A/C diagnosis is not a simple matter of reading a gauge and knowing for sure if the system is low on refrig. or overcharged - you have to troubleshoot and actually diagnose the problem - you need to know how to work on an a/c system. Same thing with all the guys pulling "codes" from the ECM on a vehicle and relating say, for instance a code indicating " O2 sensor not switching rich/lean" as to being a bad O2 sensor and go replace it without actually diagnosing anything. Many things can cause an O2 sensor to not switch rich/lean and not be a thing wrong with the O2 sensor. You can hurt yourself and/or cause more serious damage to an a/c system by adding refrigerant trying to get it to cool without knowing what you're doing. With R134a the charge really needs to be removed and accurately weighed and reinstalled for the actual system requirement - it's not the same as adding a few oz's of R12 in the old systems - they were more forgiving. You need to know actual ambient air temp and relative humidity, need to check the evaporator for air flow restrictions/dirt and condensate drainage, check the condenser for good airflow/trash accumulation etc. If your evaporator is freezing-up like you guess, it could be restricted airflow thru it, could be a sticking expansion valve if it has one or a restricted expansion tube if it has that instead - they have extremely fine screens in either and get partially clogged very easily and will cause an evaporator to freeze-up. I've seen alot of screens clogged with just the fibers from the dessicant bag inside a system from normal operation. And of course a little low on refrig can cause it as well. Ideal pressures for an R12 system will usually run around 26 -28 psi on the low side with around 225 - 275 psi on the high side, depending on ambient and humidity. These readings would be at a fast idle of around 1100 - 1200 rpms. On an R134a system the pressures will run a little higher when fully charged and operating properly - about 28 -32 psi low side and quite often the high side will be over 300 psi. Other factors come into play with these readings too, air flow across the condenser makes a huge difference especially. Now if you have a resticted expansion tube and start adding refrig to try to get it to cool and your compressor is still good you can have pressures in excess of 450 psi or more right under your nose - pretty dangerous stuff. It's usually worth it to spend the $30 checkout fee (+/-) for a professional to check your a/c system - find someone that knows how the systems operate.
