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How Do Air Conditioning Systems Work in a Car?

Dec 28, 2020 ·

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When asked how car air conditioning systems work, most people would respond, “I don’t care how they work, I just want them to work!” While that may be a common response, it does not answer the question. If you are someone who has always been curious about how car AC works, continue reading below for a deeper look into air-conditioner theory, system components and system operation.

How Does AC Work in a Car?

The air-conditioning system in a car works by manipulating refrigerant between a liquid and a gaseous state. As the refrigerant changes states, it absorbs heat and humidity from the vehicle and allows the system to give off cool, dry air.

To change the refrigerant between a liquid and a gaseous state, the air-conditioning system works to control pressure and temperature.

AC System Refrigerants

In the past, automotive air condition systems used R-12 as the refrigerant. R-12 (aka Freon) is a very effective CFC-based (chlorofluorocarbon) refrigerant that is not flammable and not poisonous to humans. During the late 1980s, scientists discovered that widespread usage of R-12 was damaging the earth’s ozone layer.

Manufacturers transitioned to R-134a in the mid-1990s. R-134a is an HFC-based (hydrofluorocarbon) refrigerant that does not have the ozone destroying properties of R-12/Freon. The newest refrigerant is R-1234yf, which produces fewer greenhouse gases. Europe requires the use of R-1234yf, and it will likely be the new standard in the United States in the near future.

Car AC System Parts

Automotive air-conditioning systems have a few key components. Listed below are the main parts and their roles:


  • Power unit of the system that separates the low-pressure side from the high-pressure side
  • Takes in low-pressure gas and compresses it into high-temperature/high-pressure gas
  • Mounted to front of engine and driven by serpentine belt


  • Reduces temperature of refrigerant while it maintains high pressure
  • Refrigerant changes from gaseous state to liquid state as it cools
  • Similar to the engine radiator, it uses forced air (fan or vehicle movement) to transfer heat
  • Mounted in front of vehicle, behind grill


  • Removes water from the refrigerant using a desiccant (drying agent)
  • Has some system-filtering properties
  • Mounted on high-pressure side of system, between condenser and metering device

Metering Device

  • Either expansion valve or fixed orifice tube
  • Lowers refrigerant pressure, which quickly drops refrigerant temperature
  • Refrigerant is still in liquid form after leaving metering device
  • Mounted on high-pressure side of system, between dryer and firewall


  • Refrigerant changes back to gaseous state in the evaporator, causing a cooling effect
  • Cabin air is cooled and dried as it blows across the evaporator
  • Only component mounted inside passenger compartment, behind dashboard

Path and characteristics of refrigerant

  1. Low-temperature/low-pressure refrigerant enters the compressor (gas)
  2. High-temperature/high-pressure refrigerant leaves the compressor (gas)
  3. Refrigerant cools and converts to liquid in the condenser
    • Still under high pressure
  4. Receiver/dryer removes water from refrigerant
  5. Expansion valve reduces refrigerant pressure
  6. Refrigerant converts back to gaseous state in the evaporator
    • Absorbs heat; when air blows across evaporator, it is cool and dry

AC System Diagram

Below is an AC system diagram that shows the main components used and how they’re connected:

ac system diagram

Common AC System Failures

Because automotive air-conditioning systems operate under pressure, they need to remain completely sealed from the surrounding environment. Anything that allows refrigerant to escape or contaminants to enter can cause a failure.

If a leak is present in any component, simply recharging the system with new refrigerant will be at best a temporary fix. Identify the leak, replace the faulty component, and then evacuate and recharge the system.

Not only will a system leak stop the AC system from cooling, it can damage the compressor. The compressor can overheat and damage itself by trying to run with too little refrigerant in the system. Compressors generally are not serviceable and are an expensive item to replace.

To operate correctly, the AC condenser needs a steady flow of air through its fins. Road debris and dirt may reduce airflow, causing system malfunction. The condenser is mounted directly behind the vehicle grill, leaving it somewhat exposed and at risk for partial blockage.

What Does This Mean for Your Career?

Almost every new car sold in the U.S. is equipped with an air conditioning system. What was once considered a luxury is now an expectation. Automotive AC technicians diagnose and repair AC systems on a regular basis at dealerships and independent shops. Technicians often hear customers complain about warm air, loud noises, and even water leaking inside the car!

Train to Become an Automotive Technician

Does an automotive technician career sound like the right fit for you? Students at Universal Technical Institute(UTI) learn about every system of a vehicle, including auto air conditioning.1 In less than a year, you could have the skills and knowledge to purse this career.7 To learn more, visit our program page and request information to get in touch with an Admissions Representative today.

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1) UTI is an educational institution and cannot guarantee employment or salary.

2) For program outcome information and other disclosures, visit

7) Some programs may require longer than one year to complete.

Universal Technical Institute of Illinois, Inc. is approved by the Division of Private Business and Vocational Schools of the Illinois Board of Higher Education.


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