I've noticed when I have the direction of air flow set to blow on the floor or floor/cab and accelerate the air flow swithces to the defrost section. I've tested this based on hearing the same "flap closing sound" that I hear when I switch the system from defrost to the floor/seat or to floor mode. I don't know if the following has anything to do with the above mentioned problem, about the same time I lost my cruise control. I only state this because I think both might have some type of control via the vacuum system of the engine. Thanks.
Dodge Q&AAsk Your Question
1996 Dodge Ram 1500 Question: cab air flow direction switching.
Answer #1Andy Y May 06, 2009, 16:52Master
The heating/AC system is controlled by vacuum as you mentioned. There's an old service bulletin that involves adding an additional vacuum check valve to prevent this. Also make sure you don't have any new vacuum leaks creating the problem. I believe your speed control (cruise control) servo is vacuum actuated so a line may have popped off. Check out this link, it's got the images that will make more sense
When the heating/AC controls lose vacuum they default to defrost (during acceleration being the perfect example)
hope this helps!