* Faulty Blower Motor Resistor: This is the most common cause. The resistor controls the fan speed. If it fails, it can short out and allow the fan to run constantly at full speed or a single speed, even with the ignition off. The resistor is usually located behind the glove box or under the dash.
* Stuck Relay: A relay controls the power to the blower motor. If the relay gets stuck in the "on" position, the fan will continue to run. Again, it's usually located under the dash, often near the fuse box.
* Short Circuit in the Wiring: A short in the wiring harness supplying power to the blower motor could provide a constant power source, even after the ignition is off. This is harder to diagnose and requires tracing the wiring.
* Faulty Blower Motor Switch: While less likely, a faulty switch could fail in a way that keeps the power on, even when it's turned off.
* Problem with the Control Module (if equipped): Some vehicles, even from that era, might have a more sophisticated control module. A malfunctioning module could cause this issue. This is less probable but possible.
* Aftermarket Radio Installation: Poorly installed aftermarket radios sometimes tap into the wrong power wires. This could inadvertently power the fan.
Troubleshooting Steps:
1. Check the fuses: Start by checking the fuses related to the blower motor. A blown fuse might indicate a short circuit somewhere in the system. However, if the fan is running *and* a fuse is good, it likely points to one of the other issues listed above.
2. Listen carefully: Does the fan run at full speed or at a lower speed? Full speed often points to a failed resistor.
3. Locate the Blower Motor Resistor: Accessing this usually requires removing the glove box or parts of the dash. Inspect it for any signs of damage or burning. You might be able to test it with a multimeter (if you have experience with electrical testing).
4. Locate and check the relay: Once you find the relay, try tapping it gently (with the car OFF, of course). A stuck relay might disengage.
5. Visual Inspection: Check the wiring harness for any obvious damage, frayed wires, or loose connections near the blower motor and the resistor.
Important Safety Note: Working with car electrical systems requires caution. Always disconnect the negative battery cable before working on any electrical components to prevent injury from accidental short circuits.
If you are not comfortable working on car electrical systems, it's best to take your Bonneville to a qualified mechanic for diagnosis and repair. It’s cheaper to pay for a proper diagnosis than to replace parts unnecessarily.
