Be sure you're putting it in the low side. When the system is off the pressures in the low and high sides will equalize. As long as there's enough refrigerant in the system that some is liquid, the pressure will be somewhat close to the air temperature.
There's no accurate way to know how much refrigerant is in the system unless you recover everything, pump it into a vacuum for half an hour, then charge it with a measured amount. Chryslers use a sight glass on most of their models and you just add until the vapor bubbles are gone. Older Fords used a sight glass too but they still had bubbles when it was fully-charged. The best you can do is to get an approximation by the low-side pressure. It should not drop below about 40 pounds when the compressor cycles on. If you think more refrigerant is needed and the can doesn't become empty in about two minutes, place it upright in a pot of very hot water.
Be aware that professionals wear safety glasses, gloves, AND face shields. Refrigerant can freeze your eyeballs and cause frostbite.
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Saturday, March 30th, 2013 AT 12:31 AM