i noticed on my car that when the Hvac controls are on the last option (window defrost) it makes the engine run really different then anny of the other controls. is there any reason for this? i thought it was kind of odd...

 

You may have a vacuum leak that goes away when in defrost. Or, it may be the AC compressor kicking in in defrost mode.

 

Fivekid.............

"...compressor kicking in in defrost mode..."

Mine does that! What the heck causes that?

 

the a/c compressor is turning on causing a strain on the motor for a few seconds,. thats why the idle goes down for a few second then should jump back to normal. the a/c compressor turns on when you turn on defrost because the cold air defogs the window faster than warm air.

if im wrong on that stuff someone correct me, but i believe it to be right

 

the AC should turn on to remove moisture from the air.

 

Well, the compressor on GM cars of this vintage definitely kicks on in defrost mode. But it isn't because cold air defrosts better--it defrosts WORSE, usually. It's because the A/C also has the side effect of dehumidifying the air.

On a VW we have you have to manually turn the A/C on in defrost mode. Surprise, the best defrost is usually with the temp lever centered (if it's warm out, I can't take hot air blowing out the vents at me when it's 85* and rainy) or full hot (if it's cold out) and the A/C switch on.

 

That's funny; I thought the A/C clutch cycled on to keep the compressor "alive" during the cold months. My a/c's been pulled out for years, the windows still defrost fine. Never gave it a second thought!

Er; wait a min. When the heater door is swung to "hot", air doesn't go over the evaporator coil. Does it?