1. Blower Motor Resistor: This is the most common cause. The resistor controls the speed of the blower motor. When it fails, it often gets stuck in the "on" position, regardless of the switch setting.
* How to check: Locate the blower motor resistor (often under the dashboard, sometimes behind the glove box). It's usually a small, rectangular box with several wires connected. Visually inspect it for any obvious signs of damage, burning, or loose connections. A multimeter can be used to test its resistance, but this requires knowing the specific resistance values for each speed setting (your car's repair manual will have this information). Often, just visually inspecting it for burning or damage is enough to determine if it's the problem.
* How to fix: Replace the resistor. They're relatively inexpensive and readily available online or at auto parts stores.
2. Blower Motor Switch: The switch itself could be faulty, remaining "on" even when you've turned it off.
* How to check: This is harder to check without a wiring diagram and a multimeter. You'd need to test the continuity and voltage at different points in the switch circuit. If you're not comfortable with electrical testing, it's best to skip this step or have a mechanic do it.
* How to fix: Replace the blower motor switch. This is usually more involved than replacing the resistor, often requiring dashboard disassembly.
3. Wiring Issues: Damaged or shorted wiring can also cause the blower motor to run continuously. This could be anywhere from the switch to the resistor to the motor itself.
* How to check: Carefully inspect all wiring related to the blower motor system for any frayed wires, exposed conductors, or signs of melting. A multimeter can help to identify shorts or open circuits, but this requires electrical knowledge and a wiring diagram.
* How to fix: Repair or replace any damaged wiring. This might involve tracing wires and possibly splicing in new sections.
4. Blower Motor Relay (Less Likely): While less common, a faulty relay could also keep the blower motor powered. The relay acts as a switch controlled by the blower motor switch.
* How to check: Locate the relay (usually in a fuse box). You can try swapping it with a similar relay (if you have a spare of the same type) to see if the problem follows the relay.
* How to fix: Replace the relay if it's faulty.
Troubleshooting Steps:
1. Start with the easiest and most common: Check the blower motor resistor first. It's the most likely culprit.
2. Visual Inspection: Carefully examine all the wiring and components related to the blower motor.
3. Use a Multimeter (If Comfortable): If you have experience with electrical testing, a multimeter can be invaluable in diagnosing the problem.
4. Consult a Repair Manual: A repair manual specific to your 1991 Pontiac Bonneville will provide wiring diagrams and detailed information on testing the system.
5. Seek Professional Help: If you're not comfortable working with car electronics, take it to a qualified mechanic. Improper electrical repairs can cause damage to other components.
Remember to disconnect the negative battery terminal before working on any electrical components in your car to prevent shorts or injury.
