This is in my '88 Camaro.

When I push the "dial" all the way to the right (with the defrost symbol), and then put the temp selector all the way to hot (it actually won't go all of the way to the right for some reason) in order to defrost my windshield in the morning, my A/C compressor turns on. The air will still be hot, but obviously the A/C doesn't need to be on for that to happen.

Also, if I turn the dial to "heat", even with the fan all the way up, barely any air comes out of the vents (a very slow trickle, at best). So, whenever I want heat, I have to turn the dial to vent with the temperature as close to hot as it will get. It works fine like that, I just wonder what the difference between vent and heat are...

Thanks

 

The compressor is supposed to run while the defroster is on. That's how the compressor stays lubed in the winter time.

 

Actually, it is designed that way because the AC strips incoming air of its moisture, thus increasing the effectiveness of the Defrost function.