2007 3.0R Subaru Outback - the hot & cold climate air work OK. I've checked the clutch gap which is within the tolerance specified by the factory service manual.

Often when climate temperature set low and idling at traffic lights we hear the A/C clutch clicking every three or four seconds. Is it normal for the compressor clutch to pull in and out so often ?

2 Answers 2


This can be a sign you are getting low on refrigerant.

There may be a pressure sensor on the low pressure side of the system that is detecting that the pressure is dropping too low. This turns off the compressor. When the pressure raises again, the compressor Is turned back on and the cycle repeats.

  • Thanks I've isolated the 3 second cycling down to asking the A/C for it's coldest air with the fans on low and windows closed. This is whilst idling at traffic lights in ambient weather of 25C/77F and no problems with engine idle. Would you expect the compressor to alternate longer periods in this case ?
    – ajayel
    May 21, 2020 at 20:58
  • 1
    3 seconds sounds like a very short on time, if it were simply due to demand needs. My guess is that you are low on refrigerant and that the low pressure side is starting to pull a vacuum. That is just my thoughts based on the information available. Best to get it checked by someone with the equipment.
    – HandyHowie
    May 21, 2020 at 21:26

The compressor is engaged or not according to the load or demand on the system, much like a hot water boiler in a house.

So if the ecu decides the compressor needs to work it will engage it, but the ecu may disengage the compressor for other reasons like high engine power demand.

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