Question Regarding AC/Heater/Defrost control system
 

I've noticed that I can only run my defrost with the system pulling in outside air. Is this normal? I hate this because I can't drive in slow moving traffic without pulling in exhaust fumes, let alone sit at an intersection.

I have an 04 and have had it since March, if that's relevant.

Is this something that can be fixed with a cabin air filter? I'd like to not be sucking in CO all winter, so any help will be great.

l'd love a fix for this too. I hate having to make the cool dry air inside into warm humid air when defogging or defrosting with it raining or snowing outside. Just makes next time worse and worse.

The less humid the air is in the cabin, the less of a fog problem you will have. I have taken to popping the moonroof a crack and blasting as much hot air as possible into the cabin, against windows, etc, drying the air as much as possible, and 'forcing' it out the moonroof. It still only works till the next time I have to defog/frost.

I agree with the OP, its irritating for me bc I have a trans leak right now and the fluid is dripping onto my midpipe.......not a pleasant scent.....

I've never had a vehicle that worked otherwise. Defrost always turns off Recirc. When it gets cold (oh wait, it never gets cold in Virginia :icon_no2: ), what I do is switch to the forward vents once the windshield is properly defrosted and then turn recirc back on. That may not do much good though in extremely cold climates.

I've never had a car that works this way. My grand prix, my buddies truck, my dad's truck and a few other cars I've been in all run the defrost with recirc.

So...will a cabin air filter fix this? Is there always one installed in these cars? I know my GP had the place for one, but they didn't install one from the factory.

Anyone have any fixes for the smell?

I believe that when you choose "recycle" the air is drawn back in through the windshield vents, so when you choose the windshield vent to defrost, it can't recycle the air since it's blowing air out through them.

That's the way my previous cars worked anywho

Ah, that makes sense. My grand prix pulled from the passenger side floor. It had a separate intake so all of the HVAC functions could be used on all settings

The reason for this is simple. Cabin air gets stale- and humid- quite quickly, so using recirculation on DEF makes little sense. It won't be effective.

Most every car I've owned for some time now automatically defaults to fresh air when the defrost setting is selected.

it makes plenty of sense when you're in an area that freezes in the winter. Hot defrost = no ice on the windshield. Hot air on the windshield also keeps it from fogging up in the winter when ice isn't an issue. It's very effective under some circumstances.

Cabin air gets stale- and humid- quite quickly, so using recirculation on DEF makes little sense.

That is exactly the way it DOESN'T work.

The a/c kicks on in defrost and dehumidifies the air, recirc will clear the misty glass quickest. The a/c is on even in defrost, even when the little light says it isn't - this is all explained in your manual. I read it.

S

its quite ingenious really, its also a ford/mazda thing.

Its quite annoying when you have a trans leak :(

Sorry, but I have to revise this. Both you and MP3Guy are right. Defrost automatically turns ON A/C AND changes intake mode to pull in FRESH AIR. Fresh Air helps dehumidify a foggy windshield in addition to the dehumidifying effects of the A/C.

I agree with most of the posters here that the nanny HVAC controls are annoying. Clearly, designers need to design with consideration for the lowest common denominator.