Usually protocol on recharging the AC (At least here at the auto repair shop I manage) is you hook up the lines to the machine on the high and low side air AC lines through the service port which are basically schrader valves not much different than a valve stem on a tire. Then the machine recovers whatever freon is in the system, if any. Then vacuums the system, installs a leak detection dye that in the event of it leaking out can be seen with a special pair of glasses and a UV light by a glow wherever the leak is. The leak test may or may not detect a leak and that's why the dye is typically installed. Then some oil gets installed then the freon is installed and AC operation is restored and the technician monitors the high and low side AC pressures. Most cars have some safety precautions built in and if the pressure is too high or low it will not let the AC compressor cycle to protect it from failure. If the service ports themselves are leaking, the leak test will not detect a leak because the AC machine is using them at the time. If die was installed a tech can determine the source of a leak by the UV light and glasses be it the service ports, AC condenser etc. There are also things that you cannot readily see like the evaporator which is typically located under the dash. Also as noted earlier a bad cooling fan can cause the pressures to go to high and not let the AC Compressor cycle. A restricted condenser can also cause high pressures resulting in the same thing. Not all cars have these safeguards and if the pressure is too high will lead to catastrophic failure. It's not a scam and would probably be helpful if you didn't go straight to the scam mentality especially given the problem hasn't been diagnosed yet. Take it back and ask them if they installed dye and it can probably be diagnosed pretty quickly.
From a shop manager's standpoint this isn't really true. Unless there is evidence of a profuse leak, AC leaks can be very hard to detect thus the purpose of the die. They don't have die in them from the manufacturer making it even more difficult. I even have a probe with a sniffer on it that will detect freon leakage with an audible warning when probing for leaks that are very small. But the only way there will be freon in the system that has leaked it all out to even be able to detect it is if I put freon in it. In that respect, the place to start is is recharging the system, installing the dye etc. On top of that most cars nowadays take a very small amount of freon meaning if it has a leak that is very small, it won't take long for it to leak out even though seeming profuse and passing a leak test. Used to be that cars held a lot of freon like 5 lbs now you are seeing a lb or less as the capacity of the whole system. You'd get more time out of something with a large capacity that might work for months with a small leak by comparison. Freon also will naturally leak out over time and the smaller the system the faster it will happen enough to the point that the low pressure switch will prevent the AC compressor from engaging even if there is freon in the system. From my experience with people, most people don't give a damn about all the technical information I can spew to them and just want it fixed. Some people do want to get bombarded with technical info but those people are rare.
Most people =
