* Faulty wiper motor switch: While the switch is "off," it might be internally failing and sending a spurious signal to the motor. This is a common failure point in older vehicles. A short circuit within the switch itself could be constantly energizing the motor.
* Short circuit in the wiring harness: A short in the wiring between the switch and the wiper motor could complete the circuit, even when the switch is off. This could be caused by chafing, water damage, or rodent damage to the wires. Look carefully at the wiring loom under the dash and around the steering column.
* Faulty wiper motor: While less likely, a faulty wiper motor could be drawing power even when it's supposed to be off. It might have a shorted internal component that keeps the motor running.
* Body control module (BCM) malfunction: The BCM controls many electrical functions, including wipers in some vehicles. A faulty BCM could be sending the wrong signal to the wiper motor, causing it to run continuously. This is less common than the other problems but possible.
* Park/Neutral switch (if applicable): Some vehicles have interlocks that prevent the wipers from operating unless the vehicle is in park or neutral. If this switch is malfunctioning, it might be sending a signal that allows the wipers to run regardless of the wiper switch position. This is less likely, but possible.
Troubleshooting Steps:
1. Check the fuses: Start with a visual inspection of the related fuses in the fuse box. If a fuse is blown, replace it with one of the same rating. However, if the fuse blows immediately after replacement, there's a more serious short circuit somewhere.
2. Inspect the wiring: Carefully examine the wiring harness leading from the wiper switch to the wiper motor. Look for any signs of damage, fraying, or corrosion. Pay special attention to areas where the wiring might rub against other parts of the vehicle.
3. Test the wiper switch: This usually requires removing the switch and testing its continuity with a multimeter. This is a more advanced troubleshooting step.
4. Test the wiper motor: Disconnect the wiring harness from the wiper motor. If the wipers still run, the problem is in the wiring or switch. If they stop, the motor itself is suspect.
5. Consider the BCM: If other electrical issues are present, the BCM could be the culprit. Diagnosing this generally requires advanced diagnostic equipment.
Since this involves electrical work, if you're not comfortable working with car electrical systems, it's best to take your F-150 to a qualified mechanic for diagnosis and repair. A persistent short circuit can cause a fire hazard.
