Here's why:
* Blend Door Actuators: These small motors control the direction of airflow. There are typically several actuators responsible for directing air to the vents, floor, and defroster. If one or more of these actuators fails, it can get stuck in the defroster position, preventing air from going anywhere else. This is a very common failure point in older vehicles.
Other possibilities, though less likely given the sudden nature of the problem, include:
* Vacuum leak: The system might use vacuum to control the blend doors. A leak in the vacuum lines would prevent proper operation of the actuators. This is less likely to suddenly occur than actuator failure.
* Wiring issue: A short or broken wire to one of the actuators or the control unit itself could also be the problem. This is also less likely to be a sudden issue unless there's been some physical damage.
* Control unit failure: While less probable than a faulty actuator, the control unit itself could have malfunctioned. This is less common as the actuators are the more mechanically stressed components.
Troubleshooting Steps:
1. Check fuses: Start with the simple stuff. Make sure the relevant fuses aren't blown.
2. Listen for actuator noises: With the ignition on (engine off is fine), cycle through the different airflow settings. Listen carefully for any clicking or whirring sounds near the dashboard area. A lack of sound from a specific actuator is a strong indicator of failure. This is usually the best place to start.
3. Visual inspection (if possible): If you're comfortable removing parts of the dashboard, you might be able to visually inspect the actuators for obvious signs of damage or failure. However, this is a more involved process.
4. Professional diagnosis: If you're not comfortable with automotive repairs, it's best to take your Blazer to a mechanic for proper diagnosis and repair. They have tools and experience to accurately pinpoint the problem.
The blend door actuators are a common and relatively inexpensive repair, but the labor cost to access and replace them can be significant due to the location within the dashboard.
