90 GMC Defroster

Defrost/defog depends largely on the air conditioning system to remove moisture from the air. The compressor should be cycling while you are using the defrost setting regardless of the outside temp.

Hey Bob My 02 S-10 will defrost no matter what the temp. Good luck Pete

Check to make sure you don't have the air flow on recirculate. You need outside air to do a good job keeping the windows clear.

Thanks to all who responded. I thought my friend might be wrong. I'll have the system checked out.

Bob

Another poster mentioned the need for outside air which is very true also. My 03 Trailblazer runs the compressor when I have the heat set to recirculate. This is to remove moisture in the air to help prevent window fogging.

This is a common view, becuase houses don't work that way, but in a car the defrost will force the AC on so the condenser will dehumdify the air (as was already said). Now, if you've got a '90 GMC, it's possible that sometime like the AC condenser has gone out, because thats what the air-flow system uses to draw in outside air. A new condenser isn't all that big of a project to do.

I don't know where you live, but where I grew up in Labrador, and here in Ontario, fresh outside air is the best, most reliable way to help keep the window clear. I also wonder how the preferred solution running the A/C, and taking the hit on gas consumption.

When I grew up not many cars had AC, but they ALL defrosted the windows. Outside air, and heat. If you have to, roll a window down a tad. Roll, no, push the button:)

Running the AC to dehumidify the air is a big help, but not the only way.

Al

I had many beaters without a/c in my youth. Yes they would defrost but to defog was a pain in the ass when it was raining and you had to keep the windows up. Like I said in more modern vehicles with a/c defrost/defog depends for the most part on a/c removing moisture from the air before it hits the windows.

I always wondered why that is, because i had an 88 nissan sentra, and as you can immagine it had no A/C. the wife at the time had a honda civic with a/c, and he windows would always be fogged... and mind never would... same thing in my 89 ford f150, no fog on those windows... .(no a/c), and the same thing in my 85 crown vic (had a/c but did not work) no fog...

i don't buy the a/c thing.

Windows haven't been foggy for the 50 years previous when a/c was not a standard option.

The storyline is crap.

Compressors are cycled in "modern" automotive machines during winter to keep them from sitting unused through so many months of the year. The idea is that cheaper compressors are more apt to fail within the warranty period if they're not spun for 9 or 10 months a year.

Even at that they don't seem to outlast the warranty by a whole lot. They usually start leaking at the shaft seal. And how many today are actually shop repairable?

The other story about "drying out the air" is so much crap, just another urban legend that's been bought into many.

Heck, I remember cars, trucks, and buses that didn't even have a heater, just a