Here's a breakdown of the likely causes and troubleshooting steps:
* Blend Door Actuators: These small motors are responsible for opening and closing the blend doors that direct the air. One or more of these actuators might be malfunctioning, stuck in a position that only allows air to the floor and windshield, or failing to receive the proper signal from the control panel. The most common failure is a failure of the actuator itself, and it would likely require replacing the faulty actuator(s).
* Blend Door Linkage: The actuators are connected to the blend doors via a linkage. This linkage can become disconnected, broken, or bind up, preventing the doors from moving to the desired position. Inspecting for broken, disconnected, or binding parts of this system is crucial.
* Vacuum System (Less Likely): Some older vehicles use a vacuum system to control the blend doors. If there's a leak in the vacuum lines or the vacuum pump is failing, it could affect the blend doors' operation. However, this is less likely given that your cold air works correctly - the cold air system would also likely be affected by a vacuum leak.
* Control Panel/Head Unit: While less likely, a faulty control panel or head unit could be sending incorrect signals to the actuators. This is usually diagnosed by elimination after checking the actuators and linkage.
Troubleshooting Steps:
1. Check the HVAC controls: Ensure you are properly selecting the desired vents. Try different settings to see if anything changes. Does changing the temperature affect the air distribution?
2. Listen for actuator noises: With the engine running and the HVAC system on, listen closely around the HVAC unit (usually under the dash). You might hear unusual clicking, grinding, or humming sounds from a faulty actuator.
3. Visual inspection (if accessible): This is difficult without removing parts of the dash, but if you can safely access any part of the blend door linkage, check for any obvious damage or disconnections.
4. Consult a repair manual: A repair manual specific to your 1991 Buick Park Avenue will provide diagrams of the HVAC system, including the location of the actuators and blend doors. This will help tremendously in diagnosing the problem.
5. Professional diagnosis: If you're not comfortable working on your car's HVAC system, taking it to a qualified mechanic or auto AC specialist is the best option. They have the tools and experience to diagnose and repair the problem accurately and efficiently.
Important Note: Working on the HVAC system can be complex and require specialized knowledge. Always exercise caution and disconnect the battery negative terminal before starting any work to avoid electrical shocks. If you're unsure about any step, consult a professional.
