Front defog- A/C sometimes on, sometimes off
Thread Starter
Instructor
Joined: Feb 2008
Posts: 147
Likes: 1
From: Huntsville AL
Front defog- A/C sometimes on, sometimes off
Car is a 98 A4 with Auto Climate Control. This is only the 2nd winter I've had the car and am again having window fogging issues on cold rainy (humid) days. When the HVAC is commanding the compressor to run I don't have any problems. But sometimes it refuses to run the compressor for no apparent reason. This is the case regardless of whether the unit is in AUTO mode or DEFROST/DEFOG mode. One minute the compressor is running and all is fine... the next I begin to smell the moisture in the air, look down to find the light on the A/C button OFF, and very shortly the windows all begin to fog up. If I push the A/C button it just blinks a time or two and stays off. I have to crack the windows to get enough fresh (and cold!) air into the car to get the moisture to evaporate off the windows. A few minutes later the A/C will come back on and I can close the windows. Based on some posts I've read on here I thought the HVAC was ALWAYS supposed to run the compressor when Defog was selected? I've never checked the freon charge in the system but it cools fine in summer on a 95 degree sunny day (and the car is triple black). Any thoughts?
Racer
Joined: Apr 2002
Posts: 352
Likes: 11
From: Glendale AZ
Have you noticed if it is below 40 degrees outside when the compressor doesn't seem to work properly? My owners manual says the compressor will not come on when the outside air temp is below 40 degrees.
DG in AZ
DG in AZ
Tech Contributor
Joined: Dec 2006
Posts: 10,962
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From: Van Buren Arkansas
Wounded Warrior Escort '11
I agree. Somewhere in the 38 - 42 range, the compressor will not turn on if that's what is going on. Otherwise, see if you have any codes present just in case and lastly, take it to an HVAC specialist and have them check the charge of the refrigerant.
Safety Car
Joined: Sep 2000
Posts: 3,572
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From: Taildragers rule! USA1
It is one of the most stupid things Chevrolet ever did. The A/C compressor simply will not come on under about 38° .
One moment you're driving along and everything is fine, then BAM! A/C off and windows fog up instantly.
AFAIK there is no fix, maybe a heater near the temp sensor?
One moment you're driving along and everything is fine, then BAM! A/C off and windows fog up instantly.
AFAIK there is no fix, maybe a heater near the temp sensor?
Thread Starter
Instructor
Joined: Feb 2008
Posts: 147
Likes: 1
From: Huntsville AL
thanks for all the replies. maybe the outside temp is what's getting me. The times i've experienced the problem in the past week very well could have been down near that temperature. But it's interesting to me that the A/C will be on, then turn off (with no apparent change in outside temp), and then within a few minutes, will be ON again, even though the outside temp didn't warm up. I'll have to pay more attention to the system behavior with ambient temp in mind.
I'm thinking 12V directly to the compressor clutch coil, wired though an inconspicuously placed switch, would solve this problem. I realize this would place additional load on the motor that the PCM would be unaware of (since it wouldn't have asked for the compressor to be on), but I can't imagine it would make any noticeable difference so long as I'm driving down the road with the engine above an idle.
I'm thinking 12V directly to the compressor clutch coil, wired though an inconspicuously placed switch, would solve this problem. I realize this would place additional load on the motor that the PCM would be unaware of (since it wouldn't have asked for the compressor to be on), but I can't imagine it would make any noticeable difference so long as I'm driving down the road with the engine above an idle.
Tech Contributor
Joined: Dec 2006
Posts: 10,962
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From: Van Buren Arkansas
Wounded Warrior Escort '11
I have to wonder about GM's reasoning behind the shutoff of the compressor at the low temps. Maybe it causes the condensor to get too cold therefore drawing moisture and freezing the air passages while you are driving...maybe it gets so cold that it ruins the compressor itself. They put this shutoff in there for a reason otherwise they wouldn't have wasted the money.
