chiph9 makes good suggestion. you could also get one of those 134a charging bottles from autozone or the like, one with gauge on it. if you havent serviced the AC in a long time it probably needs some charge. the can stuff also has lubricants in it too. just be sure to follow the directions on can. the can usually wants the compressor on during fill, but thats not always possible, so i guess you would put AC in some "on" position and then slowly fill (as needed) until compressor engages, then continue the fill to proper gauge range (as needed), etc.
I'm actually not in favor of the DIY recharge kits. The modern A/C systems require a precise amount of refrigerant and oil, and the kits usually get it wrong.
read up on 134a systems, they are all the same, they take same amount of PSI, the system is based on the 134a properties. if the system is bigger then you need more charge to get there, etc. this is why they can sell the generic canned stuff with a simple gauge on it. as for amount of oil, that could be something to look out for. but duly noted, a good dual gauge tool monitoring hi & low ports can be used to diagnose other internal AC issues (bad valves, issue with pump, etc etc).
do you have any reports or links to reports showing the can gauges got it wrong? i am interested because i do use the can stuff every 2 or so years on several vehicles.
I took the GX470 into our local Sunoco repair shop this morning. It turned out to be the A/C Blower Motor. Thank God it was only $95 for the motor.
while it was in there we had them look at the tire inflation sensors, because our tire inflation light was blinking, but I knew it was wrong because I had the pressure perfect. Those dang sensors are $145 each! Only one needed to be changed, the others cleared once they reset the system.
Our total repair bill was $325, including all the labor.
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Jeff Ostroff, CarBuyingTips.com