defroster/AC

In my '97 Ram Van 2500, why does engaging the defroster seem to be activating the AC? I noticed that using the defroster causes the freon pipe to be stone cold & causes much
condensation to drip off from the AC "box." When I use the heater, the AC gear doesn't get engaged; I thought the defroster would just be the heater gear pointed up to the front glass.
Ultimately, I'm trying to troubleshoot an overheating problem.
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You can't de-frost/de-fog a windshield with humid air... it will just re-condense on the inside of the cold glass.
This is normal operation, and is intended.
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Actually, it was started by the manufacturers to reduce AC system warranty repairs come the following summer (although a side benefit was quicker defrosting). They figured you running the defrost through the winter would keep the oils flowing through the system and keep the O-rings and seals from drying out. Remember when you could buy a car w/o AC? maybe not if your young enough, but I had plenty of them as a teen...in fact i didn't have (working) AC until I bought a used 93 Saturn! hehe, anyway...just another, your van came with A/C so it was designed to handle the added heat the A/C system would put on the cooling system. Whether it's on or not, the trouble shooting steps are the same.
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Warm air holds more water vapor than colder air (basic adiabatics). When warm air contacts cold glass and cools, which lessens the ability of the air mass to keep the water vapor in suspension. This causes the air mass to release the water vapor as liquid condensation. Chilling the air before it strikes the glass causes the airmass to cross the dew point and shed the water in the evaporatorator, not the windsheild. Once the engine is at adequate temperature and the airmass is first heated, it can absorb even more moisture as vapor. Then the air is cooled as stated before, shedding even more water at the evaporator, which rapidly dries out the air inside the vehicle. Steve

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