How Do Vehicle A/C Systems Work?

A common air conditioning myth is that A/C systems create cool air to circulate throughout the vehicle’s cabin. Though the A/C system does use cold refrigerant to cool the air, the refrigeration cycle does not produce cold air, but rather changes the state of refrigerant from a gas to a liquid and back again. The refrigeration cycle is the process of transferring heat from one place to another — using a fluid such as refrigerant — to cool an area by drawing heat, causing condensation and cooling. There are two types of vehicle A/C systems, each composed of similar parts to cycle refrigerant throughout the system and cool the air that flows through your vehicle. The air conditioning system is made up of two distinct sides, which include high-pressure and low-pressure components.

There are two main types of automotive A/C systems: Thermal Expansion Valve systems (TEV, TXV, or TX) and Clutch-Cycling Orifice Tube systems (CCOT). Both of these are closed systems, meaning that refrigerant is not lost or gained within a vehicle, and they rely on a proper charge of refrigerant in a leak-free system to work properly. Automotive A/C systems are divided into high-pressure and low-pressure sides that change the state and temperature of the refrigerant as it moves through the A/C system. The three main parts of your vehicle’s A/C system are the compressor, condenser, and evaporator. These, a few other components, and their purposes are listed below by their position in the A/C system.

High-Pressure A/C System Components

Compressor

A/C compressors start the high-pressure side and begin the refrigeration cycle. Compressors are driven by the engine’s accessory drive belt and pressurize the low-pressure refrigerant gas, creating a hot, high-pressure gas that is propelled through the system.

Condenser

After the refrigerant gas is pressurized, it moves on in the cycle to the condenser. The condenser cools the refrigerant to change its state from a high-pressure gas to a high-pressure liquid. The condenser is similar to a radiator, and cools the refrigerant using a series of coils surrounded by heat-exchanging fins. Heat is dissipated as air passes through the grille of the car or from the cooling fans that move air across the fins. As the refrigerant gas cools, it condenses into a warm liquid.

Condenser Fan

In order to quickly cool the refrigerant in the condenser, or to keep the A/C system working at idle, a cooling fan pushes or pulls air to help dissipate the heat created at the fins of the condenser. Depending on the vehicle, you may have a specific condenser fan, or you might rely on the cooling fan for the radiator to draw air through both heat exchangers.

Receiver-Drier

Receiver-driers are exclusive to thermal expansion valve A/C systems. After the liquid has been cooled in the A/C condenser, the high-pressure and low-temperature refrigerant will move from the condenser to the receiver-drier. The receiver-drier filters debris and absorbs any moisture that may have gotten inside the A/C system at any point during this cycle using an internal desiccant material. Excessive moisture in your A/C system can cause corrosion and can clog the receiver-drier, which is why it should be replaced any time your A/C system is opened for service. This desiccant filter keeps your A/C system clean and stores excess refrigerant until it is needed at the expansion valve.

In-Line A/C Filter

In some clutch-cycling orifice tube A/C systems, refrigerant filtering is accomplished by an in-line A/C filter. However, in-line filters are only included on select vehicle models, so you may need to refer to your service manual to determine if your vehicle is equipped with an A/C filter that needs regular maintenance.

Expansion Valve

The expansion valve is the point in a thermal expansion valve A/C system that splits the high-pressure and low-pressure sides of the refrigeration cycle. This valve allows the high-pressure, warm liquid refrigerant to expand, resulting in a cold, low-pressure liquid. To prevent the evaporator from freezing, the expansion valve can open and close to vary the amount of refrigerant entering the evaporator based on its temperature.

Orifice Tube

The orifice tube takes the place of the expansion valve in clutch-cycling orifice tube systems. Like the expansion valve, it allows the warm, high-pressure liquid refrigerant to expand into a cold, low-pressure liquid before it reaches the evaporator. However, the orifice tube is unable to regulate the amount of refrigerant entering the evaporator, as it has no moving parts. The increased flow from the orifice tube causes warm liquid refrigerant to exit the evaporator instead of warm refrigerant vapor, and the accumulator on CCOT systems collects the liquid refrigerant until it vaporizes to prevent damage to the compressor and to regulate cycling.

Low-Pressure A/C System Components

Evaporator

The evaporator core is located in the cabin, and acts as the heat exchanger for the cold, low-pressure liquid refrigerant. The heat from the evaporator case, the compartment in which all of your heating and air components are stored, is absorbed by the cold refrigerant. This provides a source of cold air for the cabin as the blower motor blows air over the evaporator’s fins, and also heats the refrigerant to cause it to evaporate into a high-temperature gas before being sent to the compressor.

Accumulator

Accumulators replace the receiver-drier on vehicles with a CCOT system. If your vehicle has an orifice tube in its refrigeration system, it will also have an accumulator. Accumulators filter and store refrigerant, similar to the way a receiver-drier operates, but are larger to accommodate the unregulated flow of refrigerant through the orifice tube. The accumulator is located after the evaporator to stop the liquid refrigerant from entering the compressor, which also allows the refrigerant time to heat into a gas.

