Frequently cycling AC compressor in 2009 Subaru WRX



The AC compressor clutch in my new WRX engages and disengages up to several times a minute (whether at idle or speed), even when it is warm outside and in the car. The AC air entering the car cools quickly (usually within a minute or so) to 40F, and then the cycling begins: about 10 seconds on (40F), then 5 secs off (warming towards 50), 10 secs on (cools to 40), 10 off … and on and on indefinitely. I’ve had it to two different dealers: both say pressures and functions are fine (one evacuated and refilled the system), and both say that this is “normal” for the AC "because it is so efficient it doesn’t need to run continuously.? Neither dealer had another new WRX to compare it to. If indeed this is normal, it is a very annoying design! If not, anybody have a suggestion for what it might be and how to get it fixed? Thank you!


Did either check the functions of the low-side and high side cut-off switchs? These switches are what cause the cycling, and if they are out of calibration, could be doing just this. The switches main function is to save the compressor from damage if the pressure gets too high on the high side, or too low on the low side. Out of calibration could mean they are turning the compressor on and off based on the wrong pressure tolerances. These switches are typically inexpensive, and can typically be replaced without evacuating the system again.


Thanks! Both dealers said “we checked it all out,” but obviously they may not have checked the switches (and these aren’t mentioned on the work report). Sounds like excellent advice and I will specifically request this when I take it back.


Mine does the same thing. It is quite annoying, I cant imagine that would be part of the design. Makes it interesting when you are cruising and then the air condition steels power every few seconds…


Indeed. Or when you’re coasting to a stop and can’t rely on a constant deceleration to predict where and when you’ll need to brake (for those of us who like to predict such things while driving!). Also, would seem likely to wear the compressor clutch out early (dealer says it won’t…) and it decreases cooling efficiency. I’ll keep bugging the dealer about it and will let you all know if I ever learn anything more definitive. I would certainly be interested to hear if others have noticed the same problem (or, probably more importantly, if others do NOT have the same problem).


“…warm outside…” is not a very precise descriptor. Please give temperature and approximate humidity conditions. Just for example, at 80 F and 20% RH, I would expect more cycling than at 102 F or 99% RH.


Regarding “warm outside,” it has been happening from 70-100 degrees (I haven’t tried it under 70) and generally high dewpoints (60-70; so, for example, 85 degrees and a relative humidity of 60%). But also does it at lower dewpoints.