How to recharge AC on Auto/Truck/car?

If your A/C system is low on refrigerant, recharge is a DIY-able task. Be aware that your system is leaking and may not hold a recharge very long. Also, if the system is too low for the compressor to run, it may not recharge by this method.
Below is a diagram of a typical system that I copied from familycar.com:


For more details on system operation, see http://www.familycar.com/ac1.htm
Typical leaks occur in the evaporator, condenser or compressor. A good shop can isolate and replace the leaking component.
There are still 2 types of auto A/C systems out there. The older systems use R-12, a.k.a. Freon. Freon is no longer available on the open market, but I believe licensed A/C mechanics can get it for a high price. All newer cars use R-134a, which is not as effective but is environmentally friendly. You can buy R-134a in any parts store or even at WALMART. You will also need a hose to recharge your system. Buy at the same location or borrow from a friend.
If you have the old R-12 system (indicated on the compressor tag), you will need a conversion kit to make your system run on R-134a. Even if you convert your system, you may or may not be happy with the performance of R-134a in a system that was designed for R-12.
Back now to recharge: the recharge hose uses a quick-disconnect fitting and will only fit on the low pressure port. The low pressure port is somewhere between the evaporator (inside the car) and the compressor (attached to the side of the engine near the front (end with the belt on it). Trace the aluminum pipes coming out of the engine compartment firewall (usually on the left side of the firewall as you look in from the front of the car) until you find the port on the one that goes to the compressor. Sometimes the dryer is on the low side and the port may be on the dryer. Here is an picture of a case wherein the low pressure port is on the dryer. The refill hose is attached via the quick disconnect fitting.

(Note, the high pressure port is on the line going from the radiator area back to the compressor and is a larger port)
Remove the cap from the port, pull back the ring on your hose QD fitting, shove it onto the port and release the ring--it should seat without leaking. Next, attach the can of R-134 to the other end of your hose (some of these have the can screw in and others use a cam to lock it in), close the valve on the hose (clockwise) all the way. It should pierce the can and open a hole to release the R-134. Now open the valve fully. The R-134 should not leak if you have the can seated in the hose fitting properly.
Now start the car and turn the A/C on max. You are now recharging your system. Jiggle the can without putting tension on the QD connection. Follow direction on the can. If there is oil or sealant mixed in the R-134, the can may instruct you to turn the can sideways occasionally to get everything into the system.
Note, some hoses come with a gage that you can use to determine when the system is full. I usually just go by how cool the air is and by the formation of frost on the low pressure line. If your can empties and the air is not as cold as you are used to getting from the system, put another can on. If you cannot empty the can after say 20 minutes, the system is probably full. In this case, close the valve. In either case, once your system is full or you are happy with the performance, disconnect the QD and replace the cap on the port. If you have R-134 left over, use it in another car. If you just leave it sitting, it will eventually leak out of the can thru the valve or at the fitting to the hose.

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