* Resistor Pack: This is the most likely cause. The resistor pack controls the different blower speeds. When it fails, the blower often defaults to only the highest speed (which bypasses the resistor). The resistor pack is usually located behind the glovebox or under the dash. It's a relatively inexpensive and easy-to-replace part.
* Blower Motor Switch: Less likely, but possible. The switch itself could be faulty, preventing the lower speed signals from reaching the resistor pack or motor.
* Wiring Issues: A broken or corroded wire in the circuit supplying power to the resistor pack or blower motor can cause intermittent or complete failure of lower speeds. This is less likely than a bad resistor, but needs to be checked if replacing the resistor doesn't solve the problem.
* Blower Motor (less likely): While possible, a failing blower motor is less likely to *only* affect the lower speeds. Usually, a failing motor would show more general problems like weak airflow on all speeds or intermittent operation.
Troubleshooting Steps:
1. Check the Resistor Pack: This should be your first step. Locate the resistor pack (consult a repair manual or online diagrams for your specific model year). Visually inspect it for any obvious signs of damage (burn marks, melted components). If you see damage, replace it. Even if it looks okay, it's still the most likely culprit.
2. Check the Wiring: Once you've located the resistor pack, carefully inspect the wiring harness connected to it. Look for any broken, frayed, or corroded wires. If you find any damage, repair or replace the affected wires.
3. Test the Blower Motor Switch (if resistor replacement doesn't work): This is more difficult to test without a multimeter. You could try tapping on the switch while the system is running to see if you can get the lower speeds to work intermittently (this would suggest a loose connection or failing switch).
4. Consult a Repair Manual: A repair manual specific to your 1999 Ford Explorer will give you detailed diagrams and troubleshooting steps. This is highly recommended.
Important Note: Before working on any electrical components, disconnect the negative battery terminal to prevent accidental short circuits. If you're not comfortable working with car electrical systems, it's best to take your Explorer to a qualified mechanic.
