* Resister: The blower motor resistor is a common failure point. It controls the fan speeds. If it's shorted or failing, it can cause the fan to run constantly, even at its highest speed, regardless of the switch position.
* Blower Motor Control Module (if equipped): Some vehicles, even older ones, might have a dedicated control module. A faulty module could send a continuous "on" signal to the blower motor.
* Wiring Harness: A short circuit or damaged wire in the wiring harness leading to the blower motor can also cause it to run continuously. This could be a frayed wire, a pinched wire, or corrosion in a connector.
* Blower Motor Switch: Although less common than the resistor, a faulty blower motor switch can sometimes stick in the "on" position, even when turned off.
* Body Control Module (BCM): The BCM manages many electrical functions in the car. In some cases, a malfunctioning BCM could inadvertently keep the fan running. This is less likely but possible.
Troubleshooting Steps:
1. Check the obvious: Make absolutely sure the blower motor switch is turned completely to the "off" position. Sometimes, it might not feel fully off.
2. Listen carefully: Try to locate the source of the noise. Is it definitely the blower motor, or could it be something else (like a relay clicking)?
3. Fuse check: While less likely to cause constant operation, a blown fuse *could* be the problem in a roundabout way (perhaps causing a short circuit elsewhere). Check the fuse related to the blower motor.
4. Resistor check: Locate the blower motor resistor (usually found behind the glovebox or under the dash). Inspect it for any signs of burning, melting, or damage. You might need to test its resistance with a multimeter to be sure, but visual inspection is a good first step. Replacing this is often the solution.
5. Wiring Inspection: Visually inspect the wiring harness leading to the blower motor, looking for any damaged or frayed wires. Pay attention to the connections.
6. Professional Diagnosis: If you're not comfortable with electrical work, taking your DeVille to a qualified mechanic or auto electrician is the safest bet. They have the tools and expertise to properly diagnose the issue and make the necessary repairs.
Safety Note: Disconnecting the battery's negative terminal is a good idea before working on any electrical components in your vehicle. This will prevent accidental shorts and protect you from electric shock.
