Here's a breakdown of why:
* Resistor Pack Function: The resistor pack is a component that reduces the voltage going to the fan motor. Each speed setting on the fan switch sends a different amount of current through the resistor pack, resulting in varying speeds. Speed 4 usually bypasses the resistor entirely, sending full voltage to the motor.
* Resistor Pack Failure: If the resistor pack is faulty (burned out, corroded, or internally damaged), it can't regulate the lower voltages correctly. This means the fan motor only receives full power, thus only operating at its highest speed.
* Other Less Likely Possibilities: While less common, other possibilities include:
* Wiring Issues: A short circuit or broken wire in the wiring harness leading to the resistor pack or fan motor could prevent lower speeds from functioning.
* Fan Motor Issues (though less probable): While a failing fan motor *could* cause this, it's less likely than a bad resistor pack. A failing motor usually leads to inconsistent performance across all speeds or complete failure.
* Fan Switch Problem: A faulty fan switch could also be the culprit, although this is less common than the resistor pack failure.
Therefore, the most likely culprit is a faulty resistor pack. Replacing it is usually the most straightforward and cost-effective solution. You can find replacement resistor packs online or at auto parts stores. However, if you're not comfortable working on your car's electrical system, it's best to take it to a mechanic.
