How to Use Freon Gauges

by Chris Stevenson

A vehicle owner who wishes to charge his air conditioning system soon finds out that the procedure does not involve simply hooking a can and a hose up to the engine and turning a valve. Some AC units have to be completely evacuated, then pulled for vacuum and then recharged. The use of AC gauges might seem daunting, but the vehicle owner who knows how to hook them up and follow directions can complete an evacuation and recharge properly, without damaging the AC system.

Refer to your owner's manual for the location of the high and low side fittings on your AC lines. Typically R134a refrigerant has replaced the outdated R22 refrigerant, so the fittings on your vehicle will likely have quick release fittings. Hang the AC gauges from a hood flange or hole. Hook up the high side, or red hose on the AC gauge to the high side fitting on your AC line location. It has a push-on quick release.

Notice that the high side gauge hose will not fit to the low side on your car, therefore no mistakes will be made during the connection. Hook up the low side, blue gauge hose to your low side fitting on your AC line. Open both knob valves on the blue and red gauges. The needles will move. With the engine and AC controls off, look at both gauges. A normal (static) reading will be between 80 and 105 pounds per square inch. Check your owner's manual for your exact figure.

Have an assistant start your engine and bring the idle up to 800 rpm. Have him turn on the AC unit to full speed at the maximum setting. Look at the low side blue gauge. A normal reading will indicate 25-35 psi. Some Chrysler vehicles call for 15 to 25 psi, so be careful to check your owner's manual.

Look at the high side, red gauge. The high side should range between 300 to 350 psi. Readings outside or inside these ranges will indicate a problem with the evaporator, a leak or refrigerant discharge-overcharge condition. Have your assistant place a thermometer in the central AC vent. The temperature should read between 42 and 55 degrees for normal AC operation.

Look at both gauges. If both gauges read below normal while the engine runs and at maximum AC, it means a low refrigerant condition exists. To charge the system with a can of freon, the yellow middle hose on the gauge must be connected to a freon can. The hose end has a twisting puncture valve on it that connects securely to the freon can. After puncturing the can, turn the valve knob open on the hose and can to release refrigerant into AC compressor on your vehicle.

Look at both gauges. If the high and low side readings appear higher than normal, the system has too much refrigerant in it -- an overcharge condition. It could also point to a defective condenser fan, or an extremely hot engine.

Check both gauges. If the low side has dropped to indicate a vacuum and the high side gauge reads above normal, it means the expansion valve has failed or an orifice tube has clogged. A blocked condenser, although not common, could also be the result of this reading.

Look at both gauges. If the low side reads too high and the high side reads too low, it points to a failing compressor. It means the pump can not put out enough force or pressure to circulate the refrigerant. A slipping AC belt can also cause this reading, so be sure to check the belt.

Hook the screw fitting of the yellow hose from the gauge to the in-port nozzle on the vacuum pump if you need to pull vacuum from an AC system, or recover refrigerant from an existing system. Turn the vacuum pump on to "Vacuum" and open both the red and blue knob valves on the gauge. Some AC gauge sets come with an additional yellow hose. Hook this hose from the out-port side of the vacuum pump and the other end to the top valve of a recovery tank. Open both valves to recover refrigerant.

Items you will need

About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.

Photo Credits

  • photo_camera pressure gauges image by laurent dambies from Fotolia.com