Defrost without A/C??
 

Just purchased an '08 Trailblazer, and the owner's manual states that when the defrost mode is selected for the windshield that the a/c compressor automatically kicks on when the outside air is above 40 degrees.
(Dual climate control system).
This is a huge problem. Here in FL, we have 100% humidity and temps near 80 degrees in the summer before the sun comes up. Having a windshield cooler than the outside air, precipitates fog on the outside of the glass resulting in a windshield you literally can't see through. I crank both temp knobs to 90 degrees but air coming through the vents is still cooler than the outside air. I need HOT air on the windshield to warm it- not cool it. Currently I need to open all the windows and sit for 10-15 minutes so the windshield glass temperature equalizes and the fog on the outside of it no longer forms.

Is there a way to override the a/c compressor kicking on whenever the defrost mode is used? This is a HUGE problem and design issue IMO.
Thanks in advance for any suggestions.

I see the issue you are dealing with. My truck has manual AC controls, so I can run the blower in the defrost (windshield) position with the AC compressor turned off.

That said, I tend to be hot natured, and crank the AC most of the year. It's very humid and hot much of the time here in Alabama as well. If my windshield fogs up in summer on the outside (and it does), I turn on my windshield wipers on a slow interval to wipe the fog away, and proceed with my driving. I sure wouldn't sit for 10-15 minutes. Fog on the inside would be a different story, but fog on the outside - can't your wipers take care of it?

+1 Defrost is for inside, windshield wipes for outside. And the a/c running helps de-humidify insode air. You could also try a "no fog" product like RainX to decrease condensation build up

It's odd...
Passenger side, wiper mostly clears it away (though it reappears in a few seconds so they need to be left on). Driver side, where I actually need to see through- it's near impossible, Like I said, very odd. Never seen anything like it. I'll see if I can get a pic, passenger side clear, driver side completely opaque. Even dry paper towels won't remove it. I'd swear there was something "on" that side of the windshield, but no evidence of it.
Even put on a fancy Rain-X Latifude Water-Repellency wiper which I hoped would help as it's supposed to leave a coating on the glass- no difference.

I have the Rain-X products- including the Anti-Fog spray which they state is for the interior only. What could be the problem with using it on the exterior of the windshield, other than it would probably get removed in short order by the wipers?

I'm about to order an inexpensive little 12v heater to place on the dash to warm the windshield...

Well, any car I've ever owned, even with fancy climate control systems, would let you set the defroster to HOT versus cold. I'll assume you've tried that too, and still have this issue, or does it just take time for the engine to warm up and produce heat? I would try that before putting a space heater on the dash...

It does seem like something is on the glass on your driver's side, if the passenger side clears fine with the wiper. Maybe you just need to grab Windex or isopropyl alcohol and take it down to bare glass on both sides, and take it from there.

I have experienced the same condition in the Midwest and in the Mid-Atlantic regions of the USA as well.

Easiest way I found to deal with the exterior fogging is to first clean the exterior side of the glass with a regular window cleaner, then clean the exterior surfaces with vinegar, clean again with regular glass cleaner, and then a final cleaning with isopropyl alcohol. Finally, apply Rain-X to the cleaned exterior glass surfaces.

If that does not work or you really want to clean the windshield to as close to being nothing on the surface as is humanly possible, clean it using Bon-Ami, followed by lots of water, then glass cleaner and then Rain-X.

Good Luck!

I can max out the temp (90), but as I stated the a/c compressor operates above 40 degrees outside temp and cannot be disabled. Could possibly be an issue with a blend door (I don't know the Chevy system yet, just bought it), but the air blowing out the defroster vents is cold. Not ice cold- but not even warm which is what is needed.

I'll had used a vinegar/water/little bit of alcohol spray and wipe down (found it online) but I'll try the different method mentioned here followed by the Rain-X anti-fog. Ordered a dash heater as a backup if all else fails. Shouldn't be too much more of this 100% humidity in the am.

Even with a/c running (as it's designed to do) you should be geting hot air quickly at the defrost outlets

^^^ Then it must be a blend door problem.
Anyone know if there's a service manual for this vehicle online in PDF format?

All the info should be buried in here somewhere:

https://charm.li/Chevy%20Truck/2008/...4WD%20V8-5.3L/

I think they are calling it either the mode actuator or the air temperature actuator:

https://charm.li/Chevy%20Truck/2008/...20%2F%20Motor/