The expansion valve releases and regulates refrigerant flow into evaporator. After the valve opens, the refrigerant passes through a small orifice causing temperature and pressure to drop. The refrigerant is then converted from a hot high-pressure liquid to a cold low-pressure. The cold liquid boils in the evaporator coil, cooling the air blowing into the truck cab. The proper amount of refrigerant flowing thru the evaporator is managed by the expansion valve, giving maximum cooling.