground Circuit Check Verify the sensor's ground circuit has less than 0.5 ohms resistance to chassis ground. Poor grounds are extremely common in older machines and cause intermittent high-voltage readings.

Step 5: Component Replacement and Verification After replacing faulty components, clear the fault code using diagnostic software compatible with Sumitomo equipment. Perform a heat cycle test to verify the sensor reads correctly across temperature ranges before returning the machine to service.

---

Disclaimer: This guide provides general diagnostic procedures for Code 108. Always consult the official Sumitomo service manual for your specific machine serial number. If you're uncomfortable performing electrical diagnostics, contact a certified heavy equipment technician to prevent further damage to the ECM or electrical system.