On a 1994 Chevy C1500 5.0 V8 it is the top valve with a black cover next to the coolant resivoir
look for the port with the black cap on it, it's the low pressure port.
The diagnostic port on a 1994 Chevy S-10 is located under the dash, near the hood release. It allows an OBDII code reader to be connected for troubleshooting problems.
You can find the air conditioning ports on top of the air conditioning compressor. There will be a low pressure port and a high pressure port.
on the evaporator / dryer
You can add Freon to your 1999 Chevy cooling system through the low pressure port. Connect your Freon bottle to the low pressure port and release the Freon into the system.
It is on the accumulator.
The 1994 Honda Civic air conditioning low pressure port is located on top of the air conditioner compressor. The low pressure port will have a blue top.
The 1994 Mercedes-Benz low side port can be found on the top of the air conditioner compressor. The low side port will be labeled as the low pressure port.
The smallest with a green cap on it.
The 2002 Chevy Cavalier low side service port can be found on the top of the air conditioner compressor. The low side service port will be labeled as the low pressure port.
The 1994 Buick Century air conditioning low side port can be found on top of the air conditioning compressor. The low side port will be labeled as the low pressure port.
Follow lines from compressor-Larger line is low pressure--port may be at or around the accumulator/drier by passenger firewall