St George Auto Repair

776 So Bluff Street


How to repair your a/c

You will need a set of manifold gauges to check you freon pressure.

Here are some general guideline for A/C system pressures and temperatures based on ambient outside temperature. Remember that these are a guideline and your actual temperatures and pressures will vary depending on humidity in the air and the condition of your system. When running the car at idle to check the freon level it is a good idea to have a water hose handy and spray water on the a/c condencer. The condencer is located in front of the radiator.

For vehicles currently running with freon R-134a only

Outside-----low side-------High side------Center vent temp

60 F -----28-38 psi----130-190 psi ----44-46 F

70 F ------30-40 psi ---190-220 psi ----44-48 F

80 F ------30-40 psi ---190-220 psi ----43-48 F

90 F ------35-40 psi ---190-225 psi ----44-50 F

100 F -----40-50 psi ---200-250 psi ----52-60 F

110 F -----50-60 psi ---250-300 psi ----68-74 F

120 F -----55-65 psi ---320-350 psi ----70-75 F


You can also use this good rule of thumb with R134a, run fan on High with all windows open and engine at 1500 rpm. High side pressure in PSI should be about 2.2 to 2.5 times outside air temperature in degrees F. Example if it's 90 degrees F, high side should be no higher than 225PSI

Here is a useful trouble shooting chart for systems currently running on freon R-134a

Trouble shooting

Low Compressor Discharge Pressure

1. Leak in system 2. Defective expansion valve 3. Suction valve closed 4. Freon shortage 5. Plugged receiver drier 6. Compressor suction valve leaking 7. Bad reed valves in compressor


1. Repair leak in system 2. Replace valve 3. Open valve 4. Add freon 5. Replace drier 6. Replace valve 7. Replace reed valves

High Compressor Discharge Pressure

1. Air in system 2. Clogged condenser 3. Discharge valve closed 4. Overcharged system 5. Insufficient condenser air 6. Loose fan belt 7. Condenser not centered on fan or too far from radiator


1. Recharge system 2. Clean condenser 3. Open valve 4. Remove some refrigerant 5. Install large fan 6. Tighten fan belt 7. Center and check distance

Low Suction Pressure

1. Refrigerant shortage 2. Worn compressor piston 3. Compressor head gasket leaking 4. Kinked or flattened hose 5. Compressor suction valve leaking 6. Moisture in system 7. Trash in expansion valve or screen


1. Add refrigerant 2. Replace compressor 3. Replace head gasket 4. Replace hose 5. Change valve plate 6. Replace drier 7. Replace drier

High Suction Pressure

1. Loose expansion valve 2. Overcharged system 3. Expansion valve stuck open 4. Compressor reed valves 5. Leaking head gasket on compressor


1. Tighten valve 2. Remove some refrigerant 3. Replace expansion valve 4. Replace reed valves 5. Replace head gasket

Compressor Not Working

1. Broken belt 2. Broken clutch wire or no 12v power 3. Broken compressor piston 4. Bad thermostat 5. Bad clutch coil 6. Low Refrigerant - low pressure switch has cut off clutch power


1. Replace belt 2. Repair wire or check for power 3. Replace compressor 4. Replace thermostat 5. Replace clutch coil 6. Add refrigerant

Tune up that A/C system!