Now that is getting cold I'm starting to need my defrost. But I noticed that my A/C compressor starts running when I put on the defrost even if I have it set all the way to the heat side. Is there a sensor somewhere and could be bad?

 

Seems to me--and it's been a long time since I messed with that business--that the above is normal. Someone else will no doubt chime in.

EDIT: Curious, I peeked at my shop manual. "When an air conditioning mode (max, norm, bi-level, defrost) is selected, electrical energy is supplied to the compressor clutch coil."

JamesC

 

Hmm ok. I just thought the lever had to be all the way to the cold side for the a/c to come on. Would that be good for the a/c to be running when its 20* out side? Also seems like a lot of wasted gas since the compressor would be running to cool the air just to get heated up by the heater core again.

 

The AC acts as a dehumidifier when the defrost is selected.....it's like that in all cars.
One thing to watch is, if you drive with you defrost on all the time in the winter you are killing your gas milage because of the compressor being engaged.

 

it keeps the windows from fogging up by removing moisture from the air.

 

The only thing the hot/cold slider does it open/shut the door to the heater core, and maybe open/close the heater diverter valve with a vacuum on cars that have it (don't know if the hot/cold slider sends the vacuum out or if the heat selection sends it out).

 

Ok thanks, I just never realized that it worked that way. My a/c didn't work last winter so I never heard it come on then. Still defogged the windows just fine. Thanks though!

 

It will make sense when it gets colder and you will see them defrost 1/3 time faster.

 

What you are seeing is normal and proper. Just about every car that has air conditioning will turn the compressor on when defrost or defog (defog = defrost + floor heat.....not applicable in our cars) is turned on. This is to speed up the dehumidifying of the windshield.

I've had cars that never had air conditioning. In this instance, you were relying on heated air to lower the windshield humidity enough that the fog would disappear. It worked good enough, but not great.

But with A/C being added at the same time, the removal of windshield fog is way wayyyyy quicker. Granted, this does depend on outdoor temperatures. If it's 45°F outside in the morning, an A/C equipped car will defrost the windshield much faster. If you're in 20°F (or colder) weather, it won't make a lick of difference.

Why is this? Because the compressor in our cars shuts off at around 26 psi. This works out to aprox 40°F. So anytime the outside weather is below 40°, you can select DEFROST on the dash and the compressor won't run. The air conditioning refrigerant will be below 26 psi and as such not turn the compressor on. You could set it on MAX A/C or NORM A/C and see the same result. The compressor would stay off.

There isn't much fuel being wasted by running the compressor when the Defrost is turned on. That's because the compressor is going to run very sparingly during wintertime. It's simply too easy for it to reach it's low pressure cut off point. (26 psi) It's not like in the summertime when the compressor may run continuously in city traffic and cycle once every couple of minutes on the highway.....During wintertime, the compressor may only run for 5 seconds and then shut off for 20 seconds.


As for running the car with the A/C and moving the cold-hot slider away from cold? We do that all the time in the humid south. During the fall and spring, it's often moderate temps, but utterly humid. Something like 74° and 92% humidity (especially at night time). In this case, you need the dehumidifying of MAX A/C but you don't need the cooling. So you move the cold-hot slider towards the middle. The A/C coil is still running ultra cold and dehumidifying like crazy since it's still set on MAX A/C. But the duct temps blowing into the car are comfortable instead of frigid.

 

Yeah it's 100% normal. I unplug my compressor in the winter to save compressor clutch life. It doesnt get much under 40 here ever so I live with slow defrost times.