What is Hyundai R805LC-7 Fault Code 108-4?

Fault Code 108-4 on the Hyundai R805LC-7 excavator indicates a "Boost Pressure Sensor Circuit Low Voltage" condition. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally low voltage signal from the turbocharger boost pressure sensor, typically below the manufacturer's specified threshold of approximately 0.5 volts.

The boost pressure sensor monitors the intake manifold pressure generated by the turbocharger and sends this critical data to the ECM. This information is essential for proper fuel injection timing, air-fuel ratio calculations, and overall engine performance optimization. On a large mining-class excavator like the R805LC-7, maintaining accurate boost pressure readings is crucial for achieving maximum hydraulic power output and fuel efficiency during demanding digging operations.

Common Symptoms

When Code 108-4 is active on your Hyundai R805LC-7, you may experience:

• Check Engine Light or malfunction indicator lamp illuminated on the instrument cluster
• Noticeable power loss or engine derate mode, especially under heavy loads or during bucket breakout operations
• Black smoke from the exhaust due to improper air-fuel mixture calculations
• Rough idle or hesitation during acceleration when increasing engine RPM
• Reduced hydraulic response and slower cycle times due to diminished engine output

Potential Causes

The most common technical reasons for Code 108-4 on used R805LC-7 excavators include:

• Damaged or corroded wiring harness between the boost pressure sensor and ECM, particularly at harness routing points near the engine block where vibration and heat cause deterioration
• Failed boost pressure sensor due to internal circuit breakdown or contamination from oil vapor in the intake system
• Corroded or loose electrical connectors at the sensor or ECM connection points—common in machines with high operating hours
• Broken or chafed wires at known rub points near the turbocharger heat shield
• Faulty ECM internal circuitry (rare, but possible in high-hour machines)
• Poor ground connection on the sensor circuit

How to Troubleshoot and Fix Code 108-4

Step 1: Visual Inspection Begin with a thorough inspection of the boost pressure sensor located on the intake manifold near the turbocharger. Check the three-pin connector for corrosion, bent pins, or moisture intrusion—extremely common on used excavators. Inspect the wiring harness from sensor to ECM for obvious damage, particularly where the harness passes near hot or moving components.

Step 2: Electrical Testing Using a digital multimeter (DMM), perform a voltage supply check. With the key on and engine off, verify the sensor receives approximately 5 volts on the reference voltage wire. Check the signal wire voltage—it should read between 0.5-4.5 volts depending on atmospheric conditions. Measure resistance to ground on the ground wire; it should be less than 5 ohms. Any deviation indicates wiring issues.

Step 3: Sensor and Harness Testing Disconnect the sensor and measure its internal resistance according to Hyundai specifications (typically 50-150 ohms at room temperature). If readings are out of range, replace the sensor. For used excavators, perform a wiggle test on all harness connections while monitoring voltage to identify intermittent connection failures caused by vibration damage or connector wear.

Step 4: Advanced Diagnostics Connect Hyundai Hi-MATE diagnostic software or equivalent to perform active tests and monitor real-time sensor data. Compare boost pressure readings at various engine speeds against specifications. Check for previous fault code history that might indicate chronic wiring problems.

Disclaimer: This guide provides general diagnostic information. Always consult the official Hyundai R805LC-7 service manual and consider professional assistance from certified Hyundai technicians for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.