Most Likely A Dumb Question

[gogmorgo]

:dunno: This morning I opened my door, reached in and started the truck without actually getting in, unplugged the block heater and brushed the snow off. I'd left the vents on heat (the floor) last night, and even though I started out with a clear windshield, it had frost on the inside by the time I got back in, although not much. It doesn't much matter what the dew point is of the warm air, as it will cool down almost instantly when it hits the cold windshield, and deposit some of the humidity.

 

 

At any rate, it doesn't much matter what's going on at 30°F. As I said above, if it's that cold, your a/c compressor won't be running anyhow. It's simply too cold for it to function safely.

 But say you're in a hot, humid, environment, like 95°F and 95% humidity, and it starts raining. The rain cools your windshield, so the warm humid air inside the cab fogs up the windows. The air coming in is still roughly the same temperature and humidity, and won't really do much to clear the windows, if anything at all. And you're not going to be warming it up, cause it's already 95°. But if you have the a/c compressor running, it will dehumidify the air which will definitely help clear off that windshield. And this, in my opinion, is the only reason I can see that the a/c would run in the defrost. Anything else is just a happy coincidence.

[Eagle]

At any rate, it doesn't much matter what's going on at 30°F. As I said above, if it's that cold, your a/c compressor won't be running anyhow. It's simply too cold for it to function safely.

 

Sounds great in theory, but the fact is -- the a/c DOES cycle when wnen it's cold. I don't know which of the two reasons being discussed is the reason the system was designed this way, or if both reasons apply. The point is -- the OP asked why his system was malfunctioning, and the answer is that his system is functioning as designed. If y'all want to argue about why the engineers made it that way, have at it.

 

From the 1988 MJ FSM:

 

In addition to the above functions, the air conditioning system provides an evaporator for cooling and dehumidification of incoming outside air prior to blending with the heated air and is thus referred to as a "reheat" system. The evaporator is in operation during the A/C mode and also in the defrost mode for defogging purposes.

[Oyaji]

Thanks, Eagle.

 

No manufacturer includes items or function without good reason. Cost is so much a factor in design and manufacturing that every single nut, bolt, washer, weld, and dab of sealer has a purpose, and wouldn't be there if it didn't. You can be sure that the A/C running with the defroster is no exception.

[gogmorgo]

Yes, the a/c compressor cycles in the defrost. But it DOES NOT cycle below a certain point. Mostly it's just pointless to do so, as your a/c system won't really be able to make it any colder than outside temperature, and you're wasting fuel, and as pointed out above you're not really doing much dehumidifying. Below a certain point, however, it becomes dangerous to run the compressor, so it won't cycle. With decreasing temperature the refrigerant pressure decreases and eventually becomes liquid, and your compressor isn't designed to handle that. Is there an actual temperature switch? I'll admit, I don't actually know. It's probably the low-pressure safety shutoff.

 

Some GM owners' manuals have a temperature given. This is from the owner's manual for a 2004 Impala.

The right knob can also be used to select defog or defrost mode.
[...] It should be noted, that the air conditioning compressor will be engaged unless the outside temperature is 40° F (4°C) or below.

 

I realize that this is a r-134a system, not r-12, and many years newer, but that doesn't really make all that much difference. R-12 boils at -22°F. R-134a boils at -15°F. Basically an insignificant difference. Pressure will be far too low for operation of either system long before that point. Older compressors will also shut off. This is also normal operation. Perhaps if the a/c was overloaded with freon, it would work down to a lower temperature. But I'm not going to be the one finding out, as the risk far outweighs any possible advantages to running the a/c at lower temps.

 

Sorry OP for the  :hijack: .