Automotive Air Conditioning Information Forum

Like the thread title says, do you have to worry about introducing air into the system when connecting the AC manifold gauges in order to check pressures?

I realize that the AC system is under high pressure, and even the low side will be way higher than atmospheric pressure, so I would imagine that the refrigerant simply 'pushes' up against the air in the AC MGS hoses and keeps it from actually being sucked into the system. However, I also started to wonder if maybe the air in the low-side "blue" hose might get sucked into the system when the engine/compressor RPM goes up and the low-side pressure suddently drops (like, say, from 30psi to 20psi). FYI, I'm referring to r134a systems if it makes a difference.

I thought about maybe trying to vacuum out the AC MGS & hoses before connecting them as a way to keep air out, but then wouldn't that simply cause me to lose more refrigerant inside the hoses, since the vacuum would suck out & fill them with refrigerant?

What's the best way to manage this for the DIY guy?

No need to make it more complicated than it needs to be. Hook gauges to the system. Close the valves, and bleed a little refrigerant through the center charging line—open valves, pump/vacuum down the system, and charge to spec.

Thanks for the feedback, but I guess I wasn't clear.
I already charged up my system. I did it completely by weight, liquid, on the high-side with the engine off. I didn't even hook up the low side at all.

My question was about just hooking up gauges in general, on a working system (not on an empty one or one that needs to be charged), in order to measure pressures. Say, for example, I want to double check pressures on a really hot day a month from now...

Do I need to be worried about getting air into the AC lines simply by hooking up my gauges to read pressures? Should I be vacuuming out the manifold before hooking it up?

csrf wrote: ↑Wed Oct 04, 2023 7:44 pm My question was about just hooking up gauges in general, on a working system (not on an empty one or one that needs to be charged), in order to measure pressures. Say, for example, I want to double check pressures on a really hot day a month from now...

I'm not an AC professional, but I say "NEVER touch an AC system that's operating properly/cooling well !!!"

No need to double check pressures. Measure vent temperatures after 15 minutes while driving down the road. As Yogi Berra says "if you're cool, that's cool !!!"

"Do I need to be worried about getting air into the AC lines simply by hooking up my gauges to read pressures? Should I be vacuuming out the manifold before hooking it up?"
No.
The system pressures will be higher than atmospheric (both hi and lo sides) at ordinary temps. For example, the PT chart tells us the pressure of 134 at 60*F is
57.4 psi.

But as Cusser says, no need to be checking pressures all the time. If it is blowing cold, leave it alone. And unless you know how to bleed the red hose back into the blue hose, you are gonna lose the 2 oz of 134 that is contained in the red hose when you disconnect.

hi
Just make sure your gauge/hose set is in good order , replace seals , lubricate o rings etc etc vacuum and pressure test . If your doing A/con often enough you get to no your tool characteristics anyways . I `ve always found gauge sets donot age well
Don`t forget to service your vacuum pump regular .