I have never in my life messed with ac systems other than blend doors and simple things.
When you connect the manifold gauge set, keep in mind that the hoses may be full of "air" instead of "refrigerant". You do not want that air in the A/C system.
Connect the hoses to the Schrader valves with the engine OFF. You should see pressure on the two gauges. Often, the hoses themselves (Blue low-side, Red high-side) have a valve in them; those hose-valves have to be open. With the middle (generally yellow) hose open on it's far end, open the valves on the manifold set briefly. Pssssst! from the low-side valve, and then close that valve. Pssssst! from the high-side valve, and then close that valve. Now the manifold gauge set hoses have been purged of air, they're filled with refrigerant. Close the valve in the yellow hose (if equipped) to seal the refrigerant in the hoses.
NOW you can take your pressure readings with the engine running and compressor pumping, or add refrigerant without fear of air contaminating the system.
The A/C refrigerant system is relatively simple. Most of the complexity is involved in controlling the compressor, and controlling the air-doors. But don't forget that you'll make Greta cry (and maybe even show up at your home along with the entourage of Lamestream Media, Hollywood Hotshots, a whack-job black-and-white photographer, etc.) if you vent excess refrigerant, and refrigerant boils at something like -40 degrees, which can be really painful if you're in the way of it. The system also operates under considerable pressure, so
don't EVER open the high-side manifold valve with the compressor running. In fact, unless you intend to add refrigerant TO the system, or remove refrigerant FROM the system, don't open the high- or the low-side manifold valves at all except to purge the hoses.
Probably want to watch (multiple) Youtube videos; and/or read up on the A/C service procedures in the Service Manual.
BE CAREFUL. A/C work "can" be dangerous. You don't want this showing up at your door.
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A gen-you-wine Scandahoovian Native American.
