What is Hyundai R805LC-7 Fault Code 755?

Fault Code 755 on the Hyundai R805LC-7 excavator indicates a hydraulic oil temperature sensor circuit malfunction or an abnormal hydraulic oil temperature reading. This diagnostic trouble code (DTC) is triggered when the Electronic Control Unit (ECU) detects either an out-of-range voltage signal from the hydraulic oil temperature sensor or when the hydraulic system temperature exceeds or falls below programmed thresholds.

In the R805LC-7's sophisticated hydraulic management system, this sensor plays a critical role in protecting expensive hydraulic components from thermal damage. The ECU uses temperature data to adjust pump output, activate cooling fans, and prevent operation outside safe temperature parameters. On used excavators, this code often appears due to sensor degradation, wiring issues from years of vibration and heat exposure, or actual hydraulic system problems that cause overheating.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument cluster
• Reduced hydraulic performance or automatic derate mode limiting machine functions
• Cooling fan running continuously at high speed regardless of actual temperature
• Erratic temperature gauge readings that fluctuate abnormally or show implausible values
• Machine may enter limp mode restricting swing speed and boom/arm movements to prevent damage

Potential Causes

Sensor-Related Issues:

• Failed or degraded hydraulic oil temperature sensor (common in machines with 5,000+ hours)
• Corroded or damaged sensor connector exposed to hydraulic fluid or moisture intrusion

Wiring and Electrical:

• Chafed or broken wiring in the sensor harness, particularly where it routes near the hydraulic tank (known rub point on R805LC-7)
• Poor ground connection at the sensor mounting point or ECU ground terminals
• ECU internal fault (less common but possible in high-hour machines)

Actual Hydraulic System Problems:

• Genuine hydraulic oil overheating from clogged hydraulic oil cooler fins or failed cooling fan
• Contaminated or incorrect viscosity hydraulic oil causing increased operating temperatures
• Restricted hydraulic return filters forcing excess heat generation

How to Troubleshoot and Fix Code 755

Step 1: Visual Inspection and Initial Checks Start by inspecting the hydraulic oil temperature sensor location on the hydraulic tank. Check for oil leaks, damaged connectors, or obvious wiring damage. On used excavators, pay special attention to connector pins for corrosion—clean with electrical contact cleaner if necessary. Verify the actual hydraulic oil temperature using an infrared thermometer to determine if overheating is real or sensor-related.

Step 2: Electrical Testing Disconnect the temperature sensor connector and use a digital multimeter to check sensor resistance. At room temperature (68°F/20°C), resistance should typically read between 2,000-3,000 ohms (consult service manual for exact specifications). Check for supply voltage from the ECU (usually 5V reference) at the connector harness side. Inspect wiring continuity between the sensor and ECU pins and verify ground circuit integrity (should read less than 1 ohm resistance to chassis ground).

Step 3: Component Replacement and System Verification If sensor readings are out of specification or wiring shows damage, replace the faulty component. For used machines, always inspect harness routing for wear points where cables contact frame members—use protective sleeving or reroute if needed. After repairs, clear the fault code using Hyundai diagnostic software (Hi-MATE or compatible scan tool) and perform a function test running the machine through full hydraulic cycles while monitoring live temperature data.

Step 4: Address Root Causes If the sensor and wiring test good, investigate actual hydraulic overheating. Clean the hydraulic oil cooler, verify cooling fan operation, check hydraulic oil level and condition, and inspect for system restrictions causing excessive heat buildup.

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Disclaimer: This guide provides general troubleshooting information for experienced technicians. Always consult the official Hyundai R805LC-7 service manual for exact specifications and safety procedures. If you lack proper diagnostic tools or experience with high-voltage electrical systems, consult a certified Hyundai heavy equipment technician.