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You can locate the low side charging valve by following the line that runs from the compressor back to the firewall; it is often on one end or the other of this hose. If not, then look in the line running back from the firewall towards the condenser in the front of the car. These 2 lines make up the low pressure side of the system, and the low serice port must be in one of them.
The one you have found is the high pressure side; it is a different size to prevent you from accidentally connecting a charging adapter for the low side to it; if the system is operating and the refrigerant can were connected to it, the can would explode violently from the 300 psi pressure in that port.
<smile> now that I have answered your original question, allow me to offer a little professional advice?
You really should think twice before attempting this particular service. The kits you buy in the stores have no provision for measuring the amount of refrigerant you are adding, or of removing air from the line so that you don;t push it into the system along with the refrigerant (an extremely bad thing to do; moisture in teh air causes corrosion damage to internal components, and will cause it to not cool). To determine if refrigerant is actually needed, and how much, requires a manifold gauge set; to add refrigerant accurately requires a digital scale accurate to less than one ounce. The reason is that the system refrigerant capacity is absolutely critical to correct operation; if you ahve 2 ounces too much or 2 ounces too little in it, the system will not cool properly. Too much refrigerant will cause compressor damage. Since you have no way to determine charge level or to measure the refrigerant into the system, the odds of making it cool correctly without causing damage down teh road are not very good; I do this for a living and would never attempt to charge a modern system using 14 ounce cans.
One other consideration is to make sure that what you are putting in contains no "sealer" type of material. These sealer materials spread throughout the system, and the theory is that they will work like radiator stop leak does: when they contact moisture in the outside air they harden into a solid material. The problem is, once this stuff is in the system, it can never again be repaired without replacing a lot of expensive components. If you spring a larger leak and it goes empty, or if the system must be drained and opened for something like an O-ring replacement, this stuff hardens everywhere, essentially destroying the A/C system on the vehicle. ALso, it cannot be evacuated and recharged ever again by most repair shops; it this sealer should enter a shop's AC charging machine, it does the same thing; essentially destroying a $5000 piece of equipment. For this reason, most shops perform a chemical test for such sealers before working on a ve3hicle. If they are found, many repair shops turn the vehicle away, as they are unable to empty the system to work on it.
I hope this is helpful to you, and saves you some $$; if so, an accept would be most aprpeciated! Thanks!