How to Recharge an Auto AC

by Lee Sallings

Recharging an auto air conditioning system is a fairly simple task. However, it does require some understanding of the system. The common charging hoses available at auto parts stores do not provide all the needed information. To successfully recharge the system, a gauge set is needed. A gauge set provides the information needed to recharge the system as well as diagnose problems.

Attach the A/C gauge set to the service ports. The blue hose is the low-pressure side and is attached to the service port located on the accumulator (a cylindrical aluminum part attached near the firewall) or low side line between the compressor and accumulator. The red hose is the high pressure side and is attached to the service port located on the high side line (the smaller of the two lines on the back of the compressor) between the condenser and fire wall. Make sure both valves on the gauge set are closed and then connect the hoses. The remaining yellow hose is for adding refrigerant or pulling vacuum on a system that has been opened.

Start the engine and set the A/C controls to medium blower, max A/C and recirculating air. The gauge readings on a fully charged system will be 35 psi to 45 psi on the low side and 200 psi to 350 psi on the high side. This is temperature related, with the lower readings occurring on a cold day and the higher readings occurring on a warm day. If the readings are lower than this, add refrigerant.

Add refrigerant to the system by installing a can tap to the yellow service hose and attaching a can of refrigerant to the tap. Screw the thumbscrew all the way in and all the way out to open the can. Then, with the engine running, open the blue valve on the gauge set to allow refrigerant to enter the system. Close the valve after half the can has been emptied and check the gauge readings. If the readings are still low, continue adding refrigerant until the system pressures show full on the gauges.

Turn off the engine and allow the gauge readings to stabilize before removing the hoses from the system. This will prevent damage to the high side service port. Replace any caps that were removed from the service ports and restart the engine. With a thermometer, check the temperature at the A/C vent. It should be at least 40 degrees cooler then the air outside. If not, you may have moisture or some other problem in the system.


  • check Use refrigerant that contains dye. This makes finding leaks easier.


  • close Never try to add refrigerant to the high pressure side. The pressure developed in that side is more than the can can handle and it could explode, causing injury.

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About the Author

Lee Sallings is a freelance writer from Fort Worth, Texas. Specializing in website content and design for the automobile enthusiast, he also has many years of experience in the auto repair industry. He has written Web content for eHow, and designed the website. He began his writing career developing and teaching automotive technical training programs.