BACKYARD MECHANI:C: Slow A/C is normal, growling pulley is not

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This article was published 15/07/2011 (5231 days ago), so information in it may no longer be current.

QUESTION: The air conditioning on my Chevrolet Cavalier takes a long time before it starts to blow cold air. This only happens if the car has been sitting overnight or longer. Once the system has been turned on for a few minutes the air does blow cold enough, but it just takes a long time to get there. Also recently there has been a growling noise coming from the air conditioning compressor. The noise is worst when the air conditioning is off and quiets down when I turn it on. Is the compressor causing all my problems and do I need a new one?

ANSWER: The air conditioning compressor is what is causing your concern, but the delay in blowing cold air is a normal condition of this type of compressor. The Cavalier uses a variable displacement compressor or pump to move the refrigerant through the air conditioning system. The variable displacement compressor reduces loads on the engine to give better fuel economy but it does take a couple minutes to start working properly after the vehicle has been sitting.

To enable the compressor to deliver a variable output of refrigerant, a plate inside the compressor changes the length of stroke of the pumping pistons. A pressure control valve and refrigerant pressures inside the compressor are what change the angle of the plate as the compressor runs. When the engine is shut off, the liquid refrigerant collects in the lowest part of the system, which is the compressor, and prevents the plate from changing its angle.

The compressor operates at its minimum capacity until the liquid refrigerant inside the compressor has had a chance to bleed into the system through a small orifice. Then the cold air really starts to blow!

The growling noise you hear at the compressor is most likely a bad pulley bearing. When the air conditioning is turned off, the pulley is still rotating and is supported by a bearing inside the pulley. After the compressor engages, the load on this bearing changes because now bearings inside the compressor are holding some of the load.

Changing the pulley bearing is a common repair and most shops have the equipment to do it. The system does not usually have to be discharged and refilled but special pullers are needed to remove and install the pulley. If you try to use anything other than the proper tools, you will likely bend the shaft on the compressor and then a new compressor is required.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists Association of Canada.

kerr.jim@sasktel.net