1. Blower Motor Resistor: This is the most common culprit. The resistor controls the different blower speeds. If it fails, the blower often defaults to the highest speed.
* Troubleshooting: Locate the resistor (usually under the dash, sometimes behind the glove box). It's a small, rectangular unit often with several wires attached. Visually inspect it for burn marks or other signs of damage. If it looks burnt or damaged, it needs replacement. A multimeter can be used to test its resistance (though this requires some electrical knowledge).
2. Blower Motor Control Module (often part of the HVAC control head): This module receives signals from the climate control switch and regulates the blower motor. A faulty module can cause the blower to run continuously, even at high speed.
* Troubleshooting: This is more difficult to diagnose without specialized equipment. You'll likely need to have it checked by a mechanic or get a replacement if the resistor is good. Sometimes, just unplugging and replugging the connector to the module can temporarily fix the issue if it's a loose connection.
3. Wiring Issues: A short circuit or damaged wiring in the blower motor circuit could also cause the problem.
* Troubleshooting: This involves carefully inspecting all wiring related to the blower motor, resistor, and control module for any damage, frayed wires, or corrosion.
4. Ignition Switch Problem (less likely): In rare cases, a faulty ignition switch can send power to the blower motor even when the key is out of the ignition.
5. Aftermarket Radio Installation: If you recently had an aftermarket radio installed, there might be a wiring error causing the problem. Check all the connections related to that installation.
What to do:
1. Start with the blower motor resistor: It's the easiest and cheapest part to replace. Many auto parts stores have diagrams showing its location.
2. If replacing the resistor doesn't solve the problem: You'll likely need professional help to diagnose the control module or wiring issues. A mechanic can perform a proper diagnosis with specialized tools.
3. Disconnect the battery negative terminal: Before working on any electrical components, disconnect the battery's negative terminal to prevent accidental shorts and shocks.
Important Note: Working on car electrical systems can be dangerous if you're not comfortable with it. If you're unsure about any of these steps, it's best to take your Trailblazer to a qualified mechanic. They have the tools and knowledge to diagnose and repair the problem safely and efficiently.