Blower Motor

The blower motor pushes air over the cold evaporator’s fins as the refrigerant absorbs heat from the climate control case under the dash. This creates cool air that blows through the vents and into the cabin of the vehicle, completing the A/C refrigeration cycle. Learn how to replace your blower motor or blower motor resistor in our helpful article to help you get the cool air you need into your car’s cabin.

High- and Low-Pressure A/C Hoses and Return Lines

A/C hoses and lines are both high-pressure and low-pressure components depending on where they are located in the A/C system. The refrigeration cycle relies on A/C lines and hoses to move refrigerant to and from each A/C component. In most A/C systems, there are three types of hoses: The suction line, the discharge hose, and the liquid line. The suction line pulls A/C vapor from the evaporator or accumulator (depending on the system type) to the compressor. The discharge hose then carries pressurized refrigerant vapor to the condenser, and the liquid line carries liquid refrigerant from the condenser to the expansion valve or evaporator. Each hose and return line is responsible for moving refrigerant in its different states throughout the refrigerant cycle, and the A/C fittings, O-rings, or hoses can suffer damage over time. If you are experiencing refrigerant loss, checking the condition of your A/C hoses is often the most obvious place to start looking for signs of refrigerant leaks or other problems.

Your vehicle’s A/C system relies on two types of fluid to run properly: refrigerant and refrigerant oil. Without refrigerant and oil, your A/C system is not able to build adequate pressure for cooling or remain properly lubricated. Refrigerant changes from a liquid to a gas throughout the system, and this produces cool, dry air by absorbing heat from the climate control system of the vehicle. Refrigerant oil is responsible for lubricating and sealing the moving parts in your A/C system, ensuring optimal performance and preventing the A/C compressor from suffering friction or damage that could lead to failure.

One of the most common causes of A/C system failure is low refrigerant levels. If your vehicle is too low on refrigerant, the system may not be able to build the pressure needed to appropriate temperature regulation, causing your vehicle’s A/C system to blow hot or lukewarm air. If the moving parts of your A/C system are not properly oiled, especially during extended periods running your air conditioner, components may begin to show signs of wear or inefficient cooling over time. Refrigerant oils are only compatible with certain types of refrigerants, and you should always ensure you’re using the right type of refrigerant and oil when performing A/C system service. If you’re not sure what type of refrigerant oil or refrigerant your A/C system needs, you can consult your vehicle’s owners manual, or visit O’Reilly Auto Parts and let our parts professionals help you find all the best air conditioning parts and chemicals for your vehicle.

Because the A/C system is closed and should remain sealed, low refrigerant levels or pressure often indicate a system leak. A/C system leaks can develop in any of the system components, hoses, or fittings. When it comes to refrigerant loss, a little goes a long way. A loss of just 10% of the refrigerant from your A/C system can be as harmful and potentially damaging as losing 40% of your vehicle’s motor oil. However, these leaks can be hard to identify, as refrigerant leaks rarely occur as a liquid and usually escape as a vapor. Often, drivers may notice water underneath their vehicle after running the air conditioner and think their A/C is leaking — but this is just condensation from the evaporator. Since A/C system leaks are hard to detect, using an A/C leak detector can help detect any escaping refrigerant vapors quickly. You can also detect refrigerant vapor with UV dye and a special light, or use a specialty refrigerant sniffer or detector to determine the source of the leak. It’s also important to remember that any A/C system leak should be repaired before recharging the A/C system to prevent refrigerant waste and ensure the overall health and performance of your air conditioner.

What Are The Types Of Refrigerant?

• Refrigerant R-134a is the refrigerant required for use in most modern vehicles. This replaced R-12 refrigerant used in older vehicles, and cars made before 1995 can often have their A/C systems retrofitted to use R-134a.
• Refrigerant HFO-1234yf or R-1234yf is the newest type of refrigerant formulation and was designed as a replacement for R-134a. To significantly reduce global warming potential, most newer vehicles are now equipped with R-1234yf systems.

What Type Of Refrigerant Oil Does My Car Need?

• Refrigerant Oil PAG or Polyalkylene Glycol oil is the most common oil used for automotive A/C systems. PAG oil is compatible with both R-134a and R-1234y refrigerants.
• While ester oil is sometimes used for older R-12 A/C systems, it is not as commonly-used as PAG oil. Ester oil is compatible with both R-12 and R-134a A/C systems, and can be mixed with mineral oil originally used in R-12 systems. However, ester oil has fewer viscosity options and offers significantly less component protection than PAG oil.

Eliminate a stuffy, hot vehicle cabin and restore a cool, comfortable ride with parts and tools from O’Reilly. Whether you’re looking for instructional or informational articles and videos in our How-To Hub, refrigerant, or a variety of A/C replacement parts, our parts professionals are ready to assist you with your next repair or maintenance task. If you need a specialized tool for one-time use, check out our selection of A/C rental tools available through our Loaner Tool Program, where only a refundable deposit is required. Don’t feel comfortable tackling an A/C repair job yourself? We can recommend professional shops near you to help you get the job done right.