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Old 08-28-2003, 06:34 PM   #10
Scooby Newbie
Member#: 42480
Join Date: Aug 2003
Location: woodinville wa
2001 911/996 C4
wimbledon green

Default Fogging up

The problem you describe has always existed since the advent of A/C in automobiles, but became much more of a nuisence with the advent of non-freon refrigerants that have a much lower cooling efficiency than freon.

To make up for the loss in efficiency the A/C evaporator had to be redesigned. As a result modern day A/C evaporators can act very much like a sponge. In order to increase efficiency the evaporator had to become much more complex, more metal vanes, cooling surfaces exposed to the airflow, all stacked into a very tight space under your dash.

In simple terms, a SPONGE.

When you park your car for the evening after a humid day of heavy A/C use, the evaporator will likely be saturated, throughly CHOKED, with moisture. During the day as the system continued to operate the condensed mositure would accumulate into droplets large enough for gravity to overcome viscosity and then run out the drain tube provided for tha very purpose.

But not your car is sitting there with a very thin firm of moisture covering about 10,000 square inches (92 Lexus LS400) of cooling vane surface.

If you park inside it will help tremendously if you can lower all of the windows during the night. Then, unless the garage is quite cool, the thin film will evaporate into the local atmosphere and much of it will then end up outside the vehicle.

Another aspect of this that you might take serious note of is the fact that during the winter when outside temperatures decline below freezing the vehicle's A/C cooling capability will become totally and completely inoperable. Many modern day systems actually disable the A/C compressor altogether when the outside temperature declines to 35F and below.

It's cold outside, so who cares, you say.

Well, you do care.

Many, if not most, modern day automatic A/C systems will operate the A/C system all year around in order to more closely regulate the cabin temperature and humidity.

But even if that is not the case almost ALL systems will automatically activate the A/C when the defrost/defog/demist mode is activated.

So there are two things to remember.

First, if you happen to use the A/C for defogging during fairly cool weather then when you exit that mode the thin film of moisture left will oftentimes act like a MOISTURE BOMB. Shortly after you deactivate defog mode that thin film of mositure will begin to evaporate back into the incoming airstream in which case you might end up with higher cabin humidity, and possibly a more severely fogged windshield, than before the first use of the defog function.

Second, and this is for those of you with automatic A/C systems or climate control, as you drive your vehicle into an area of declining temperatures the A/C control system may, spontaneously and without any indication to you whatsoever, disable the A/C compressor completely due to outside temperatures reaching 35F or below.

Imagine driving along on a cold but bright winter's day, four people in the car warm and cozy. Now as you approach the higher mountain elevations the A/C compressor shuts down due to the declining temperature. At 65 MPH the windshield interior surface is becoming thoroughly chilled due to the impinging COLD airflow.

AND now that thin film of moisture is evaporating into the incoming airstream and being combined within the cabin atmosphere with the moist byproducts of human metabolism.

Very soon you will begin to small wisps of condensation at the lower corners of the windshield due to rising cabin humdity and a chilled windshield surface. Remember, now, the A/C is disabled
and cannot be used to dehumidify the incoming airstream. And even if it could be activated it's usefullness at dehumidifying an already cold incoming airstream is negligible at best.

But go ahead, activate the defrost/defog/demist function.

The result will very likely be exactly what I encountered in the winter of 92/93 in my brand new Lexus LS400.

A quite thoroughly and completely fogged over windshield.

When you activated the defrost/defog/demist function the result was that now ALL of the humid system airflow is routed toward the cold windshield surface.

What you REALLY need to do FIRST, is WARM that chilled windshield surface to raise it's dewpoint and begin to evaporate the already condensed moisture.

You can do that best by turning the heat demand of the system to MAX (the blower speed will usually increase automatically as you do this, if not...), and now once the heat is really flowing and the blower is at full speed then activate the defrost/defog/demist mode.

There is an aftermarket product available that about an hour after the vehicle is shut down will begin to run the system blower intermittently to disperse the moisture outside of the A/C plenum as it begins to evaporate from the evaporator vanes.

I understand that Ford is aware of your problem and will oftentimes recommend that the dealer install this device if the customer complains.

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