on my 99 dodge ram depending on the truck the ac will work fine but then you hear a pop sound and the heater is turned on sometimes it doesnt happen sometimes its right away sometimes ill drive for 20 min and it will do it i was wondering how the blend door is controlled in our trucks

 

vacuum. do a search, there are many threads on this subject

 

Wild vents aka crazy vents. it's all inner related so just do a search for either one of them and you will get a ****load of threads

 

well from what i am reading it seems that it does it only while driving because it will do it to me at idle but if i turn it off and on it comes right back so the vacum canister/check valve will fix that?