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Odd defrost / air-conditioner behaviour...

1176 Views 9 Replies 6 Participants Last post by  gfinockio
So let me explain - I understand that the A/C will often come on when using the front defrost button with Foresters (and other vehicles). That isn't my concern.

In my 2018 XT, when I run the front defrost, it seems to work fine (and there is no indication the A/C is on but I assume it is), but when I click the "Mode" button after the defrost is off, it turns on the A/C automatically regardless of what the temperature inside the vehicle is.

Now normally I wouldn't worry too much about this... but I don't really want the A/C coming on when I'm driving around in -25C weather!

If I turn the A/C off manually, and flip through the modes, it doesn't come on again. This only happens *after* I use the front defrost.

Thoughts? Explanations?
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I think that the A/C kicks in so it draws outside dry air to assist in the defogging.
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@grantyoung Yes - when the defrost is ON.

That doesn't explain why the A/C comes on after the defrost is turned OFF.
Weird, not residual cold air from AC cooling the condenser? I dislike automatic stuff and don't want cold dry air everytime I direct air in through windshield 'defrost' vents, so on my 2010 and 2015 I disassembled the console and disabled the AC always on with defrost setting. 2010 just removed dogleg white plastic piece behind the vent selector knob and reassembled. 2015 used sharp knife to cut sharkfin micro switch off flush with the housing so that the cam on backside of vent knob didn't depress the switch, dab of CA on switch afterwards so that it should never come on unless I push the AC button. HTH :)
When you turn on the "defrost windshield" function the a/c turns on automatically to dehumidify the air flowing into the cabin.

If you don't use the "defrost windshield" function, if you choose the air outputs, on some cars if you direct air towards the windshield, you have to have air output to the floor as well, because they assume the passenger compartment is cold if you need to blow air onto the windshield.

If I want to blow air onto the windshield only and not towards the floor, what I have to do in my 2018 Forester is turn on "defrost windshield" and then press a button to turn off a/c. Then I can choose temperature and fan output as desired. I think things worked the same in my 2007 Forester but I don't remember.

If you want to remove moisture from the insides of the windshield and other front windows (and the cabin and the other windows) turn the a/c on which will remove moisture from the incoming air. The incoming air is passed over the cold a/c lines which cools the air, but also removes moisture from the air because condensation will form on the a/c lines and that condensation is water removed from the incoming air. It takes some electrical power to run the a/c compressor, and it takes some heat from the incoming air, but using the a/c and heat at the same time is the quickest way to clear the windows. Sometimes, when multiple people come into the vehicle from the rain, it will get humid in the passenger compartment, and everything in there will get damp, due to condensation and there being a lot of moisture in the car. I will have the a/c on along with a mild amount of heat, and after five minutes or so the cabin is dry enough and I will turn off the a/c but leave the heat on, and drawing air in from outside (outside air is also almost always drier than inside air, so draw outside air when you want to clear the windows). Usually drawing outside air will remove enough humidity that I don't need to keep the a/c on.
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When you turn on the "defrost windshield" function the a/c turns on automatically to dehumidify the air flowing into the cabin.
Again - I am aware that the defrost option turns the A/C on. My concern is that, when the defrost option is used and then turned off, and I change the mode from floor/windshield to front blowing, the A/C comes on regardless of the temperature. For example, I used the defrost this morning to try and defrost my windshield at -27C... after 10 minutes, I turned off the defrost and switched to front/floor blowing mode, and the A/C came on.

This is the concern, not the fact that the A/C comes on when using defrost - it's when it's NOT ON DEFROST that is the concern.
@gfinockio Not that you are saying this, but MANY people think A/C is always cold. If you have the temperature set on heat, it will be warm.

Just because it is running doesn’t mean it is chilling the air - it is “conditioning” it...🤔
@Firetrux Sure... except there is cold air being blown through the vents, not warm...
Change the Mode to bottom(floor) and set the temperature to maximum.

Laughing at oneself and with others is good for the Soul
@Yoda_One I can also just hit the A/C button to turn it off. But I shouldn't have to do that every time I use the defrost and switch into a different mode afterwards.
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