The case of the strange-acting A/C controls

Wheels: Ron writes by e-mail: "I am having two problems with my wife's 2009 Chevrolet Malibu that has approximately 43,000 miles on it. I have made certain to change the oil and filter every 5,000 miles with 5W/30 oil and filter. I made sure to rotate the tires every 5,000 miles, front end alignment and all, twice a year – in other words, everything the owner's manual says to do.

The first problem is on the A/C controls. We will turn the A/C on and within a few minutes it flips over to outside air. We have to hold the A/C button for about five minutes before it stays on A/C. Even doing this, it still flips back to outside air, and then back to A/C. Only when the car has been on and driven for 20 minutes or longer will it stay on A/C.

Halderman: This sounds as if the A/C system is low on charge. Once the temperature increases, the pressure in the system increases enough for the low pressure switch to close, thereby allowing the compressor to work. The A/C system uses a low pressure switch that will only allow the compressor to operate when there is enough refrigerant in the system to move the oil through the system. Refrigerant oil (R134a system) does not mix with the refrigerant but instead uses the movement of the refrigerant through the system to lubricate the compressor.

Have an automotive-related question? Write to Jim at jim@jameshalderman.com

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