Question about the HVAC controls

[bringselpup]

Last week I noticed when I was driving in pouring rain that my a/c was on blowing cold out the defrost. But I didn't have the switch engaged to turn on the a/c. No green light. I checked it out yesterday and have determined if I have the middle control set to any setting that routes the flow through the upper defrost vents the compressor is engaged. But not if I put it say to only the panel vents.

Then it only comes on if I push the a/c switch.

What's up with that?

[hardisk_c]

Mine does that too, and I think its normal operation. If you have the interior controls set to defrost & have the temperature guide set to cold, it engages the AC compressor to dehumidify the air before hitting the windshield w/ it. But if you have the controls set to cold & the vents, it just vents unless you have the AC button pressed.

My on a '94 JGC Laredo

[bringselpup]

I kinda figured it would be "normal" operation too, but I went back and put it on defrost and turned the dial to heat and the compressor stays engaged.

[hardisk_c]

Prolly the same deal, allow me to amend meself. . . I did some checking w/ a mechanic here, he said that during defrost it'll engage the compressor for dehumidification. . . . not just when on defrost & cold

[bringselpup]

I can see the logic behind all that because air conditioners do in fact suck the water out of the air. But that means even in the dead of winter when I've scraped the ice off my windshield that compressor is running if I put the heat through the defrost vents? OUCH I tend to leave it set on defrost all the time just to push some air at the windshield. No wonder I'm on the low end of what people report for gas mileage for the 4.0, that compressor is there adding a little extra drag all the time.

Well you gotta love them Jeeps but I might just start unplugging the damn thing when the weather is cool enough to not need the AC.

[ChiefJ]

Actually since about the mid 80's most auto manufactuers have been using the A/C compressor for the defroster system to reduce humidity. They actually plan for the power draw of the compressor in the designs.Pretty much nowadays you get better gas mileage with the windows up and the A/c on due to less wind resistance. I think the area of the country you live in and the blends of gasoline for the region play more of factor in the mileage number. I know here in the northeast most gas stations are required to sell 10% ethanol in the gas as an EPA pollution control requirement. Thank the Feds for the mileage

[ChiefJ]

Originally Posted by bringselpup

I can see the logic behind all that because air conditioners do in fact suck the water out of the air. But that means even in the dead of winter when I've scraped the ice off my windshield that compressor is running if I put the heat through the defrost vents? OUCH I tend to leave it set on defrost all the time just to push some air at the windshield. No wonder I'm on the low end of what people report for gas mileage for the 4.0, that compressor is there adding a little extra drag all the time.

Well you gotta love them Jeeps but I might just start unplugging the damn thing when the weather is cool enough to not need the AC.

I think that LoadWarrior on top is probably placing more drag on your I6 that the A/C compressor.

[bringselpup]

Point taken.

That Yakima is a nicely designed piece of gear and is made to come off easily. I suppose if I want to get completely obsessive about it I can do trials with it on and off and with the compressor pumping or not.

But I'm betting I can think of plenty of other stuff to do instead.

[Bob D.]

It's not just Jeeps. I've had that happen in most of the vehicles I've owned and was pretty sure the compressor turned on when in defrost mode to keep the AC lubed up and seals from drying out during the winter months.