What is Hyundai R335LC-9T Fault Code 108-4?

Fault Code 108-4 on the Hyundai R335LC-9T excavator indicates a High Signal Voltage condition in the Atmospheric Pressure Sensor circuit. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally high voltage reading from the atmospheric pressure sensor, typically exceeding 4.5-4.8 volts when normal operation should remain between 0.5-4.5 volts.

The atmospheric pressure sensor is critical for the R335LC-9T's Electronic Control System because it provides essential altitude and barometric data to the ECM. This information allows the engine management system to adjust fuel injection timing, air-fuel ratios, and turbocharger boost pressure for optimal combustion efficiency. When this sensor circuit fails, the excavator's performance and fuel economy can be significantly compromised, especially at varying elevations.

Common Symptoms

When Code 108-4 is active on your Hyundai R335LC-9T, operators typically experience:

• Check Engine Light illuminated on the instrument cluster with possible stored fault code
• Reduced engine power or hesitation during acceleration, particularly noticeable during hydraulic work cycles
• Hard starting conditions or extended cranking time, especially in cold weather or at higher altitudes
• Black smoke from the exhaust due to improper fuel-air mixture calculations
• Poor fuel economy as the ECM defaults to open-loop operation without accurate atmospheric data

Potential Causes

The most common technical failures causing Code 108-4 in used R335LC-9T excavators include:

• Open circuit in the atmospheric pressure sensor wiring harness, often caused by rodent damage or physical wear near the main harness routing along the engine frame
• Corroded or damaged connector pins at the sensor plug—a frequent issue in machines operating in marine, coastal, or high-humidity environments
• Failed atmospheric pressure sensor internally shorted to 5-volt reference supply
• Damaged ECM wiring where the harness passes through the bulkhead near the cab mounting points (known wear point on LC-9T models)
• Faulty ECM providing incorrect reference voltage, though this is less common than sensor/wiring failures

How to Troubleshoot and Fix Code 108-4

Step 1: Visual Inspection Begin with a thorough inspection of the atmospheric pressure sensor, typically mounted on the engine ECM or nearby firewall. Check the electrical connector for corrosion, bent pins, moisture intrusion, or physical damage. On used excavators, pay special attention to harness routing where cables contact frame members—this is a common failure point on machines with 5,000+ operating hours.

Step 2: Electrical Testing Using a digital multimeter (DMM), disconnect the atmospheric pressure sensor and measure voltage at the harness connector. With ignition ON and engine OFF, verify approximately 5 volts on the reference voltage pin and proper ground continuity (less than 0.5 ohms resistance to chassis ground). If reference voltage exceeds 5.2 volts or shows no voltage, suspect ECM issues or open circuits in the main harness.

Step 3: Sensor Resistance Check Measure resistance across the sensor terminals. Specifications vary, but typical atmospheric pressure sensors show resistance values between 20-80 ohms depending on current barometric pressure. Infinite resistance indicates an open internal sensor circuit, while zero resistance suggests internal shorting.

Step 4: Signal Wire Continuity Test continuity between the sensor signal pin and the corresponding ECM terminal (consult Hyundai service manual wiring diagrams specific to R335LC-9T). Any resistance above 5 ohms indicates damaged wiring requiring repair or harness replacement.

Step 5: Component Replacement If sensor and wiring test satisfactorily, replace the atmospheric pressure sensor with a genuine Hyundai OEM part or verified equivalent. After installation, clear fault codes using Hyundai Hi-MATE diagnostic software or compatible scan tool, then perform a test cycle to verify repair.

Critical for Used Machines: Before replacing components, thoroughly clean all electrical connections with electrical contact cleaner and apply dielectric grease to prevent future corrosion—a simple step that resolves many intermittent fault codes on older equipment.

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Disclaimer: This guide provides general diagnostic information for Code 108-4. Always consult the official Hyundai service manual for your specific machine serial number and software version. If you lack proper diagnostic tools or technical experience, contact a certified Hyundai excavator technician to prevent misdiagnosis and unnecessary parts replacement.