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How to Test an Auto AC Compressor

by Aaron Ratliff

An air-conditioning compressor is a key component in a vehicle equipped with an AC controlled climate system. It is responsible for compressing the refrigerant in the system so it can be cooled and circulated into the evaporator. Air then passes over the evaporator before traveling though your vehicle's air ducts and into the passenger area. If you suspect the air-conditioning system in your vehicle is not functioning properly, there are several things to test that will determine if the compressor is at fault.

Check the Compressor's Belt and Pulley

Open the hood of your vehicle, and locate the air compressor. In most cars the air compressor is located near the upper left section of the engine compartment.

Check the belt connected to the compressor for any tears, rips or notches on both the inner and outer sides of the belt. Also, check to ensure that the belt is not overly smooth or shiny on the inside, as this is an indicator of a belt that has been stretched out. If any of the above symptoms can be seen, the belt should be replaced before testing any other components.

Move the air conditioner selector knob in the climate control section of your car to the off position.

Start your vehicle's engine. With the hood still open, turn the air-conditioner selector knob to the on position.

Listen for any unusual noises such as a screech or groan. Both of these are symptoms that your air compressor belt or pulley needs to be replaced.

Test the Compressor's Clutches

Open the hood of your vehicle, and locate the air compressor. It is typically located in the upper left hand side of the engine compartment.

Turn the air conditioner selector knob in your vehicle to the off position, and then start the engine.

Connect the voltage tester to the negative connection on your vehicle's battery.

Locate the wires coming off of the ac compressor, and unplug them. All of the wires should be connected with one wire feed. Place the voltage tester on each of the three connections individually. No activity should be noted on the voltage meter.

Turn the air-conditioner selector knob to the on position. Place the voltage tester on each of the three connections. The middle connection should produce activity on the voltage tester, indicating the connection is "hot." If no activity is noted, the air compressor clutch and wire should be replaced.


  • Exercise extreme caution when checking the belts and electrical connections of an air compressor, as the moving parts of an engine can cause injury.

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About the Author

Based in Asheville, N.C., Aaron Ratliff started writing as a journalist for his hometown radio station in 1997. He is currently a North Carolina licensed Emergency Medical Technician and a certified personal trainer. Ratliff is pursuing a Bachelor of Science in health promotion at Appalachian State University.

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