if it's 15 degrees outside will the a/c compressor still turn on when Defrost mode and A/C is selected?

 

mine is cycling on for a few seconds, then turns off for an extended period. is that due to the really cold outside temps?

 

i can already here the comments about a/c being used in the winter and months away from seeing 60 degrees, yes I know this sounds funny.

What car is it on? If it's on a Legacy where the AC normally comes on with defrost, then yes the AC compressor will likely still come on even in cold temps, even if just for a few moments.

What car is it on? If it's on a Legacy where the AC normally comes on with defrost, then yes the AC compressor will likely still come on even in cold temps, even if just for a few moments.

forester. it's cycling...on....off....on...off, i figure it's normal. seemed to be cycling off a lot more when first started than when warm - maybe it's temp dependent.

 

GG

Yeah it can depending on the level of charge in the system. But it will act like the charge is low because the pressure is different due to the colder temp.

It always comes on with the defrost - it dry's the air and makes the defroster more effective. Also keeps the compressor from sitting idle all winter.

 

GD

It always comes on with the defrost - it dry's the air and makes the defroster more effective. Also keeps the compressor from sitting idle all winter.

 

GD

right - i'm curious as to the cycling of the compressor, not that it comes on. title of the topic is a bit misleading.

 

is that pressure or temperature controlled and would it cycle more in winter?

The cycling will likely be based primarily on temperature. You've got the temperature probe in the evaporator that monitors the temperature leaving the evaporator. Since the inlet temp is low, the control unit will switch the compressor off to keep it from icing up.

 

As a test, if you move the selector to recirc, the inlet air should be warmer, and the compressor will likely have a longer running cycle. However, that's probably not the best scenario for defrosting though.

Do make sure that BOTH radiator fans are running when the compressor comes on. If only one or no fans come on, it can cause short cycling.

 

I suppose you could find someone to throw a pressure gauge on the AC to check the state of charge or look for bubbles in the sight glass... (does it have a sight glass? We've been mercifully free of AC problems with our Subies....)

no sight glass on these i don't believe. i have a set of gauges so i could do that. i was kind of curious though about how temp's affect it cycling.

It may be coming on even when the air is dry and very very very cold to lubricate itself, this is common to a lot of AC systems.

 

Otherwise it does come on with the defrost.

Lower ambient temps will cause the compressor to short-cycle because the condenser core essentially becomes more efficient. The compressor's cycle is controlled by both temp and pressure. Since you're not getting the same pressure drop across the evap core, the compressor doesn't have to run as much.

 

Short version, this is normal behavior. As ambient temps come up, the compressor's duty cycle will get closer to 50%.