Defrost with heater or using ac compressor??

pippo on Thu July 17, 2014 8:21 PM

People,

Wondering at about what year/when did cars started being engineered with the 'defrost" mode utilizing the ac compressor, actually producing COLD defrost temps top clear up that fogy windshield as opposed to older cars (1970s??) which never did this but instead defrost mode used warm/heated air. Anybody know? or am I misinformed in that they always used the COLD ac to defrost?

Thanks, people.

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beware of the arrival

pippo on Thu July 17, 2014 8:22 PM

ops, sorry for posting thread 2 times. I goofed....

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beware of the arrival

GM Tech on Thu July 17, 2014 10:24 PM

It is NOT the cold that clears the windshield-- it is the dry air--you run defrost with temp on warm and the air is dried out by removing the moisture in the evaporator pass thru- yes the air gets cold in evap- but is warmed by the heater core. The evap removes heat and moisture from air that passes through it.

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

pippo on Fri July 18, 2014 6:54 PM

Quote
Originally posted by: GM Tech
It is NOT the cold that clears the windshield-- it is the dry air--you run defrost with temp on warm and the air is dried out by removing the moisture in the evaporator pass thru- yes the air gets cold in evap- but is warmed by the heater core. The evap removes heat and moisture from air that passes through it.

OK, GM, but if its like you say that you set the temp on warm, how can the ac comp engage then to produce cold air which is supposed to dry out that air? Im stuck in my mind somehow.......thanks.

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beware of the arrival

GM Tech on Fri July 18, 2014 7:03 PM

Outside Ambient Moist air enters through air inlet- passes through evap where air is cooled and drops its moisture in form of condensate, the air is passed across heater core to warm the now dry air and force it onto your windshield.

You may be hung up on the fact that the air is cold coming into your air inlet- but alas, a/c compressor function is limited to system pressure above 47 psi- which in turn equates to about 40 degrees F out side temp.. so the a/c is NOT on below freezing even though the defrost has enabled it- the pressure switch says no - not until ambient is 40 or above-- (or system pressure is above 47 psi.

climate contgrol systems with ouside temp sensors- use 40 degf as their bogey- a/c enabled above 40, not below 40degf-- been that way since early 60s....

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

pippo on Fri July 18, 2014 7:50 PM