Auto Repair Q&A



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AC Performance Testing


The most basic of air conditioning system tests is the AC performance test. This is fairly easy to do and anyone can conduct this test, with or without certification, in most areas. It does not require breaching the AC system or any complex tools beyond a thermometer.

If you plan to do an AC system performance test, you should acquire an automotive A/C thermometer specific to the task. Regular kitchen thermometers and the like will not likely be calibrated or shaped correctly to work with your car's system. That said, any thermometer capable of reading temperatures from about 40 degrees Fahrenheit to about 150 or so. Most thermometers will read from 0 to 200.

The test is very easy. Begin by letting your thermometer adjust to the interior temperature of the car when it's not running. This is your baseline or "ambient temp." Just place the thermometer into the center-most vent and let it sit long enough to stabilize at whatever the temp in the car might be.

Now start the car and turn on the A/C. Set it to the lowest temperature option and turn off recirculation (open vents). Within two or three minutes, the temperature should bottom out. Compared to the baseline reading, this new reading should be 30-40 degrees (F) cooler. The actual reading may vary, depending on humidity and altitude, but inside that range is the best-case, "working properly" point for your car.

This simple performance test can go a long way towards figuring out whether your AC system is malfunctioning or if the problem is really just the time of year. Areas under high humidity will have less performance from the AC system than will dry, high-altitude areas. If you live in the deep south on the Gulf Coast, for example, you can expect your AC to only drop temperatures by 20-30 degrees during the most humid times of year. If you live in the high deserts of Wyoming and Montana, though, you can expect the temperature drop to be near 50 degrees or more. So adjust your expectations and assessment accordingly.