* Blower Motor Resistor: This is the most common culprit. The resistor is a component that regulates the speed of the blower motor. When it fails, it often defaults to only the highest speed. The resistor itself can burn out, or the wiring to it can become corroded or damaged.
* Blower Motor: While less likely than the resistor, a failing blower motor *could* cause this issue. A motor nearing the end of its life might only function at its highest speed setting.
* Wiring Issues: Damaged, corroded, or loose wires in the circuit that controls the blower motor speed can also interrupt the lower speed settings. This could be anywhere along the path from the switch to the resistor and motor.
* Blower Motor Switch: Although less frequent, a problem with the blower motor switch itself could also prevent lower speed selection. It's possible a contact within the switch is faulty.
* Fuse or Circuit Breaker: A blown fuse or tripped circuit breaker specifically for the AC blower motor (though less common for only high speed failure) should also be checked.
Troubleshooting Steps (in order of likelihood and ease):
1. Check the Blower Motor Resistor: This is usually located behind the glove box or under the dash, often near the blower motor itself. Look for a small black box with several wires connected. Inspect it visually for any signs of burning, melting, or corrosion. If you find damage, replacing it is the most likely solution. These are relatively inexpensive and easy to replace.
2. Check the Wiring: Carefully inspect the wiring connected to the blower motor resistor and the blower motor for any damage, corrosion, or loose connections. Repair or replace any damaged wiring.
3. Test the Blower Motor (with caution): This requires some electrical knowledge and safety precautions. You'll need a multimeter to test the motor directly for continuity and proper operation. If the motor is defective, it needs replacement.
4. Inspect the Blower Motor Switch: This is more involved and requires removing the switch from the dash to inspect for damage or faulty connections.
5. Check Fuses and Circuit Breakers: Locate the relevant fuses and circuit breakers in the fuse box (usually under the hood and/or inside the cabin) and check for any blown fuses or tripped breakers.
Before you start: Disconnect the negative battery terminal before working with any electrical components in your vehicle. If you're not comfortable working on car electrical systems, it's best to take your S10 Blazer to a qualified mechanic.
