1. Check the Blower Motor Resistor: This is the most common culprit. The resistor regulates the fan speeds. A failed resistor can cause the blower to run constantly on high speed, or even just fail to turn off completely.
* Location: The resistor is usually located behind the glovebox or under the dashboard, near the blower motor itself. Consult your repair manual for the exact location in your specific model.
* Testing: Visually inspect the resistor for any signs of burning, melting, or loose connections. A multimeter can be used to test the resistance values (though you'll need to know the correct specifications from a wiring diagram). If it's visually damaged or fails the resistance test, replace it.
2. Check the Blower Motor Switch: The switch itself could be malfunctioning, stuck in the "on" position, or have a short circuit.
* Testing: This is trickier to test directly without specialized equipment. Try switching the fan speed through all positions multiple times. If the problem persists, it's likely the switch. Replacement is often the solution.
3. Inspect the Wiring and Connections: Look for any frayed, broken, or corroded wires in the blower motor circuit. Pay close attention to the connections at the resistor, switch, and the blower motor itself. A short circuit anywhere in this wiring can cause the blower to run continuously.
* Use a multimeter: Check for continuity and voltage at various points in the circuit. This requires a good understanding of automotive electrical systems or the help of a wiring diagram.
4. Check the Control Module (if applicable): Some models may have a dedicated blower motor control module. This is less likely on a 1991 model, but if the above steps don't solve the problem, it might be worth investigating. A faulty module would need replacement.
5. Blower Motor Relay (less likely): A relay might be stuck closed. While less common for this symptom, a faulty relay could keep power constantly supplied to the motor.
6. Ground Connection: A poor ground connection can also cause strange electrical issues. Check the ground connections associated with the blower motor circuit.
Before you start:
* Disconnect the Battery Negative Terminal: This is crucial for safety before working with any electrical components.
* Consult a Repair Manual: A repair manual specific to your 1991 Cadillac Sedan DeVille will be invaluable for locating components, understanding the wiring diagrams, and providing troubleshooting guidance.
If you're not comfortable working with car electrical systems, it's best to take your vehicle to a qualified mechanic. Improper repairs can lead to further damage or even create safety hazards.
