Testing automotive AC

Lets talk about testing automotive AC. Although it is still winter our thoughts will soon be thinking of spring and summer. It is a common condition that after a long winter season of not using your automotive air conditioning it may not seem to operate correctly when you do turn it on that first warm day.

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ac test guages


Testing automotive AC can be tackled in the driveway by a DIY car mechanic with some experience wearing comfortable safety glasses. Performance testing the system is a good place to start.

Performance testing provides a measurement of air-conditioning system operation and efficiency. It is recommended that a manifold pressure gauge be used to determine both the high and low pressures in the automotive AC system.

These manifold gauge sets have really come down in price over the last few years. Although professional grade sets can cost well over $100 a good DIY set can be had for fewer than 50.

This could be considered a good investment that will return big rewards over time on several different vehicles.


Automotive AC pressure readings

The desired pressure readings will vary according to the temperature during the time of testing.
Professional auto service manuals will include a temperature pressure chart as a guide to determine the proper pressure for your vehicle under the conditions the test is being performed.

You can use a basic rule of thumb that the low side readings should be around 35 psi. Also the high side should read anywhere from 150 to 200 psi. This is just a generalization and these readings will again be determined by the amount of Freon in the system and the outside temperature during testing.

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ac center duct

Another variable that will affect operating pressures would be humidity. On a more humid day operating pressures will be higher than readings obtained on a very dry day. If you are operating pressures both on the high and low side are found to be within the normal range then the Freon or refrigeration portion of the air-conditioning system is most likely functioning correctly.

This can be further confirmed with a check of the output duct temperature. You can use a thermometer inserted in the center duct to determine the discharge temperature into the passenger compartment. They do make specialized thermometers that will clip onto the vent grille and provide an accurate reading.


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AC air flow

Output temperature again will be determined by outside temperature and humidity. A good rule of thumb to use is the ac duct temperature should be anywhere between 40 to 50°. Here in Florida an air-conditioning system is seldom found to have an output temperature of less than 50°on a hot humid day.

A good tip for obtaining the most real world temperature output. For the best results place a high speed fan in front of the radiator grille to simulate road operation and provide sufficient airflow across the condenser. For the most accurate reading place your thermometer in the center duct and reduce your blower setting to either low or low 2. Placing the blower on a higher setting actually rushes the air through the evaporator and can increase output temperatures.

Lots of automotive information is available here. See the latest blog auto repair posts.


One comment

  1. AC-Expansion Valve. I was putting Freon in my ac, which was working somewhat. After my second can the Expansion valve started cycling out fluids( a mixture of oil and freon. What is the Expansion valve and how do it function. Did I overfill the unit, or did I blow the expansion Valve?

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