An AC expansion valve is a component of the air conditioning (AC) system. It is typically located near the AC evaporator core, on the firewall in the engine compartment or underneath the dashboard.
The expansion valve receives refrigerant (by way of the receiver/drier) at high pressure from the AC condenser. A tiny orifice in the expansion valve causes the refrigerant to expand rapidly as it passes through, reducing its pressure and absorbing large amounts of thermal energy (heat) from the evaporator in the process. Air blown by a fan through the cooled evaporator enters the passenger compartment.
The expansion valve is similar to a fixed “orifice tube” that serves the same function, except that the expansion valve is able to adjust the orifice size to accommodate varying conditions.