Why does the cooling fan relay fail in the Mustang?

The most common causes of cooling fan (or radiator fan) relay failure are:

1. Short circuit in the fan circuit: a short circuit in the wiring or within the fan itself can cause an excessive current that burns out the contacts or the relay’s coil.
2. Wear of internal components: over time, the contacts and the internal coil of the relay can wear out due to repeated use, which can cause malfunctioning or prevent the relay from activating the fan.
3. Vibrations and shocks: constant engine vibrations and road shocks can cause internal damage to the relay or loosen the connections, leading to intermittent or complete failures.
4. Low quality or defective relay: relays that are of low quality or have manufacturing defects may fail prematurely, even under normal operating conditions of the vehicle.

To confirm if the cooling fan relay is the issue, you can test it with a multimeter or replace it with another relay of the same specifications and check if this resolves the problem.

Other causes of cooling fan failure in the Mustang

If the relay is functioning correctly, other possible causes of cooling fan failure include:

1. Blown fuse: the cooling fan fuse may be blown. This usually happens due to an electrical problem, such as a damaged wire in contact with the vehicle’s body.
2. Faulty cooling fan motor: if the cooling fan motor is damaged or worn out, the fan will not activate, which can lead to engine overheating.
3. Defective coolant temperature sensor (ECT): this sensor measures the coolant temperature and sends the signal to the fan relay to activate the fan. If the sensor is faulty or not working properly, the fan may not turn on when needed.
4. Faulty thermostat: a defective thermostat that does not allow the engine to reach the proper temperature may prevent the radiator fan from activating at the correct time, as the cooling system will not heat up enough to trigger the fan.
5. Engine control unit (ECU) issues: in modern vehicles, the ECU manages many engine functions, including the control of the radiator fan. A problem with the ECU or its operating software can prevent the fan from turning on.
6. Low coolant level: If the coolant level is very low, the temperature sensor may not correctly detect overheating, preventing the fan from activating.