How to & Guides

Car AC Pressure Chart: Low-Side & High-Side Pressure of R134a

From opened car windows to advanced multi-zone HVAC Systems, the Air Conditioning in vehicles has undergone significant advancements. A modern car’s air conditioning system comprises multiple components, including the Compressor, Condenser, Evaporator, and Refrigerant. With such complexity, it is crucial to pay attention to the refrigerant’s pressure for proper maintenance.

In this article, we will delve deeper into Car AC Refrigerant, explore a typical Car AC Pressure Chart for the widely used R134a refrigerant, and provide troubleshooting tips related to the AC system pressure.

How Does Car AC Work?

First, let’s briefly understand how the air conditioning system in a car operates. The process of extracting heat from an area or location is known as Refrigeration. Refrigerators and Air Conditioners exemplify this system by mechanically transferring unwanted heat to an external area where it is harmless.

Refrigerators and air conditioners employ a special fluid called Refrigerant to facilitate heat transfer. This fluid continuously cycles between a high-pressure liquid state and a low-pressure gas state while absorbing heat and humidity from the car’s cabin, thus providing cool and dry air.

Car AC Working

All the components, including the compressor, condenser, accumulator (or receiver drier), and evaporator work in harmony to deliver cool air.

It is evident that the refrigerant plays a significant role in cooling the cabin. At low temperature and pressure, the refrigerant absorbs heat and undergoes a phase change from liquid to gas. To expel all the absorbed heat, the refrigerant is then condensed from gas to liquid at high temperature and pressure.

The refrigerant should possess specific properties, such as:

  • Non-explosive and non-flammable
  • Low boiling point
  • Non-corrosive to metals and miscible with oils
  • Non-hazardous, safe, and non-toxic
  • High latent heat value
  • Easy to liquefy or vaporize and moderate its pressure and temperature
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Refrigerants, including R-12, R-134a, Freon, Freeze-12, GHG-HP, Hot Shot, Ikon-12, and R-406A, are chosen based on these properties and other industry-approved requirements.

R134a Refrigerant

Previously, R-12 (also known as Freon-12) was widely used as the primary refrigerant in automotive and household refrigeration. However, due to its detrimental impact on the ozone layer, it has been phased out since 1994.

Since then, the industry has adopted R134a as the standard refrigerant for automotive ACs and other refrigeration and freezing applications. R134a has thermodynamic properties similar to R-12 but significantly lessens ozone layer depletion.

At atmospheric pressure, R134a has a boiling point of -26.8oC. When subjected to 10psi pressure, its boiling point rises to -10.6oC. If released into a room at atmospheric pressure and room temperature, R134a would instantly convert into gas.

The refrigerant’s temperature and pressure are directly related. For instance, if the pressure of R134a is 35psi (241kPa), its temperature will be 4.4oC. Therefore, it is evident that temperature and pressure are vital factors in the condensation and expansion of R134a.

Low-Side Pressure and High-Side Pressure

Before examining the AC Pressure Chart for R134a Refrigerant, let us understand why measuring and monitoring pressure is essential in a car’s air conditioning system.

In an automotive AC system, pressurized refrigerant continuously transitions between liquid and gas states in a closed loop. The refrigerant’s pressure is directly correlated with the temperature.

Based on the refrigerant’s pressure at any specific point in the loop, the system can be divided into the High-Pressure Side and Low-Pressure Side.

Low-Side and High-Side Pressure

The compressor receives low-pressure gas from the evaporator and compresses it into high-pressure gas, causing its temperature to rise. Since this is where the high-pressure refrigerant gas begins, the High-Pressure measuring point is located after the compressor.

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The high-pressure gas then flows to the condenser, where the radiator fan blows cool air onto it. Consequently, the high-pressure gas is converted into high-pressure liquid as it leaves the condenser. At this stage, the refrigerant releases the heat it had absorbed at the evaporator.

The high-pressure liquid passes through the receiver/dryer, which filters out any moisture in the refrigerant. It then traverses a thermal expansion valve, where it expands and transforms into low-pressure liquid. From there, the low-pressure liquid enters the evaporator, where it starts boiling and converting into low-pressure gas.

This low-pressure gas absorbs heat from the cabin and is then sent back to the compressor, initiating the cycle again. As this is the exit point for the low-pressure refrigerant gas, the Low-Pressure measuring point is located after the evaporator.

AC Pressure Chart

To determine the low-side and high-side pressure of the refrigerant, a specialized testing device equipped with two pressure gauges (one for the low side and one for the high side) connected to a manifold assembly is employed. The high side pressure gauge and its connecting pipe are typically colored red, while the low side pressure gauge and its connecting pipe are colored blue.

AC Pressure Manifold

The following table presents the AC Pressure Chart for R134a Refrigerant. It includes the typical high-side and low-side pressure values of R134a at specific temperatures.

AC Pressure Chart

How to Use the AC Pressure Chart?

If you notice that your car’s AC is not cooling adequately, it is essential to take your car to an AC technician. They will likely recommend checking the refrigerant’s pressure and ensuring that the readings for both the high side and low side fall within the optimum range specified in the Car AC Pressure Chart mentioned above.

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Based on the pressure gauge readings, you can further diagnose any issues using the following table.

AC Pressure Chart Uses


This article provided a simple overview of the Car AC System, R134a Refrigerant, and the automotive AC Pressure Chart for R134a. By properly caring for and maintaining your car’s AC system, you can expect it to serve you well without major repairs or replacements. We recommend regularly checking the refrigerant levels by testing the high-side and low-side pressure at a trusted AC technician or workshop.

For more information about OnSpec Electronic, Inc., visit OnSpec Electronic, Inc.

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