* Blower Motor Resistor: This is the most frequent cause. The resistor controls the different fan speeds. If it's faulty, the higher speeds might not work. The resistor often burns out due to age and heat. It's usually located somewhere near the blower motor, often under the dash. You'll need to consult a repair manual or online resources specific to your Bonneville to find its exact location.
* Blower Motor: The blower motor itself could be failing. It might still have enough power to run on low speed but not enough to handle the higher amperage draw required for high speed. A failing motor often makes unusual noises (whining, grinding).
* Wiring and Connections: A break, short, or loose connection in the wiring harness leading to the blower motor or resistor can interrupt the higher speed circuits. Inspect the wiring carefully for any damage.
* Fuse or Circuit Breaker: Though less likely if the low speed works, a blown fuse or tripped circuit breaker specifically for the higher blower speeds could be the problem. Check your owner's manual for the fuse box location and diagram.
* Control Module (HVAC): Less common, but a faulty HVAC control module could be sending the wrong signals to the blower motor or resistor. This is usually the most expensive repair option.
Troubleshooting Steps:
1. Check the Fuse/Circuit Breaker: This is the easiest and quickest thing to check.
2. Inspect the Blower Motor Resistor: Locate it (consult a repair manual!), and visually inspect it for any obvious signs of damage (burn marks, melted plastic). You might even be able to test its continuity with a multimeter if you're comfortable with that.
3. Listen to the Blower Motor: When on low, does it sound normal? Any unusual noises (scraping, whining) suggest a failing motor.
4. Check Wiring: Visually inspect the wiring harness leading to the blower motor and resistor for any cuts, breaks, or loose connections.
5. Test the Blower Motor (Advanced): If you have a multimeter and are comfortable with electrical testing, you can test the blower motor itself to see if it's receiving power and working correctly on higher speeds.
Important Considerations:
* Repair Manual: A repair manual specific to your 1998 Pontiac Bonneville SE will be invaluable. It will show you where the resistor and blower motor are located, wiring diagrams, and troubleshooting steps.
* Safety: Working on car electrical systems can be dangerous. Disconnect the negative battery terminal before starting any work.
If you're not comfortable working on car electrical systems, it's best to take your vehicle to a qualified mechanic for diagnosis and repair. The cost of a professional diagnosis is usually less than the cost of replacing parts unnecessarily.
