* Faulty Blower Motor Resistor: This is the most common culprit. The resistor regulates the speed of the blower motor. If it fails, it might short circuit and provide power to the motor even when the key is off. A shorted resistor will often smell burnt or even be visibly damaged.
* Faulty Blower Motor Control Module: In some vehicles, the module controls the resistor and other aspects of the blower motor. A malfunctioning module can cause the motor to run continuously. This is often integrated with the resistor itself.
* Wiring Problems: A short circuit in the wiring harness supplying the blower motor could bypass the switch and resistor, allowing constant power. This is less common but possible, especially if there's been previous damage to the wiring near the blower motor or under the dash.
* Stuck Relay: While less likely, a relay related to the blower motor could be stuck closed, providing continuous power.
* Bad Ignition Switch: Although less probable, a faulty ignition switch might not completely cut power to certain circuits, including the blower motor.
Troubleshooting Steps:
1. Check the fuses: While unlikely to be the sole cause of the problem (since the motor is running), a blown fuse in the blower motor circuit could be a symptom of a bigger electrical problem.
2. Inspect the Blower Motor Resistor: Locate the blower motor resistor (usually under the dashboard, sometimes behind the glove box). Look for signs of burning, melting, or damage. This is often the easiest and most cost-effective component to replace.
3. Check the wiring: Carefully inspect the wiring harness leading to the blower motor resistor and the blower motor itself for any signs of damage, fraying, or shorts.
4. Test with a multimeter: If you're comfortable with basic electrical testing, use a multimeter to check the voltage at the blower motor and the resistor terminals with the key off. A reading of 12V indicates a short circuit somewhere.
5. Professional Diagnosis: If you're not comfortable with electrical repairs, it's best to take your car to a qualified mechanic for a proper diagnosis and repair. They have the tools and expertise to pinpoint the problem accurately.
Remember to disconnect the battery's negative terminal before working on any electrical components in your vehicle. This is crucial for your safety.
