With the truck at operating temperature put the heater controls around the middle of the dial (medium temperature and floor heat)
Run the rpm up to about 1700 and hold steady
roll the window down so you can hear the engine
turn the heater control all the way to defrost and wait a couple of seconds......








the AC compressor kicks in and drags the rpm's down about 100 rpm. It is amazing how much of a parasite running around with the defrost on can be to your horse power.....

 

Not only a parasite on horse power. How about fuel milage.

 

Most companies are wiring their vehicles to run A/C when in Defrost mode to reduce moisture in the air inside the vehicle.

 

Do I need to roll the window down? It is cold out today and snow is expected.

 

I noticed that when I first got my truck. You can turn the AC compressor off and still run hot or cold air through the defrost vents. My .02

 

This whole A/C running while in defrost isnt something new, cars and trucks have been doing this for years, its just alot of people never really took notice, I had an 83 Buick regal that did the same thing. Its also designed this way because when you dont use your A/C for all those cooler months when its not needed, the compressor doesnt get fully lubricated. The car designers did this because they know you will use your defroster alot in the cooler months and the winter, so they made it that when you put on your defrost, it will kick in the A/C compressor to get it running a bit, to keep everything moving and lubricated.

Kind of like a gas motor in a car, if you let it sit, with out starting it every so often, all the oil wil drain to the pan, and oil coverage will be minimal when it comes to starting it eventually, (thats where synthetic helps). Usually if I forget to start my vehicles for a while, (this would be for a few months) I sometimes pull the distributor out, and will spin the oil pump to get the oil flowing through the block for added protection before start-up.

Its either, run the A/C compressor for a bit to keep everything lubricated, or let it stay static, for a few months, and hope that nothing seizes up when you finally go to use it later on...

 

Although having the A/C compressor run in the defrost mode does help to lubricate the unit during those winter months. It is not the primary reason the manufactures wired it this way. The primary reason was to dehumidify the interior of the vehicle ie; defrost the windows. When the A/C compressor is cycled on. The evaporator (located behing your glovebox area) gets cold and with the air circulating through it and your cab it pulls the moisture from the air. Your windows defrost at two to three times as fast with the A/C compressor engaged. If you have a vehicle that has a push button to turn it on/off try it yourself both ways and see which one works fastest.
I have been in the auto A/C business for years and when a customer has a lunched compressor at the end of the summer they may say "i'll just wait until next sprind before I fix it" then they come in in December saying thier defroster does'nt for worth a crap. Go Figure!