If you are one of the guilty folks who slam in a can of AC charge to "top up", this video from FordTechMakuloco goes over how to figure out you went too far and overcharged the system. Enjoy.
How to recognize an overcharged AC system
- Thread starter Snagglefoot
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This is a lot like a oil analysis. If you don't have a baseline to go by then it's just data. If you don't have a known accurate pressure to compare against then you can't just go out and hang a set of gauges on a system and know whether it's overcharged by a lb. of refrigerant or not.
I have tried to use those temperature/humidity charts to compare against pressure gauge readings and that information is also all over the place. It seems to vary by system and moon phase.
Bottom line, evacuate the system and recharge to a precise and accurate quantity and let it go. Anything else is simply guessing. If you have no A/C and it's 95ºF outside then any sane human would drop $10 on a 12 oz. can of R134a and see if they can improve their situation.
I have tried to use those temperature/humidity charts to compare against pressure gauge readings and that information is also all over the place. It seems to vary by system and moon phase.
Bottom line, evacuate the system and recharge to a precise and accurate quantity and let it go. Anything else is simply guessing. If you have no A/C and it's 95ºF outside then any sane human would drop $10 on a 12 oz. can of R134a and see if they can improve their situation.
Although if you are too low and compressor will not kick in, then you might not get the R134a to flow from the can into the AC system.
My view is unless you have all the tools like an A/C service machine you should bring it to a shop. If your A/C system lis not working, then you probably have a leak (or leaks) that needs to be found and repaired then hook up the A/C service machine.
Oil changes are easy, properly servicing an A/C system is a lot more complicated.
The only proper way is to evacuate and charge by weight. Variable compressors that are now pretty common are impossible to charge by pressures only. It must be weighed in.
In my experience using the old R12 back in the day, no gauges, dirt cheap. This was the way to tell : System obviously low, compressor cycling and blowing 50 degree air from the vents using a stick thermometer. Bypass the compressor to make it run, dump in a 1lb can, right side up. Let it run in the heat 95 degrees, high humidity, at 1200 rpm. Check thermometer. Oh, it got colder by 8 degrees. Dump in another can, oh, almost 33 degrees ! Dump in another can, thermometer went up and high line stopped sweating. OOPSY !!! Overfilled. Disconnect and push schrader valve in to let some out. Keep checkin thermostat till you got to around 33-35 degrees, YAY 
It's full. Then throw in a can of compressor oil for the hell of it ....Good ole days
It's full. Then throw in a can of compressor oil for the hell of it ....Good ole days
I think the above was in the Ford service manual for a Pinto. And shame dumping R12 into atmosphere.In my experience using the old R12 back in the day, no gauges, dirt cheap. This was the way to tell : System obviously low, compressor cycling and blowing 50 degree air from the vents using a stick thermometer. Bypass the compressor to make it run, dump in a 1lb can, right side up. Let it run in the heat 95 degrees, high humidity, at 1200 rpm. Check thermometer. Oh, it got colder by 8 degrees. Dump in another can, oh, almost 33 degrees ! Dump in another can, thermometer went up and high line stopped sweating. OOPSY !!! Overfilled. Disconnect and push schrader valve in to let some out. Keep checkin thermostat till you got to around 33-35 degrees, YAY
But for a decent car you need to find & fix the leaks. Pull a vacuum for 30 min to check for leaks, then fill by weight.
I used to work on systems that didn't come with data like charge weights. So when charging I went by pressures.
Yes, I use gauges now. R134 is not cheap to just let fly away. R12 was dirt cheap in the early 80's.
1234yf is outrageous. Makes 134a look like chump change
Never heard of 1234yf ???? What does that apply to ???
Came in my 2020 Ranger, most manufs have been using it for a little while now.
Chemical companies keeping their iron grip on their profits instead of using cheap and unpatentable hydrocarbons or CO2.
I don't subscribe to almost all conspiracy theories, but I have strong feelings about the refrigerant market. There's no reason they can't use cheap gasses in automobiles, but they instead come up with new chemicals that are toxic in new and exciting ways....and expensive.
Just peachy...More expensive than R134A you say ???? I'm keeping my beaters that use R134A. Frig this !!!!!! Don't care how much the wife cries for a new car.......... So 2020, that means R134A is going bye bye is on the shelves, just like R12 did. Car companies are switching over to this new crap now. Time to buy a few pounds of 134 and leave it on the shelf in the garage. Wonderful 
Thanks for the heads up my BITOG friend
Thanks for the heads up my BITOG friend Pretty much everything since 2017 or so.
Haven't bought a car newer than 2017, so I had no idea. That was hush hush. Something different ...I'll find some R134A before the prices skyrocket. I'm sure this new stuff is not compatible to R134A, just like the R12 wasn't compatible to R134A. Unless you did that whole retrofit nonsense they were pushing back then.
...I'll find some R134A before the prices skyrocket. I'm sure this new stuff is not compatible to R134A, just like the R12 wasn't compatible to R134A. Unless you did that whole retrofit nonsense they were pushing back then.It's the new freon they have been using since I believe 2018. Over 20 bucks for 12oz. Needs all new gauges and everything to service and different oil, different everything
It is also blended with R32 to make 454B that is used in home a/c units.
Wow. Learn something new everyday. Cool
NO COOL...NO A/C in you're car !!!!!!..If you have cars that use R134A, you better scoop some up before it's all gone ( unless you plan on buying new cars ). It's like R12 all over again. I have PTSD from that disaster. I think a 1 lb can of R12 shot up to like $ 60 bucks when the stock was just about depleted. I just bought 4 lbs of R134A to keep in stock. Prices are on the rise already. Do you remember the R12 disaster of the 80's ???? Same thing. They switched car freons from R12 to R134A and kept it all hush hush. Same thing all over again. Except now it's R134A to 1234yf. It's all a scam by the chemical companies and bean counters.
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The 1234yf systems I've encountered take the same PAG oil, Some systems are not different at all except the service ports are different. I converted a set of 134a gauges with 1234yf couplers & charge with 134a most of the time.
