How to Check Pressure in an Auto Air Conditioning System

The automotive air conditioning system is a self-contained unit of components that uses a refrigerant to cool inside cabin air. The components include the compressor, condenser, evaporator, accumulator, along with various hoses, fittings and valves. Composed of a high and low pressure side, the air conditioning unit must maintain a certain system pressure to operate properly and deliver cool air. Most systems today use R-134a refrigerant. A vehicle owner will need a set of AC manifold gauges to check the system pressure, making sure it conforms to manufacturer's specifications.

Instructions

1

Place the vehicle in park, if equipped with an automatic transmission, and in neutral for a standard type. Set the emergency brake and raise the hood. Find the high- and low-side service ports connected to the air conditioning hoses. Refer to your owner's manual for their location. The red hose usually signifies the high-pressure side, while the blue hose denotes the low-pressure side.

2

Remove the service caps by unscrewing them. Use a thermometer to record the outside ambient air temperature, since temperature differences can affect the pounds per square inch reading on A/C gauges. Turn the blue and red knobs on a set of manifold gauges clockwise to close them. Hook up the blue low pressure gauge hose to the low-side service port. The low-side hose and fitting are smaller than the high-side hose.

3

With the engine and A/C off, open both gauge knobs counterclockwise. Check your service manual for the correct static pressure required for your system. The low side should read at least 56 psi or slightly more at 80 degrees F. The high-side reading should approximately match the low-side reading.

4

Adjust the reading to about 70 psi for an outside ambient air temperature of 90 degrees. Adjust that figure for a lower reading if the outside temperature is at 70 or colder. Your manual should have the recommended psi for a normal specific temperature. If the reading falls below 50 psi, it is safe to assume the system is low on refrigerant, but take the outside air temperature into consideration. Any reading significantly higher will indicate an overcharge, or too much refrigerant in the system.

5

Turn on the engine and set the A/C controls for maximum blower speed and the coldest setting. Set the vent control for the recirculating mode. Have an assistant raise the engine idle to slightly higher than normal, or about 1,000 to 1,200 rpm. The low-side gauge should read about 25 to 35 psi, while the high-side gauge should read from 175 to 200 or even higher according to your manufacturer's specifications. The vent air should be very cool or cold to the touch.

6

Feel for warm air at the vents if the pressure reading on both gauges is very low. Both high- and low-side lower-than-normal pressure readings can indicate a low refrigerant charge, stuck expansion valve, plugged orifice or a compressor failure.

7

Shut the engine off. Close off both blue and red manifold gauge knobs by turning them clockwise. Disconnect the high- and low-side gauge hoses from the service ports and remove the A/C manifold gauges. Screw the dust caps back onto the service port fittings.