1. Check the Blower Motor Resistor: This is a common culprit. The resistor controls the different fan speeds. A faulty resistor can cause the blower to not work at all, or only work on high speed (if at all). The resistor is usually located behind the glove compartment. You'll need to consult a repair manual or online resources specific to your 1999 DeVille to find its exact location and how to test it. A simple multimeter test will determine if it's functioning correctly. Replacement is relatively inexpensive and straightforward.
2. Check the Blower Motor Relay: Relays are electrical switches that control the flow of power to the motor. A bad relay can prevent the motor from receiving power. Your repair manual will show you the location of the blower motor relay in the fuse box (under the hood or inside the cabin). Visually inspect it for any damage. You might be able to swap it with another relay of the same amperage rating (if safe to do so) to test if it's the problem.
3. Check the Wiring and Connections: Carefully inspect all the wiring connecting the blower motor, resistor, relay, and the control switch. Look for any broken, frayed, or corroded wires. Pay close attention to the connections – ensure they are secure and free of corrosion. A poor connection can interrupt the electrical circuit.
4. Check the Blower Motor Switch: The switch itself could be faulty. This is harder to test at home without specialized equipment. You can try switching the blower speed settings to see if you hear any clicking sounds that might indicate a problem with the switch. Replacing the switch is a more involved process.
5. Consult a Wiring Diagram: A wiring diagram for your 1999 Cadillac DeVille is essential. It will show you the exact path of the electrical circuit for the blower motor, allowing you to systematically trace the problem. Online resources or a repair manual should provide this.
6. Professional Help: If you're not comfortable working with car electrical systems, it's best to take your DeVille to a qualified mechanic. They have the tools and expertise to diagnose and repair the problem accurately.
Remember safety first! Disconnect the negative battery terminal before working on any electrical components in your car.
