No heat blows out front vents, just A/C or air

Welcome to 2CarPros.

Do you get heat from any other area? It could be a blend air door actuator. It is responsible for the air temp that enters the vehicle. I will provide the directions for inspection and adjustment of the actuator, but this is for a rear wheel drive brougham. If you have the FWD model, I will need to know. The attached pic correlates with these directions. Also, here is a link that shows in general how one is replaced. It may help guide you.

https://www.2carpros.com/articles/replace-blend-door-motor

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1990 Cadillac Fleetwood Brougham V8-350 5.7L
Air Mix Valve Adjustment
Vehicle Heating and Air Conditioning Air Door Actuator / Motor Adjustments Air Mix Valve Adjustment
AIR MIX VALVE ADJUSTMENT
Programmer
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1. Remove glove box module.
2. Set temperature on Climate Control Panel (CCP) for 32°C (90°F). Allow 1 to 2 minutes for programmer arm to travel to maximum heat position.
3. Snap threaded rod from plastic retainer on programmer output arm.
4. Check air mix valve for free travel. Push valve to maximum A/C position, and check for binding.
5. Pre-load air mix valve in maximum heat position by pulling the threaded rod to ensure the valve is sealing. Programmer output arm should be in maximum heat position.
6. Snap threaded rod into plastic retainer on programmer arm. Avoid moving programmer arm or air mix valve.
7. Set temperature on CCP for l6°C (60°F). Verify the programmer arm and air mix valve travel to the max A/C position.
8. Install glove box module.

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Let me know if this helps.

Take care,
Joe

Welcome back:

No problem. The information you provide doesn't change this condition. The problem has something to do with what I've mentioned. If you decide to check it yourself, let me know.

I will watch for your reply after having it checked.

Take care,
Joe