Originally Posted by solarpower
the reason the A/C turns on when you hit the defroster is to keep it lubricated and running properly for use in the summer. it's only on for about a minute or 2.
I read that in a owners manual yeras ago when I had my first XJ. I've had 3 Jeeps since then and they all have done the same thing. 2XJ's, 1 TJ and the wife commmander all.
reason number 321354681321356003 why I like Jeeps. advanced engineering.(ok not always)
No, it's not.
The reason A/C comes on with defrost (or should be turned on for defrost in cars where you can manually switch the a/c on whenever you want) is because it improves the defogging ability about 6000%. The A/C is an excellent dehumidifier - a side effect of how it works causes it to remove moisture from the air. In cool damp climates, or with a carload of people, especially wet people, a defroster with no a/c will have a very hard time keeping the windows clear - if it even can. (Same thing with using recirc by the way.) Any time you see a car with badly fogged up windows going down the highway, you can bet good money they have selected recirc, and if they have a separate A/C switch its probably off. And in the morning when the defroster is having a tough time defogging the inside of the glass - just switch on that A/C and presto! it clears in seconds!
And it's not "only on for a minute or two." There is nothing in the control circuit for that. It will kick on and off according to a thermostat in the plenum (keeps it from freezing up the evaporator) and/or a pressure switch (also keeps from freezing up, and from overpressuring and blowing a hose.)