So... I have been having issues with getting really bad gas mileage (like 15 or 16 mpg), since its winter here and 10 degrees out right now I have been using my heat a lot. More importantly I always have it on the defrost and floor cycle... like 24/7 while driving the past few months i have it either on low or high. Either way its always on once my car warms up.

Anyways... I read somewhere about the ac compressor coming on with the defrost cycles are selected. So i checked it out and indeed the ac compressor kicks on when i turn the dial to defrost mode. Why does the compressor come on? and has anyone else noticed this? I have a 01 Monte SS NA

 

The compressor works because it helps pull moisture from the air, hence defrost

 

Totally normal behavior for the defrost. It's to pull the moisture out of the air so yes the gas mileage will suffer some. Just filled up my 2001 with the 3.4 motor and got just over 20 mpg from mostly lighter freeway rush hour driving here in Minnesota so your mileage doesn't seem that alarming.

 

I don't know the exact reason, but the compressor kicks in to assist with defogging the windows (not to defrost).
If I understand correctly, interior fog is a result of rapid high interior temp against a low exterior temp. By kicking the AC on for a short cycle it will chill the window a small bit to allow it to defog.

Others can chime in if I got some of that wrong, but that's how I understand it.

 

Haha, I just read two comments posted the same time as I. And Chibi is on the money (don't know why I forgot that, I've applied that concept on other situations non-car related).

 

Wow.. well it looks like I'm gonna have to stop using it on that mode and see if I see a change in gas millage. This is my 3rd monte 6th gen and I never realized that till now.. ugh.

 

How much has your fuel mileage dropped?
It's not just Montes, any vehicle with a functioning a/c system uses the compressor during the defrost cycle.

 

Well i bought the car in July and was getting like 280-300 miles a tank. In the past few months I was happy with getting 200. But that's only city driving and i let my car warm up for 10 min twice a day. Also i was starting my car for 10 mins on my work breaks (which im gonna stop doing). So probably a few mpg

 

Starting the car on your break is never a great idea. Since you aren't actually driving it, and it probably isn't getting fully up to temperature you are just creating condensation and wasting fuel.

 

I only run my defroster when I need too. I keep it on Bi Level in the winter time - my right foot gets too hot if I only have it coming out of the floor vents.