What is Hyundai R305LC-9T Fault Code 687?

Fault Code 687 on the Hyundai R305LC-9T excavator indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading detected by the Engine Control Module (ECM). This code triggers when the ECM receives voltage signals outside the expected range from the coolant temperature sensor, typically indicating either a sensor failure, wiring issue, or actual overheating condition.

The coolant temperature sensor is critical for the R305LC-9T's Cummins QSB6.7 engine management system. It directly influences fuel injection timing, idle speed control, and engine protection protocols. When this sensor fails or provides incorrect data, the ECM cannot properly manage engine temperature, potentially leading to severe engine damage, reduced fuel efficiency, and activation of derate modes that limit machine performance.

Common Symptoms

• Engine warning light or check engine lamp illuminated on the instrument cluster
• High temperature warning alarm sounding even when coolant appears normal
• Engine derate mode activated, limiting RPM and hydraulic power output
• Hard starting or rough idle, especially during cold starts
• Cooling fan running continuously at maximum speed regardless of actual temperature

Potential Causes

The most common causes of Code 687 on used R305LC-9T excavators include:

• Failed coolant temperature sensor due to age-related degradation or thermal cycling stress
• Corroded or damaged wiring harness connections at the sensor connector, particularly where the harness routes near the exhaust manifold
• Broken or chafed wires in the engine bay where vibration causes harness contact with sharp edges
• ECM internal fault or poor ground connection affecting signal quality
• Actual coolant system problems including low coolant level, air pockets, or thermostat failure causing genuine overheating
• Moisture infiltration in electrical connectors common in machines operating in wet environments

How to Troubleshoot and Fix Code 687

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block near the thermostat housing. Check the wiring harness for obvious damage, chafing, or corrosion. On used excavators, pay special attention to harness routing where it may contact hot surfaces or moving components. Disconnect the sensor connector and inspect pins for corrosion, bent terminals, or moisture.

Step 2: Sensor Resistance Testing Using a digital multimeter, measure the sensor's resistance. With the engine cold (approximately 68°F/20°C), resistance should read approximately 2,000-3,000 ohms. As temperature increases to operating range (190°F/88°C), resistance should drop to approximately 200-300 ohms. Values significantly outside this range indicate sensor replacement is needed.

Step 3: Circuit Voltage Testing With the ignition on and sensor disconnected, check for 5-volt reference voltage at the harness connector from the ECM. Also verify proper ground continuity (less than 1 ohm resistance to chassis ground). If reference voltage is absent or ground circuit shows high resistance, trace wiring back to the ECM checking for breaks or poor connections.

Step 4: Verify Coolant System Before replacing electrical components, confirm actual coolant level is correct and the system is properly bled. A genuine overheating condition will trigger this code legitimately.

Step 5: Clear Code and Test After repairs, use Hyundai Hi-MATE diagnostic software or compatible scan tool to clear the fault code. Operate the machine through a complete warm-up cycle and monitor for code recurrence.

---

Disclaimer: This guide provides general diagnostic information. Always consult the official Hyundai service manual for your specific machine serial number and consider professional diagnosis for complex electrical issues or if you lack proper diagnostic equipment.