* Bad Blower Motor Resistor: This is the most common cause. The resistor controls the fan speeds. If it's shorted, it can provide constant power to the motor but may have damaged the motor itself, or it might prevent the motor from receiving the proper voltage to operate at any speed. This is usually a relatively inexpensive and easy fix.
* Faulty Blower Motor Relay: The relay switches the power to the blower motor. A faulty relay could be stuck closed, providing constant power even with the key off, but it could also be preventing the motor from functioning properly.
* Short in the Wiring Harness: A short circuit in the wiring leading to the blower motor could be supplying constant power and potentially preventing the motor from working correctly. This is more difficult to diagnose and repair.
* Faulty Blower Motor: While less likely if you're getting constant power, the motor itself could be internally damaged and unable to run despite receiving power. It's possible a short within the motor is causing the constant power issue.
* Aftermarket Radio or Accessory Installation: Improperly installed aftermarket equipment could be drawing power and interfering with the blower motor circuit.
Troubleshooting Steps:
1. Check the Blower Motor Resistor: This is the easiest and most common first step. Locate the resistor (usually under the dash, near the blower motor) and inspect it for any signs of burning or damage. You can usually test it with a multimeter for continuity. A replacement is readily available and relatively inexpensive.
2. Check the Blower Motor Relay: Locate the blower motor relay in the fuse box (under the hood or inside the car, depending on the model). You can try swapping it with a relay of the same rating in a different position to see if the problem follows the relay.
3. Inspect the Wiring Harness: Visually inspect the wiring leading to the blower motor for any signs of damage, fraying, or exposed wires. This is more difficult and requires some electrical knowledge.
4. Test the Blower Motor Directly: With the key off, disconnect the power supply to the blower motor. If the power remains, the problem lies in the wiring or a short before the motor. If the power stops, the issue is either the motor or a component before it. Using a multimeter to test for power at the motor leads will be most beneficial.
Important Safety Note: Before working on any electrical components, disconnect the negative battery terminal to prevent accidental shocks or short circuits.
If you're not comfortable working with car electrical systems, it's best to take your Cadillac DeVille to a qualified mechanic for diagnosis and repair. The constant power draw could damage other components if left unresolved.
