How to Test Your Car's AC Compressor

by Tony Ehrike
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If your car's air conditioner (AC) stops blowing cold air and appears to only blow air, it may be low on Freon -- or it may require a new AC compressor. Testing the AC compressor is a relatively simple task; you must simply follow a few simple steps to determine whether the compressor requires repair or replacement.

Step 1

Turn the vehicle on, shift it into in park (for automatic transmissions) and engage the emergency brake. Turn on the AC. Connect a pressure gauge to the low side of the AC compressor, typically found on the firewall passenger side in the engine compartment. Add approximately 2 oz. of Freon to determine whether the compressor kicks on (if it does, you will hear a loud click followed by a puttering sound).

Step 2

Discontinue adding any more Freon at this point. On a 75-degree day, normal pressure should be approximately 38 lbs. per square inch (psi) -- known as the "blue zone" on most gauges. If the compressor has not kicked on, check the fuses -- usually located in the glove box. Iif you have trouble locating them, refer to your owner's manual.

Step 3

Check the leads to the compressor, the magnetic clutch and operation. To do so, grab a lead from the batter to determine whether the clutch engages. Check the high- and low-pressure switch. Check the AC control on the dash. If you cannot get the compressor to kick on and cannot find the problem with troubleshooting, you may need to replace the AC compressor.

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