AC running when set to defrost?

1999/3.5GX/96K

Hi gang. I've noticed that the air flowing from my vents, when on either defrost/feet or just defrost, seems as cold as it does when the AC is on, even though the AC is off. It doesn't seem this cold when the vents are in the forward, AC position and other settings. The air coming out is cold enough to fog up the windshield above the defrost vent.

If I turn the blue/red temperature knob towards the middle it gets more in the normal temp range, but of course at this time of year the knob is always at the coldest setting. I thought that even in the coolest setting, without the AC on, the air will only be as cool as the outside temp, but this air feels noticably colder.

So I'm basically thinking that my AC may be turning on when I put the vent on defrost or defrost/feet, though I don't know how that could occur without the AC being turned on. When the AC is turned on, I get the blinking light and random AC shutoff, so it also doesn't seem right that the cold flow would be as constant as it is.

The defrost/feet setting is what I keep it on pretty much whenever the AC is off, probably 80% of the year or more, so if the AC is somehow running when in this postion, I think it may be the cause of the awful fuel economy I've been unable to find the cause of for over 2 years now (I only put maybe 3500 on the intrigue per year) despite many trips to the shop and 2 dealers who could find no problems (without a CE light on anyway).

Before I bought the Olds, I'd read that the GL and GLS models had problems with their climate control stuff, so I was glad to have a GX with some good options instead. I don't know if maybe my temp controls could be screwy as well due to some common flaw.

Group regulars may recall my mileage dropped from a solid 19-20mpg suburbs to 14-15 a few years back, basically overnight and I've been bedevilled by it ever since.

Thaks for any thoughts, and it would help if anyone can tell me what type of suburb/city MPGs you get when you have the AC on pretty constantly. Maybe if 14-15 MPG is normal for AC running, I've found my problem. I know better than to bet on that though!

Thanks and have a good weekend!

Whenever you put it in defrost mode, the A/C automatically comes on, no matter which climate control you have. On some auto ones, though, you can disable the A/c with the econ button, but since you have manual, you just have to live with it. I also think the A/C comes on when you are in mixed mode of feet/defrost. For some reason, GM has done this on most of their vehicles in recent times.

From what I've read it is to help remove excess moisture from the air inside your vehicle. The humidity is a lot higher in the summer months and running the AC helps to reduce that and make the defrost system work more efficiently.

Ahah, thanks guys, I'd no idea it did that by design. So the hunt continues.

Next I plan to have the plug 'boots' changed and hopefully find a shop that can hook'er up to the right diagnostic equipment. That won't be for a few months though, as I'm having a few teeth crowned. Last place I went was to an Olds dealer/service dept. and they couldn't do anything for me without a check engine code.

Even if you have it set to ECON mode, the compressor will still kick in when you turn the defrost on, the ECON light will stay on, but the compressor WILL kick in. If you dont believe me, try it with the door open and listen as the compressor kicks in. As was said its to help to remove the moisture from the air, otherwise the defrost wont work as it should.