Your car’s air conditioning system can encounter various problems. A typical issue is when the unit doesn’t blow cold air, which can be troublesome in hot weather. Several causes can lead to this condition, including compressor problems, low refrigerant levels, issues with the heating or AC controls, and more. A symptom indicating a complication in the air conditioning is low pressure on the AC low side.
How to Take Pressure Readings
Several factors contribute to this condition. In fact, temperature is directly related to the system’s pressure. You can measure the refrigerant pressure at a given temperature. There is a temperature chart for refrigerant pressure that you can find available online.
To measure the pressure, connect a gauge set to the AC system and record the pressures on the low and high sides. Check the ambient temperature and see if the pressures match the numbers in the pressure chart.
Matching readings mean you don’t have to worry about the freon level. Readings higher than the chart indicate an overcharged system, while lower readings confirm a low refrigerant level and a possible leak. On the other hand, there might be a significant leak when the pressure is zero.
You can refer to the car air conditioning pressure chart below for more information:
| Ambient Temperature (°F) | Low Pressure (PSI) | High Pressure (PSI) |
| 65 | 25-35 | 135-155 |
| 70 | 35-40 | 145-160 |
| 75 | 35-45 | 150-170 |
| 80 | 40-50 | 175-210 |
| 85 | 45-55 | 225-250 |
| 90 | 45-55 | 250-270 |
| 95 | 50-55 | 275-200 |
High and Low Side Pressure in Your Car’s Air Conditioning System
The high side starts from the discharge side of the compressor and moves through the condenser where heat is transferred to condensation, then to the receiver-drier and ends at the expansion valve. The compressor creates high pressure by compressing the low-pressure vapor from the evaporator so that the high-pressure vapor can release heat at the condenser. This pressure change occurs again at the expansion valve where the refrigerant is slowed and the pressure changes.
While the low side starts at the expansion valve which creates low-pressure refrigerant that moves to the evaporator where it transfers heat from inside the vehicle and goes to the compressor where it turns into high pressure and the whole process repeats again.
AC Low Side High, High Side Low : Why Does It Happen?
The low-side pressure reading should be between 25 and 30 psi and the high side between 200 and 250 psi. But if you see the AC low side high, high side low, such as the low side is 100 and the high side is 150, there might be problems with one of the internal components.
The possible reason could be an excessive amount of freon going to the evaporator. It could also be a faulty thermal bulb unable to pick up temperature signals or a TXV stuck in the open position.
It could also be a problem with the orifice tube. This tube works as an expansion valve to regulate the amount of refrigerant flowing to the evaporator. When its opening becomes larger than the usual size, more refrigerant goes to the evaporator and creates pressure discrepancies. The same issue arises when the O-ring on the outside of the orifice tube wears out.
Before deciding to replace the compressor, other components you can check are the pressure switch, expansion valve, and compressor clutch. The switch is located near the expansion valve. Use an ohmmeter to see if it gives the correct reading. Also, check if the compressor clutch engages and spins the compressor when the AC is turned on. Similarly, the fan should also spin when you turn on the system.
When all these components seem to be working correctly and the AC low side high, high side low problem persists, you may need to change the compressor to fix the unit. However, it’s always better to consult a mechanic before deciding to replace a part.
Do You Charge the AC on the High or Low Side?
We will need to recharge the car’s air conditioning system with new refrigerant when the system isn’t cooling properly. But the question is whether you should charge the car’s AC on the low or high side? You can certainly charge your car’s AC from the high side, however, high-side refrigerant charging is sometimes dangerous and can damage your car’s system.
Watch the video to learn more:
When charging high-pressure gas, note that the engine is not running, the low-pressure valve is completely closed, and the high-pressure valve is completely open. Fill a tank with enough gas, then close the high-pressure valve. Note that it’s necessary to choose a charger suitable for the system’s capacity.
Can you charge the AC system from the low side? Yes, you can. The low-side port is important because it’s the point you connect to recharge the AC refrigerant. Its position varies in each vehicle, but in almost all cars, it will start at the compressor. It’s recommended not to charge until it’s 75 degrees or warmer for your vehicle’s AC unit to recharge.