Date: Sat, 25 Jan 2003 15:15:55 +0000
From: Grunff <>
Subject: Re: Saab 9000 fogging up

Fannie Schapiro M.D. & Daniel Krill wrote: > My wife's 97 Aero is fogging up within 20 minutes of driving on these cold > Rochester mornings lately. The mechanics she uses (Schoen Place) have > replaced the heater core twice already, specifying that as the problem. Why > would the heater core cause the interior to fog up (and ice over, lately) on > the inside of all the windows? It seems to me that this is more of an issue > of recirculated air and not getting fresh, dry air from the outside to dry > out the interior. > > Also, for some reason, the manual says to increase the temperature to 70 if > the interior fogs up. Why would this work? It seems counterintuitive, since > on normal cars to defog you turn on the air conditioning. Since there is no > independent AC control in the 9000, the only thing to do is to change from > recirculate to fresh air, or turn the temperature DOWN to get the AC going. It sounds like your'e either recirculating the air, or you have a *lot* of water leaking into the car. The reason a failed heater core causes fogging is because air passing through it gets loaded with a high water vapour content. You say that on 'normal cars' you turn the air con on to defrost. This works because when you cool the incoming air, you condense out much of it's moisture. So you end up blowing very dry air across the screen, which evaporates the condensation. This is fine, as long as it's not very cold outside. If it is, it's more effective to use hot air to evaporate the condensation. I think you need to eliminate the possibility of a leak first (easy, closely examine the carpets front and back). Then you need to look at whether you are drawing air in from outside when you switch recirc. off. Easiest way to do this without taking the dash apart is with a smoke match. -- Grunff

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