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 pressure temperature chart 2

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

What static pressures actually means

Generally speaking, if the static pressure reading is below that listed on the temperature/pressure chart, you can assume the system is low on charge. That means the system has a leak.

If the static pressure is close to the numbers shown on the chart, proceed to conduct a running pressure test.

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.

But adding refrigerant at this point without conducting a full running pressure test can cause other problems. For example, if the system is low on charge but the static pressures are still above the low-pressure switch cutoff point, you most likely have air in the system. That would show up as high high-side pressure during a running test. If you don’t evacuate the system to remove the air, it will never cool properly.

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat

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