Re: foggy windows

["Fred Munro" <munrof@isys.ca>]

-----Original Message-----
From: Jesster436 <Jesster436@aol.com>
To: quattro@coimbra.ans.net <quattro@coimbra.ans.net>
Date: Monday, January 19, 1998 4:36 AM
Subject: foggy windows


>Am I just totally stupid (don't answer that!) or does anyone else have
>problems defrosting windows with the automatic climate controls (90q20v)?
>Last week I was driving in the rain and the windows started to fog up so I
hit
>the windsheild defroster button and that caused all the windows to fog up.
I
>had to just stop in the middle of the road.  A few days later I hit the
AUTO
>button (set to 72 degrees) and same thing happened.  Then today while I was
>filling up at the gas station, a small patch of fog formed just above the
>vents on the center top of the dash.  When I got in the car I hit the
button
>and all the windows fogged up.  I thought the defroster was supposed to
>DEfrost.  Is there a problem here?
>
>Jess C. Almero, Jr
>South Pasadena,  CA
>'90 90q20v
>'86 Coupe GT


Hi Jess;

    What you are describing happens when warm, moist air from your defrost
system hits colder glass. The moisture in the air condenses on the glass,
causing fog.
    The question is, of course, why the air from the defroster is moist.
There are several possibilities:

    1. The air conditioner is O/S. When you engage defrost mode, the air
conditioner is supposed to come on. This removes the moisture in the
incoming air (it condenses in the A/C evaporator) and effectively defrosts
the windshield. If the outside temperature is too cold (below 32 F I think),
the A/C will not engage, but then it wouldn't be raining either - the theory
here is that cold winter air already has a low moisture content and the A/C
is not needed. If it is raining, cool, and the A/C is not working, the
heater will evaporate the moisture into the defrost air stream and it will
happily condense itself all over your glass.

    2. A coolant leak (O happy day! I've BTDT) in the heater core will also
cause this problem. The hot coolant saturates the air stream with moisture.
Since the heater core is downstream of the A/C evap, the moisture cannot be
removed by the A/C system. A friend with an '85 Jetta had this problem; it
got so bad that if the heat was on he couldn't see anything after driving 5
minutes in winter. Although the lad is as tight as bark to the proverbial
tree, even he had to break down and have the father-in-law install a new
core.

    I have not experienced this fogging problem with either of my Audis.

HTH

Fred Munro

'91 200q  245k km