Possible Causes:
* Fan Relay: This is the most likely culprit. The relay acts as a switch, controlled by the engine control unit (ECU). A faulty relay will prevent power from reaching the fan motor even when the engine is overheating.
* Temperature Sensor (Coolant Temperature Sensor - CTS): The ECU relies on the CTS to determine engine temperature. A faulty CTS might not signal the ECU to activate the fan, even when the engine is hot. There might be two, one for the gauge and one for the ECU.
* ECU (Engine Control Unit): While less common, a malfunctioning ECU could be preventing the fan from engaging. This is a more expensive and complex issue to diagnose.
* Wiring: A break or short in the wiring harness between the CTS, relay, and fan motor can interrupt the circuit. Look for chafed wires, corrosion, or loose connections.
* Fan Motor (less likely): While less probable given the hotwire works, a partially failing fan motor could draw insufficient current to trip the relay.
Troubleshooting Steps:
1. Check the fuses: Ensure all fuses related to the cooling system are intact. Consult your owner's manual to locate the correct fuses.
2. Inspect the relay: Locate the radiator fan relay (often in a fuse box under the hood or dashboard). Visually inspect it for damage. You can try swapping it with another relay of the same amperage rating (if safe to do so – check your owner's manual) to see if the problem moves.
3. Test the coolant temperature sensor (CTS): This requires a multimeter. You'll need to consult a wiring diagram to locate the CTS and its connector. The resistance of the CTS changes with temperature. A faulty CTS will show incorrect resistance readings.
4. Check the wiring: Visually inspect the wiring harness leading to the fan, relay, and CTS for any damage, corrosion, or loose connections. Repair or replace as needed.
5. Test the fan motor (if other steps fail): With the fan disconnected from the wiring harness, directly connect a suitable power source (e.g., a battery with appropriate fuses and wiring) to the fan motor leads. If it runs, the motor is likely fine; if not, the motor needs replacement.
6. Advanced Diagnostics (if all else fails): If you've checked everything above and the fan still doesn't work, you might need to use a scan tool to read diagnostic trouble codes (DTCs) from the ECU. This can pinpoint more specific issues.
Important Safety Note: Working on your car's electrical system can be dangerous. Always disconnect the battery negative terminal before working on any electrical components. If you're not comfortable working with car electronics, take your Beetle to a qualified mechanic. Driving with a malfunctioning cooling system can lead to engine damage.
