Here I was wondering why my car has been suffering bad MPG (barely 200 miles per fill) and stalling in the winter when my A/C wasn't even on. Found out yesterday that when you have the heater on (A/C off obviously) and turn to the windshield defroster setting, it actually activates your A/C.

No the A/C light does not turn on but you can see the gauge lights dim noticeably and starts to stall when going out of gear. Funny thing is its not even blowing cold air. Anyone notice this?

Also that is the only setting that stresses the electric system, b/c when i turn it up to the defroster/leg setting the lights brighten up and stalling goes away.

 

Yeah, I'm pretty sure Defrost spins the A/C compressor. To my knowledge there's no way around it. Stupid I know. Check your manual I believe it's mentioned there.

 

Pretty sure the defroster has to blow dry air so the windshield doesn't fog up. It does this by dehumidifying the air using the A/C.

 

The above poster is 100% correct.

 

i don't understand y your car is stalling when the a/c compressor turns on????

 

that explains my bad mileage also

 

I think its b/c of my cams. I have Comp 280's right now and it stalled in the summer whenever I turned my a/c on. The problem seemed to have gone away with driving a lot with a/c on so I guess the ecu adapted to it. I reset my ecu last week when i was installing grounding wires and now the problem is back.

 

oh ok, i guess thats sumtin i will keep in mind whenever i get to cams for my evo.

 

Wow you guys must be car noobs I would say about 100% of cars that were produced in 2000 to present once you turn on the front defrost it kicks the a/c compressor on. Why do you think mom and dad's old cars would always stay fogged up even with the front defroster on? I would tell you in the owners manuel to turn on the a/c and about 100% of people wouldn't turn it on. So the manufactures started to just make it an automatic feature.

 

I'm always amazed how few people understand that you need the A/C to de-fog the car because it dehumidifies it.

the worst one ever though was this girl who had a cobalt (or something like that) who always pushed the snowflake button on her dash whenever it was snowing. she thought it put the car in snow mode????? all she was doing was turning the A/C on and off.

 

Snow mode? haha

But for anyone who doesn't understand how this works try defogging a window with just blowing warm air at it (most likely in a 90s model or before car.) Then try it again with the air conditioner activated. The difference is night and day.