Rick's Free Auto Repair Advice

Static pressure reading

Static pressure chart for car AC

To find the static pressure reading for your car, attach to the hoses and gauge set to the high and low side ports of your AC system.

1. With the engine OFF for at least 30 minutes, read the low and high pressure gauges.

2. Measure ambient air temperature at the vehicle (do not use weather service temperatures) WITH A THERMOMETER.

3. Compare the pressure readings to the pressure temperature chart below

R-134a TEMPERATURE STATIC PRESSURE CHART
Ambient Temperature °F/°C Low-Pressure Gauge High-Pressure Gauge
65°F Low side pressure 25-35 psi High side pressure 135-155 psi
70°F Low side pressure 35-40 psi High side pressure 145-160 psi
75°F Low side pressure 35-45 psi High side pressure 150-170 psi
80°F Low side pressure 40-50 psi High side pressure 175-210 psi
85°F Low side pressure 45-55 psi High side pressure 225-250 psi
90°F Low side pressure 45-55 psi High side pressure 250-270 psi
95°F Low side pressure 50.55 psi High side pressure 275.300 psi
100°F Low side pressure 50-55 psi High side pressure 315-325 psi
105°F Low side pressure 50-55 psi High side pressure 330-335 psi
110°F Low side pressure 50.55 psi High side pressure 340.345 psi

WARNING: Ambient temp is the outside atmospheric temperature at the vehicle, NOT THE WEATHER SERVICE.

What static pressures actually means

Static pressure cannot tell you if the system is full. That’s a very common DIY misconception. That’s due to the nature of refrigerant. Refrigerant pressure is directly related to temperature, not volume. So, a 30-lb jug of refrigerant at 80°F will have the exact same static pressure reading as a the same 30-lb container at 80°F containing just with just 10-oz.

Static pressure only tells you two things

If the static pressures are close to the numbers shown on the chart, it only tells you two things:

The system isn’t completely empty

The pressure is either above or below the low pressure limit switch threshold and/or the pressure is either above or below the high pressure switch threshold.
If the static pressure is lower than the threshold for the low pressure limit switch on your particular vehicle, the AC system will not engage the compressor and you will not get any cooling.

If the static pressure is higher than the threshold for the high pressure limit switch on your particular vehicle, the AC system will not engage the compressor and you will not get any cooling.

NOTE: If the original static pressure is low and you’ve add too much refrigerant, remember that it’s illegal to vent the excess to the atmosphere. Plus, venting refrigerant can cause frostbite.

Also, if you didn’t add refrigerant and the system pressure is high, chances are you’ve got air in the system and venting will NOT solve your cooling problems. Venting will lower AC pressure, but it won’t compensate for the fact that you have air in the system.

What 0 static pressure means

The entire AC system is empty due to a major leak. Find the problem before refilling.

If static pressure is above the low pressure switch threshold

You can start the engine and turn on the AC. If the system is low on charge, the compressor will engage for a short period and then disengage. That’s because the compressor is sucking refrigerant out of the evaporator, causing the pressure to drop. Since the refrigerant charge is too low, not enough new refrigerant is replacing the suctioned refrigerant and that causes the pressure to fall below the low pressure switch threshold. So it turns off the compressor.

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat



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