* Blend Door Actuator: This is the most common culprit. The blend door actuator is a small motor that controls the flaps directing air to the vents (face, floor, defrost). If it's malfunctioning or broken, it might be stuck in the heater vent position, regardless of your selected setting. This is especially likely on a car of that age.
* Blend Door Linkage: The actuator operates a linkage system that physically moves the blend doors. This linkage can become broken, disconnected, or corroded over time, preventing the doors from moving correctly.
* Vacuum Leaks: Many older cars, including your Cadillac, use vacuum lines to control some aspects of the climate control system. If there's a leak in the vacuum lines, it can disrupt the operation of the blend doors.
* Mode Door Actuator (if equipped): Some systems also have a separate mode door actuator to control which *type* of vent is used (defrost, floor, vent). If this is malfunctioning, it could send air to the heater vents even if the blend door is working correctly.
* Faulty Control Panel: Though less likely, a problem with the climate control panel itself could be sending incorrect signals to the actuators. A short circuit or worn-out switch could cause this.
* Clogged or restricted vents: While less likely to cause air to *only* come from the heater vents, restricted vents elsewhere in the system could force more air through the less-restricted heater vents.
Troubleshooting Steps (in order of likely cause):
1. Check the Vacuum Lines: Carefully inspect all vacuum lines related to the climate control system for cracks, holes, or loose connections.
2. Test the Blend Door Actuator: This usually requires some disassembly to access the actuator. You might be able to manually move the blend door to see if it's free and responds. If the actuator is obviously stuck or broken, it needs replacing.
3. Check the Blend Door Linkage: Once you've accessed the blend door, carefully examine the linkage for any signs of breakage, corrosion, or misalignment.
4. Inspect the Control Panel: While it's less likely, check for any obvious damage or loose connections in the climate control panel itself.
Recommendation:
Given the age of your car, it's highly recommended to consult a mechanic specializing in classic cars or a knowledgeable auto repair shop. Diagnosing and repairing climate control issues can be complex, and a professional will have the experience and tools to properly identify and fix the problem. Replacing parts in a vintage climate control system often requires patience and specific tools. Trying to troubleshoot on your own could easily lead to more damage.
