On the evaparotor (black canister) on passenger side
The smaller port is the Low side port.
The low pressure air conditioner port can be found on the top of the air conditioner compressor. The low pressure port should be labeled as such.
On the side of the receiver dryer. You can find a diagram of this on the below link. Just follow the "Find your low side Port" icon.
thel ow pressure side on the ac system is the biggest valve ( which is the blue one ) the other is smaller ( red ).
The Accumulator.
Trace the lines from the compressor. Probably toward the firewall you will find the low pressure port made to a sliver pipe.
There will be two. Generally the low pressure is mounted more towards the front of the car and will most likely have a blue cap on it. If it does not have a blue cap on it compare it with the high pressure port. The high pressure port (for r134a) will be wider with different threads where as the low pressure port will be slim. If you cannot find either search deeper with a flash light. Be careful to wait till the car has cooled, although you should not have to search too much as the low pressure port is generally easy to reach. Also, check the label on your car to make sure it is r134a coolant. If it says r12 you will need to take it in to have it charged with r12.
It is behind the engine, up near the firewall.
2, a high and a low pressure port
There are two aluminum tubes coming out of the compressor. The high pressure tube goes to the condensor in front of the radiator. The low pressure tube is the other tube, and the port has a black plastic cap over it, on the low pressure tube.
standing in front of the car, the low pressure port is on right hand side near the fuse box. the high pressure port is on the left hand side near the windshield washer fluid tank.
The low pressure port is located on the left side of the car towards the back of the engine compartment. It was a blue cap on it with a capital L.