I've been around air conditioning my entire life. In the past, I avoided knowing too much, since I had a father, and brother that were experts. They'd help, so I avoided the troubleshooting. Things are different know. They're both gone, so I've sharpened my skills and do what I can.
My pickup AC started giving me trouble. It was short cycling, and before it could get cold, the compressor would shut off, only to repeat the process. I knew the process of short cycling usually indicated a low Freon charge, but I called a business acquaintance that's an expert for some information.
Long story short. His advice: "If the low pressure side is 45 psi, or so, stop hunting the problem and change the clutch cycling switch."
That wasn't the situation. It was cycling to the low pressure cutoff point and shutting down. I figured I'd find this before I started the engine due to the low pressure reading on the gauges in relation to the ambient temperature. According to the pressure of 79 pounds, it should have been much cooler than the 85 degrees Fahrenheit ambient temperature, so I wasn't real surprised when the cycle switch didn't appear to be the problem.
I started adding a can of R-134A, with dye, so I could check for leaks. Within an ounce or two, the compressor quit cycling and the low side pressure started rising. I quit at a half can with a decision to add some more the following day. The AC was blowing cold air, even though the suction pressure was still a little on the low side.
The following day, a quick glance of the pressure in reference to the ambient temperature showed my system was still low on Freon, so I added the rest of the can. After it was gone, the suction side pressure was 34 psi, which is an acceptable range. I would leave it alone for a day or two, then check it for leaks.
Early this morning, before the sun rose, I started looking on the engine for leaks. I used an ultraviolet pen light, with special glasses, to examine the different parts of the system. The condenser coils (by the radiator) showed no signs of a leak. The same for the hoses and fittings. The water dripping from the evaporator coil (in the dash) was clear and had no dye. The low side connection on the accumulator showed signs of dye, which was expected, since that's where the quick-break connections spew a little Freon when you disconnect the lines. Further examination revealed what I think is my problem: the clutch cyclic switch. It's covered with dye, but not where I would have expected the leak to be. It looks like it's internal to the switch, and not the connection. Most of the dye is around the electrical connector. I'll change the switch, and then examine everything one more time before I finish, which will require checking the high side and low side pressure and adding Freon if needed, which I think it will require.
So, after such a long post, what's my advice? Don't mess with automobile air conditioning unless you have some knowledge, or somebody to stand over your shoulder to guide you. Normally, I wouldn't even think of writing this, but after reading how some people just add Freon, without checking pressures and then adding it to no avail without checking the high pressure discharge of the compressor made me think of the possible problems that can develop. Sometimes it has nothing to do with the charge, and adding Freon can only compound the problems besides being a waste of money. Any major repair usually requires refrigerant recovery, parts replacement and a vacuum pulled on the system to verify there aren't any leaks and guarantee there isn't any moisture in the system. So, adding Freon to a system with a blockage, or bad compressor will only be wasted money.
Still, if you know what you're doing, it's good to do such repairs; at least it is for me. I like knowing how things work and I pinch pennies where I can. Even if you know enough to change parts - even major parts, such as the coils, or compressor - the final pulling of a vacuum, refrigerant charge and fine tuning of the system is usually best left to the experts. They have the equipment, specifications and experience.
I may be through with my repair, or not. The shrader valves could start leaking, which isn't uncommon. Besides wearing out, they have a tendency to start leaking when disturbed. I'll keep my fingers crossed they don't..