Here's why:
* Blower Motor Resistor: This component controls the different fan speeds. If it's faulty, it can only allow the highest speed (max AC) to function. This is the most common cause of this symptom.
* Fan Speed Switch: A malfunctioning switch could also prevent the lower speeds from engaging, forcing the fan to only work at full power. This is less common than a resistor problem.
* Wiring: There's a chance of a wiring issue between the switch, resistor, and motor. However, this is usually accompanied by other symptoms like intermittent operation at any speed or complete failure.
Troubleshooting Steps:
1. Check the Blower Motor Resistor: This is usually located behind the glove box or under the dash. Access it (consult your repair manual for location and specifics), and visually inspect it for any signs of damage (burn marks, melted parts). A simple multimeter test can confirm whether it's functioning correctly.
2. Check the Fan Speed Switch: Accessing this might require more disassembly. Again, visual inspection is the first step, then a multimeter check if you're comfortable testing electrical components.
3. Check the wiring: Carefully examine the wiring harness connected to the resistor and switch for any broken wires, loose connections, or corrosion.
Recommendation: Start by checking the blower motor resistor. It's the most likely culprit and often the easiest and cheapest to replace. If replacing the resistor doesn't solve the problem, then investigate the switch and wiring. If you're not comfortable with electrical work, take it to a qualified mechanic.
