Instead of an expansion valve, a 1993 Buick Century V6 uses a restrictor tube or orifice tube in its air conditioning system. This performs a similar function of controlling refrigerant flow. The location is within the air conditioning system's refrigerant lines, likely close to the evaporator. It's usually difficult to access without removing some AC components.
For the cooling system (engine coolant), there's no expansion valve. The cooling system uses a radiator cap that acts as a pressure relief valve and a coolant reservoir tank to allow for expansion and contraction of the coolant as it heats and cools.
To find the AC orifice tube, you'll need a repair manual specific to your 1993 Buick Century V6. A schematic diagram of the AC system will show its precise location. Trying to locate it without a manual and the appropriate knowledge could lead to damage to the system.
