Why does AC air come out of the defrost vents instead of the front vents when you accelerate?

Answer 1

Ok, on my 99 Cherokee Sport I found that the vacuum lines along the inside of the passenger side inside fender wall to the Cruise Vacuum Module. The line continues on into the grill area and ends. On mine the end was open, causing a severe vacuum leak! Plug the end of the line and the cruise, A/C, heater, etc. all work fine when you accelerate. I don't know why the factory overlooked this?

AnswerI've heard that you can add a vacuum canister in the line to act as a reservoir when extra vacuum is needed. AnswerThis occurs because the climate control doors(that direct the ventilation in the car) are vacuum operated, using intake manifold vacuum. The default(no vacuum) position directs the air to the defrost outlets. When you accelerate, the manifold vacuum drops below that required to operate the doors, and whatever you've got the temp set at blows out of the defrost. When you take your foot off the pedal, (or release the cruise control) the vacuum quickly recovers and the ventilation returns to normal. This is a design flaw, as far as I can tell, there is no fix. AnswerCould also be a vacuum leak. same thing happened to me on my Cherokee (xj) a while back AnswerWhile this might not be correct for your Jeep, I'll offer it anyway. On my Dodge van, there is a vacuum check valve in the line which feeds the climate controls. If it leaks, the controls will follow the engine vacuum and air will come out the defroster on acceleration. The idea is that since the climate control lines should be static and non-leaking, the manufacturer can use the lines and manifold as its own reservoir during periods of low vacuum.