If you have a car and know nothing about the internal running of the car, then you will surely run into problems. As simple as it may be to just turn on the air conditioner and have cool air inside your car when the temperature outside is high, the refrigerant that allows you to have such comfort needs to remain at normal pressure. It shouldn’t be too high or too low. This is why knowing the high side and low side pressures of your refrigerant is very important.
The two pressure meters (high and low side pressure) can determine the smooth running of your car’s air conditioning system. They can be used to diagnose the problem of an air conditioning compressor. There are a number of problems that can come up with the air conditioning, one of them being that the pressure on the low side is high or that the pressure on the high side is low, which is caused by other problems.
If you hook a gauge set to your air conditioning system, you can record the pressure on the high side and the low side. This record should be in line with the ambient temperature of your air conditioning system, as all pressure is temperature-dependent.
There is a chart that shows the high side and low side pressure readings in relation to different ambient temperatures. I will be showing you this chart later in this article. Before that, I will tell you what the high side and low side pressure of R12 should be, and then the high side and low side pressure of R134a and its normal operating pressures.
What Should the High and Low Side Pressures be for R12?
R12 is a refrigerant, also known as Refrigerant R12 or Freon 12. It is a widely used refrigerant for appliances that have a compressor. Apart from a car’s air conditioning system, it is used in air condition, refrigerators, freezers, dehumidifiers, water coolers, ice makers, and other cooling and refrigeration appliances.
The high side and low side pressure for refrigerants are measured in pounds per square inch (psi). For a car air conditioning system, R12 should be 170 to 210 psi on the high side and 30 to 40 psi on the low side. If the high side and low side pressures of your R12 are lower then it’s likely that it is undercharged; check for leaks and then recharge the refrigerant.
Bear in mind that the higher the ambient temperature, the higher your high side and low side pressures. This is why the chart is necessary to help you know the right pressures for each ambient temperature. To know the ambient temperature, you will need a pocket thermometer. All you have to do is hold the thermometer in front of the AC grill of your car. Doing this will allow you to measure the actual temperature of the air going through your condenser.
What Should the High and Low Side Pressure be in an R134a System?
R134a, like R12, is a refrigerant used in the compressor of air conditioning applications. It is a cooling agent. When measured, the high side pressure should be between 225 to 250 psi and the low side pressure should be 25 to 40 psi.
The Pressure Chart for R134a
The chart shows how the ambient temperature is related to the high side and low side pressures and how it affects their readings.
| Ambient Temperature (°F/ °C) | Low Side Pressure | High Side Pressure |
| 65°F / 18°C | 25-35 psi | 135-155 psi |
| 70°F / 21°C | 35-40 psi | 140-165 psi |
| 75°F / 24°C | 35-45 psi | 150-175 psi |
| 80°F / 27°C | 40-45 psi | 175-220 psi |
| 85°F / 29°C | 45-50 psi | 220-250 psi |
| 90°F / 32°C | 45-55 psi | 250-275 psi |
| 95°F / 35°C | 50-55 psi | 300-325 psi |
| 100°F / 38°C | 50-55 psi | 315-325 psi |
| 105°F / 41°C | 50-55 psi | 330-335 psi |
| 110°F / 43°C | 50-55 psi | 335-345 psi |
What are the Normal Operating Pressures for R134A?
The normal operating pressure for R134a is between 22 psi and 57 psi. This is due to the pressure of the coil. When running normally the pressure of the coil is around 22 psi and when at a very high temperature, it runs at 57 psi.
