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Strange AC behavior - On with defogger


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6 replies to this topic

#1 97subaruGT

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Posted 24 July 2004 - 08:23 AM

Hey everyone.
97 Legacy GT, 140,000 miles.
Some quick strangeness here. When I select the Windshield Defogger on the dash, the AC turns on. Regardless of where the temp setting is, hot or cold. I noticed this a few days ago. The AC button is off so the clutch should not be engaging so I'm a bit puzzled.

I checked all the fuses and they're all intact so I'm not sure where else i could look. Would I need to replace the HVAC control unit? I took it out, didn't see any obvious shorts or frayed wiring.

Any help is great..

Thanks

#2 Hocrest

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Posted 24 July 2004 - 08:36 AM

I believe it is supposed to do that...

AC helps to dry the air to aid in defogging (for the same reason, your "Recirc" button is disabled during 'Defrost" use) and it also forces use of the AC compressor during the winter months to prevent the seals from drying out.

#3 powderhound

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Posted 24 July 2004 - 01:09 PM

Hocrest is correct. That's the reason I can't accelerate with the defrost on. I'm not sure if the AC comes on when you have the defrost and feet button selected though. Just curious but did it take you 140K to notice this?

#4 97subaruGT

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Posted 25 July 2004 - 08:38 AM

Hocrest is correct. That's the reason I can't accelerate with the defrost on. I'm not sure if the AC comes on when you have the defrost and feet button selected though. Just curious but did it take you 140K to notice this?


Yea, it actually did. I usually only use it in the winter but i decided to kick it in now and felt cold air coming out and was just curious. Ignorance is bliss i guess..

#5 Buddythedog

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Posted 26 July 2004 - 06:59 PM

Yea, it actually did. I usually only use it in the winter but i decided to kick it in now and felt cold air coming out and was just curious. Ignorance is bliss i guess..


As you have heard, this is normal in most/all modern cars. The reason (may be more than one) is that the AC dries out the air in the cabin of the car, which effectively is the defrosting action, you just need to move it to warm in the winter.

#6 envy_petter

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Posted 27 July 2004 - 08:40 AM

The AC turns on with the defroster to ensure that it doesn't build up mold over the winter months. You don't use your AC over the winter, so it would get moldy. By running the AC compressor when you have the defrost on, it keeps it cleaner/fresher.

BTW, under hard acceleration the AC will turn off. It is designed to do that. Just make it down shift hard. I sold Subies for 2 years and own my second one. The first one I had for 9 years (it was a '95 Legacy). That's how they worked then and my '03 Special Edition Sedan does the same thing.

#7 Tiny Clark

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Posted 28 July 2004 - 12:02 AM

Buddy is correct, it turns on the AC to dry the air.




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