Most vehicle owners believe that charging or evacuating an air conditioning system requires some knowledge of physics, combined with magic. One look at a set of AC gauges sends them off in a panic in search of an auto repair facility that will do the job for them. Hearing terms like "high side" and "low side" only makes things worse. Then they have to consider what type or refrigerant or freon their vehicle needs, and they have no idea how to purchase and install it. Hooking up AC gauges requires only a few steps and some basic identification of what fitting goes where and why.
Set the shifter in "Park" (for automatic transmission) or "Neutral" (for manual transmission) with the emergency brake set. Raise the hood. Hang a powerful shop light from the underside of the hood or have it nearby. Take your gauge set and hook it on the underside of the hood, gauges facing out so you can see them. Refer to your owner's manual for the location of the service fittings on your AC lines that denote the high side and low side. The location of the valves varies on each vehicle.
Look for two screw-on caps, colored either brass or black that tie in to the AC lines. For the older system that uses R12 Freon, once you remove the cap on the low side fitting you will find a threaded Schrader valve that will measure 7/16 inch. The high side fitting for the R12, once the cap has been removed, will measure 3/8 inch. You only have to remember that the low side has a smaller port fitting and that you can not accidentally reverse the hook-up. Both Schrader valves look like tire valves.
Screw the AC gauge blue hose into the low side port fitting on your AC line. Screw the AC gauge red hose into the high side port fitting.
Look for quick-release (push-on) fitting ports on your AC lines if your vehicle uses R134a type refrigerant, which comes in vehicles manufactured after 1993. The low side port fitting will measure 13 mm, so you will connect the blue hose (low side) to it by shoving the hose coupler over the fitting. Connect the red hose (high side) to the high side fitting on your AC line in the same fashion. The high side fitting on your AC line measures 16 mm, so you cannot confuse them.
Locate the middle yellow hose on the AC gauges. A single can of refrigerant hooks up to the end of the hose. Place the yellow hose coupler over the refrigerant can and screw the coupler over the can lip. Turn the small valve to puncture the can, then back the valve out. Open the blue and red knob valves on each gauge to charge your AC system.
Evacuate the AC system by hooking the end of the yellow hose up to the in-port side of a vacuum pump and turn the pump on. Recover refrigerant from an AC system by hooking up an additional yellow hose from the out-port side of the vacuum pump to the top of a recovery tank. They both have screw-on fittings.
