* Faulty Fan Relay: The relay is an electrical switch that controls the fan's power. A malfunctioning relay can cause the fan to run constantly or at the wrong speed. This is a very common issue. You can test this with a multimeter.
* Bad Fan Control Module (PCM): The Powertrain Control Module (PCM) monitors engine temperature and sends signals to the relay to control the fan. A faulty PCM can send incorrect signals, leading to the fan running at high speed inappropriately. This is less common than a bad relay, but more expensive to fix. Diagnosing this requires a scan tool.
* Shorted Wiring: A short circuit in the wiring harness that controls the fan can cause it to run continuously. This might be due to chafing, rodent damage, or simply age. Careful visual inspection of the wiring is needed.
* Faulty Temperature Sensor: The engine coolant temperature sensor (ECT) tells the PCM how hot the engine is. If this sensor is faulty and providing inaccurate readings (like saying the engine is overheating constantly), the PCM will command the fan to run high speed.
* Stuck Fan Switch: Some Mustangs use a temperature-sensitive switch within the radiator that activates the fan. If this switch is stuck closed, the fan will always be on.
* Low Coolant Level: While less likely to *cause* high-speed operation, a low coolant level could make the engine run hotter, triggering the fan's high-speed operation as a safety measure. Check your coolant level!
* Cooling System Air Pockets: Air bubbles trapped in the cooling system can prevent accurate temperature readings and cause erratic fan behavior. Burping the system (properly bleeding the air out) might help.
Troubleshooting Steps:
1. Check the coolant level: This is the easiest thing to check first.
2. Inspect wiring: Look for any obvious damage to the wiring harness leading to the fan and relay.
3. Test the relay: You can swap the cooling fan relay with another relay of the same amperage (use a different relay in your fuse box for comparison), or test it with a multimeter to see if it is switching correctly.
4. Check the coolant temperature sensor: A multimeter can be used to verify the sensor's resistance values and compare them against the specifications in a repair manual.
5. Check the fan switch (if applicable): If your system uses a separate fan switch, test its continuity.
6. Scan the PCM: A professional scan tool can read codes from the PCM that might pinpoint the problem. This is usually the last step, after easier checks have failed.
Remember safety first. Always disconnect the battery's negative terminal before working on electrical components. If you're not comfortable working on the electrical system of your car, take it to a qualified mechanic. A proper diagnosis is essential to avoid replacing parts unnecessarily.
