I bought the AC Pro 20 oz today at AutoZone. My AC was blowing mildly cold air and isn't as cold as it should be, so I thought I would recharge it.

I started the Jeep, let it warm up for 5 min, and then turned the AC on full blast with recirculation. 5 minutes later, I placed the AC Pro provided gauge at my low pressure port and noticed that the pressure swayed wildly between almost 0 to the high end of the green range on the gauge. I noticed that this happened rhythmically, in concert with the AC compressor clutch. Whenever I heard the compressor working, the pressure built up. The needle rises steadily, and the moment the needle reaches the high end of the green range, the clutch shut off and the needle on the gauge dropped to almost 0. And then this repeats. Notably, the clutch pulley is also squeaking a little bit when it is not spinning.

I tried filling up a little bit from the AC Pro bottle, but it just made the needle go into the low side of the yellow section on the gauge when the compressor was working, and then still dropped to almost 0 the moment it shut off. So I was afraid to fill more and just let it be for now.

Not sure what's going on and would greatly appreciate help.

 

When the compressor cycles, yes you will get changes in pressure. I think what is probably happening is the high pressure cut off switch is cutting power to the compressor clutch, which means it might actually be overcharged. The problem with these gauges is that they don't really tell you all that much. That is why a gauge manifold hooks up to both sides. Now what you didn't say is what year your vehicle is and whether it has a big brass block with a hat on it next to the firewall with lines going into it

 

That...makes sense. I have a 98 XJ, I don't think I've seen a big brass block anywhere under the hood but I guess I've never looked for it. What is it?

Looks like I'll be heading to harbor freight for a manifold gauge set tomorrow. Thank you for your advice.
​​​​

 

That thing is a thermal expansion valve. Very forgiving piece of equipment and makes charging very easy. You have a fixed orifice and I suspect you may be overcharged, but having the gauges that have numbers on it will be much better. Also if you add R134A, do it with the can upside down. It is a blend and you need to charge with liquid so you get the right ratio of gasses.