* Faulty Blower Motor Resistor: This is the most common cause. The resistor controls the fan speeds. If it fails, the fan often runs on high speed constantly. It's a relatively inexpensive and easy part to replace, so this should be your first check.
* Blower Motor Control Module (if equipped): Some vehicles, especially those with automatic climate control, have a dedicated control module. A malfunctioning module could cause the fan to run continuously. This is less likely than a resistor issue but still possible.
* Short Circuit in the Wiring: A short in the wiring harness leading to the blower motor or the resistor can provide a constant power supply, forcing the fan to run without any control. This requires careful tracing of the wiring to locate the short.
* Faulty AC Control Head (Switch): The switch that controls the fan speed could be malfunctioning, sending a constant signal for the fan to run.
* Stuck Relay: The blower motor relay might be stuck closed, constantly supplying power to the fan.
* Bad Temperature Sensor: While less likely to cause the fan to run *all the time*, a faulty temperature sensor could potentially provide inaccurate readings to the system, leading to unusual behavior, including the fan running constantly in some situations.
Troubleshooting Steps:
1. Check the Blower Motor Resistor: This is the easiest and cheapest fix to try first. Locate the resistor (usually under the dashboard, sometimes behind the glove box) and visually inspect it for any signs of damage or burning. Replacing it is relatively straightforward.
2. Test the Blower Motor Relay: If you have a multimeter, you can test the relay to make sure it's functioning properly.
3. Inspect the Wiring: Carefully examine the wiring harness for any obvious damage, chafing, or melted insulation. Pay particular attention to the wires leading to the resistor and the blower motor itself.
4. Consult a Wiring Diagram: A wiring diagram specific to your 1991 Pontiac Bonneville SSE will be invaluable in tracing the circuits and identifying potential problems. You can usually find these online through repair manuals or automotive forums.
5. Professional Help: If you're not comfortable working with electrical systems, it's best to take your car to a qualified mechanic. They have the tools and expertise to diagnose and repair the problem efficiently.
Remember to always disconnect the battery's negative terminal before working on any electrical components in your vehicle to prevent accidental shocks or short circuits.
