Likely Culprits:
* Vacuum Leak: The blend doors (which control the mix of hot and cold air) are often operated by vacuum. A leak in the vacuum lines can cause the doors to stick in a position that allows hot air to flow. This is the most common cause. Check all vacuum lines connected to the HVAC system for cracks, splits, or disconnections. Pay close attention to the lines near the firewall and under the dash.
* Failed Blend Door Actuator(s): These small motors control the position of the blend doors. If one fails, it might stick in a position that always allows hot air to pass through. Replacing the faulty actuator is the solution. There's usually one for hot/cold and possibly another for directing air to different vents.
* Stuck Blend Door: Even without a vacuum or actuator issue, a blend door can become mechanically stuck in the "hot" position due to age, debris, or wear. This often requires accessing the HVAC system to manually free the door, which can be a challenging job.
* Heater Core Problem (Less Likely): While less likely to cause hot air with the fan OFF, a severely leaking heater core could potentially allow some coolant to bypass the thermostat and passively heat the air within the system, even with the fan off. This usually involves a noticeable coolant leak or smell.
How to Stop the Hot Air:
1. Check Vacuum Lines: This is the easiest starting point. Visually inspect all vacuum lines related to the HVAC system for any damage. Listen for hissing sounds near the lines when the engine is running (that indicates a leak). Repair or replace any damaged lines. You may need a vacuum gauge to test for leaks in the lines.
2. Listen for Actuator Sounds: When you turn the key to the "on" position, you might hear clicking sounds from the HVAC actuators. If there's a problem with one of the actuators (especially the blend door actuator), you may not hear any sound or it may make unusual clicking or grinding noises.
3. Inspect the Blend Doors (Advanced): This requires removing parts of the dashboard or HVAC housing, which is a significant undertaking. Unless you are comfortable with this level of disassembly, it's best left to a mechanic.
4. Consult a Repair Manual: A repair manual specific to your 1986 Chrysler Fifth Avenue will provide detailed diagrams of the HVAC system, including vacuum lines and actuator locations. This is crucial for successful diagnosis and repair.
5. Professional Help: If you're not comfortable working on your car's HVAC system, take it to a qualified mechanic. Diagnosing and repairing blend door issues can be challenging, and incorrect repair could worsen the problem.
Remember safety first! Always disconnect the battery's negative terminal before working on any electrical components in your vehicle. Working on the HVAC system involves dealing with sharp components and potentially hot coolant, so exercise caution.
