Hyundai R385LVS Fault Code 108-4: Technical Guide

What is Hyundai R385LVS Fault Code 108-4?

Fault Code 108-4 indicates an abnormal voltage condition in the Engine Coolant Temperature (ECT) sensor circuit, specifically a voltage signal that is above the normal operating range. This code is triggered when the Engine Control Module (ECM) detects a sensor output voltage higher than the calibrated threshold, typically above 4.5-4.8 volts.

In the Hyundai R385LVS excavator, the ECT sensor plays a critical role in engine protection and performance optimization. It provides real-time coolant temperature data to the ECM, which uses this information to control fuel injection timing, throttle response, cooling fan operation, and implement temperature-based engine derating. When Code 108-4 is active, the ECM cannot accurately monitor engine temperature, potentially leading to overheating damage or unnecessary performance limitations.

Common Symptoms

When Code 108-4 is active on your R385LVS, you may experience:

• Check Engine Light or engine warning lamp illuminated on the dashboard
• Engine derate mode engaged, limiting machine to reduced RPM and power output
• Cooling fan running continuously at high speed, even during cold startup
• Hard starting conditions or rough idle, especially when the engine is actually cold
• Inaccurate temperature gauge readings showing abnormally low temperatures or erratic fluctuations

Potential Causes

The most common technical causes for Code 108-4 in used R385LVS excavators include:

• Open circuit in the ECT sensor wiring harness, often caused by wire breakage near the engine block mounting points where vibration is highest
• Failed ECT sensor with internal circuit degradation (common after 5,000-8,000 operating hours)
• Corroded or loose connector pins at the ECT sensor plug, especially on machines operated in wet or coastal environments
• Damaged sensor ground circuit, creating a false high-voltage reading
• Chafed wiring where the harness contacts the engine valve cover or fuel lines (known rub point on this model)
• ECM internal fault (rare, but possible in high-hour machines with electrical system history)

How to Troubleshoot and Fix Code 108-4

Step 1: Visual Inspection Begin by locating the ECT sensor on the engine block (typically near the thermostat housing). Inspect the wiring harness from the sensor to the ECM for visible damage, burns, or wear points. On used excavators, check for harness rubbing against the valve cover, hydraulic lines, or engine brackets—a common failure point on the R385LVS. Examine the connector for bent pins, corrosion (green/white deposits), or moisture intrusion.

Step 2: Electrical Testing Disconnect the ECT sensor connector. Using a digital multimeter (DMM), measure the resistance across the sensor terminals with the engine cold. A properly functioning ECT sensor should read approximately 2,000-3,000 ohms at 68°F (20°C). If the reading shows infinite resistance (OL), the sensor has failed internally. Next, check the harness-side voltage with the ignition on, engine off: you should see approximately 5.0 volts at the signal wire (consult your service manual for specific pin identification). A reading above 5.2 volts suggests an ECM supply circuit fault.

Step 3: Ground Circuit Verification With the sensor still disconnected, test for continuity between the sensor ground pin and a known good chassis ground. Resistance should be less than 1 ohm. High resistance here indicates a compromised ground circuit. On used machines, corrosion at ground connection points is extremely common—clean all ground terminals with electrical contact cleaner and a wire brush.

Step 4: Advanced Diagnostics If sensor and wiring tests pass, connect Hyundai Hi-MATE diagnostic software or an equivalent scan tool. Monitor live ECT sensor voltage while gently wiggling the harness—intermittent spikes above 4.8 volts indicate internal wiring breaks. Also check for Diagnostic Trouble Codes (DTCs) in other systems; multiple electrical codes may point to ECM power supply issues rather than sensor failure.

Step 5: Component Replacement After confirming a failed component, replace the ECT sensor using OEM or high-quality aftermarket parts rated for heavy-duty applications. Apply dielectric grease to connector terminals before reassembly to prevent future corrosion. Clear the fault code using diagnostic software and perform a test run, monitoring coolant temperature behavior through normal operating cycles.

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Disclaimer: This guide provides general troubleshooting information for Code 108-4. Always consult the official Hyundai service manual for your specific serial number and software version. If you're unfamiliar with electrical diagnostics or lack proper tools, consult a certified heavy equipment technician to prevent further damage or safety hazards.