* Heater Control Switch (or Rheostat): This is the most common cause. The switch itself might be malfunctioning, allowing power to continuously flow to the fan motor even when the switch is in the "off" position. The switch could have internal contacts that are stuck closed or burned out.
* Wiring Harness Issues: A short circuit in the wiring harness leading to or from the fan motor can also cause this. A wire might be chafing against something metal, creating a short, or there could be damaged insulation allowing wires to touch that shouldn't. This is more likely if the problem started after some kind of impact or modification near the heater controls.
* Resistor (if equipped): Some models use a resistor to control the fan speed. If the resistor is failing, it might be shorting out and sending power to the fan continuously, even with the switch off. This is less likely to completely disable the ability to turn it *off* but could be a contributing factor.
* Ignition Switch: While less likely, a faulty ignition switch could be providing power to the heater fan circuit even after the key is turned off. This is usually accompanied by other electrical anomalies, however.
* Relay: There might be a relay in the circuit responsible for powering the fan. A stuck closed relay would keep the fan running.
Troubleshooting Steps:
1. Check the obvious: Make sure the heater control switch is actually in the "off" position. Sometimes, the switch can feel like it's off when it's not fully disengaged.
2. Fuse Check: Although unlikely if the fan is running constantly, check the fuse for the heater fan circuit to rule out a blown fuse. If the fuse is blown repeatedly, it points to a short circuit somewhere in the system.
3. Test the Switch: This requires some electrical knowledge and tools (multimeter). You'll need to disconnect the wiring harness from the switch and check for continuity with the switch in the "on" and "off" positions. Continuity when the switch is off suggests a bad switch. Caution: Disconnect the negative battery terminal before doing any electrical testing.
4. Inspect the Wiring: Visually inspect the wiring harness going to and from the blower motor for any signs of damage, chafing, or fraying. Look particularly closely near the heater controls and the blower motor itself.
5. Relay Check (if applicable): Locate the relay for the heater fan (consult your wiring diagram), and see if it's stuck closed.
If you're not comfortable working with automotive electrical systems, it's best to take your Buick to a qualified mechanic. Incorrectly diagnosing and repairing electrical problems can lead to further damage or even a fire hazard.
