What is LiuGong Fault Code 687?

Fault Code 687 indicates an Engine Coolant Temperature (ECT) Sensor Circuit High Voltage condition in LiuGong excavators. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally high voltage signal from the coolant temperature sensor, typically reading above 4.8 volts for more than 2 seconds.

The ECT sensor is a critical component that monitors engine operating temperature and sends real-time data to the ECM. This information directly affects fuel injection timing, idle speed control, and engine protection systems. When Code 687 activates, the ECM cannot accurately determine engine temperature, potentially leading to improper fuel delivery, overheating risks, or unnecessary engine derating. For used LiuGong excavators, this code often signals sensor degradation or wiring harness damage from years of exposure to heat, vibration, and contaminants.

Common Symptoms

When Fault Code 687 is active, operators may experience:

• Check Engine Light or malfunction indicator lamp illuminated on the instrument cluster
• Engine derate mode engaged, limiting machine performance to prevent potential overheating damage
• Inaccurate temperature gauge readings, often showing extremely low or no temperature reading
• Hard starting conditions or rough idle, especially during cold starts
• Cooling fans running continuously at maximum speed, even when engine is cold

Potential Causes

The most common technical reasons for Code 687 in used LiuGong excavators include:

• Open circuit in ECT sensor wiring harness, often caused by wire chafing near engine mounting points or exhaust manifold
• Failed coolant temperature sensor with internal resistance breakdown (common after 5,000+ operating hours)
• Corroded or damaged sensor connector pins, particularly on machines operated in marine or high-humidity environments
• Broken or disconnected ground wire in the sensor circuit
• ECM internal fault (rare, but possible in high-hour machines with electrical system stress)
• Aftermarket sensor installation with incorrect resistance specifications

How to Troubleshoot and Fix Code 687

Step 1: Visual Inspection Begin with a thorough physical examination of the ECT sensor and its wiring harness. Locate the sensor (typically installed in the engine block or cylinder head near the thermostat housing). Check for damaged wiring, melted insulation from exhaust heat exposure, or signs of coolant leakage around the sensor body. On used excavators, pay special attention to harness routing near moving components or sharp edges where friction wear commonly occurs.

Step 2: Electrical Testing Disconnect the ECT sensor connector and use a digital multimeter to measure sensor resistance. At room temperature (68°F/20°C), resistance should typically read between 2,000-3,000 ohms (consult LiuGong service manual for exact specifications). Test the wiring harness by measuring voltage at the connector with ignition on—you should see approximately 5 volts reference voltage from the ECM. If voltage reads near battery voltage (12V+), the circuit has an open condition.

Step 3: Connector and Ground Verification Inspect the sensor connector terminals for corrosion, bent pins, or moisture intrusion. Clean connections with electrical contact cleaner and apply dielectric grease. Verify the ground circuit has less than 0.5 ohms resistance to chassis ground. In used machines, ground connections often corrode, creating high-resistance faults.

Step 4: Component Replacement If testing confirms sensor failure, replace with an OEM-specification ECT sensor. Ensure proper torque (typically 15-20 ft-lbs) and use thread sealant rated for coolant contact. After replacement, clear the fault code using LiuGong diagnostic software or compatible scan tool and perform a test cycle to verify repair.

Disclaimer: This guide provides general troubleshooting information. Always consult the official LiuGong service manual for your specific machine model and serial number. If you lack proper diagnostic tools or experience with electrical systems, seek assistance from a certified LiuGong technician to prevent further damage to the excavator's electronic systems.