The orifice tube on a 1990 Ford E-150's air conditioning system is
located within the high-pressure side of the AC system, usually near the receiver/dryer. It's not easily accessible without some disassembly of the AC components. You'll likely need to remove some parts to see it. Precisely where depends on the specific configuration of the vehicle's under-hood layout, but it's almost always tucked away somewhat.
Because working with automotive A/C systems involves handling refrigerants that can be dangerous, it's strongly recommended to have a qualified mechanic perform any work on the orifice tube or other AC components. Improper handling of refrigerants can lead to injury or environmental damage.
