Where is the orifice tube 1990 Chevy truck?

On a 1990 Chevy truck, the orifice tube is located within the A/C system's high-pressure side, usually near the receiver/dryer. It's not something you'll easily see without disassembling part of the A/C system. It's a small, usually metallic tube with a precise restriction. You'll likely need a shop manual or a detailed diagram of your specific truck's A/C system to pinpoint its exact location.