What is Hyundai R805LC-7 Fault Code 110-4?

Fault Code 110-4 on the Hyundai R805LC-7 excavator indicates an Engine Coolant Temperature Sensor Circuit High Voltage condition. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally high voltage reading from the coolant temperature sensor circuit, typically exceeding 4.8-5.0 volts when normal operating range should be 0.5-4.5 volts.

The coolant temperature sensor is critical for the R805LC-7's electronic engine management system, providing real-time data that influences fuel injection timing, engine warm-up enrichment, and cooling fan activation. When this circuit fails high, the ECM loses accurate temperature monitoring capability, potentially causing improper engine operation, increased emissions, and possible thermal damage to the Cummins QSX15 engine powering this machine.

Common Symptoms

• Engine warning light illuminated on the instrument cluster, often accompanied by a distinct error message on the LCD display
• Engine operating in derate mode or fail-safe mode, limiting power output to protect components from thermal damage
• Cooling fan running continuously at maximum speed regardless of actual engine temperature
• Hard starting or rough idle, especially when engine is cold, due to incorrect fuel mixture compensation
• Reduced hydraulic performance as the ECM limits engine RPM to prevent overheating conditions

Potential Causes

The high voltage condition for Code 110-4 typically stems from these specific failures common in used excavators:

Open circuit in the coolant temperature sensor wiring harness, often caused by wire breakage near flex points where the harness routes over the engine block or through the frame rail. The R805LC-7 has a known rub point near the left engine mount bracket.

Faulty coolant temperature sensor with internal circuit failure, creating an open condition. Sensors on machines with 8,000+ hours frequently develop corrosion on internal elements.

Damaged or corroded connector terminals at the sensor itself or at the ECM connection point C101, especially on machines operated in coastal or high-humidity environments.

Intermittent short to voltage from damaged harness insulation contacting 12V or 24V power wires in the main engine harness bundle.

ECM internal circuit failure (less common) affecting the sensor reference voltage circuit or analog-to-digital converter.

How to Troubleshoot and Fix Code 110-4

Step 1: Visual Inspection and Connector Check Begin by disconnecting the battery negative terminal for safety. Locate the coolant temperature sensor on the engine block (typically positioned near the thermostat housing on the QSX15 engine). Inspect the sensor connector for corrosion, bent pins, or moisture intrusion. Check the wiring harness from sensor to ECM for visible damage, particularly at known flex points and rub areas. On used R805LC-7 excavators, pay special attention to harness routing near the turbocharger and engine mounts where heat and vibration accelerate wear.

Step 2: Electrical Testing with Multimeter With the sensor disconnected, use a digital multimeter (DMM) to measure resistance across the sensor terminals. A properly functioning coolant temperature sensor should read approximately 2,000-3,000 ohms at room temperature (decreasing as temperature increases). An infinite reading indicates an open sensor requiring replacement. Next, measure voltage at the harness connector (engine side) with ignition on: you should see approximately 5 volts reference voltage from the ECM. If voltage reads battery voltage (12V/24V) or fluctuates erratically, suspect harness damage or ECM issues.

Step 3: Harness Continuity and Repair Test continuity from the sensor connector through to ECM connector C101 (consult service manual for exact pin locations—typically Pin 23 for signal and Pin 45 for ground). Resistance should be less than 5 ohms. For used excavators, inspect the entire harness routing, checking for chafing against brackets, especially where protective loom has deteriorated. Repair any damaged sections using proper gauge wire, heat-shrink connectors, and protective sleeving. If harness tests good and sensor is faulty, replace the OEM-spec coolant temperature sensor (Hyundai part number 39210-45000 or equivalent) and apply dielectric grease to connector terminals.

Step 4: Clear Code and Verify Repair After repairs, reconnect all components and use Hyundai diagnostic software (Hi-MATE) or compatible scan tool to clear the fault code. Start the engine and monitor live data, verifying coolant temperature readings correspond with actual engine temperature (use infrared thermometer for comparison). Perform a function test by allowing engine to reach full operating temperature while monitoring for code recurrence.

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Disclaimer: This guide provides general troubleshooting information for Code 110-4. Hydraulic and electrical systems on large excavators can be complex and potentially dangerous. Always consult the official Hyundai R805LC-7 service manual and consider professional diagnostic assistance, especially for ECM-related issues or if you lack proper diagnostic equipment.