The expansion valve is placed at the evaporator inlet tube. The expansion valve is used to control refrigerant flow into the evaporator. Excessive refrigerant flow into the evaporator can cause evaporator flooding, which can result in diminished air conditioning performance and possible compressor damage. Too little refrigerant flow can cause evaporator icing or diminished air conditioning performance.

The expansion valve contains a variable orifice that is controlled by a sensing bulb placed inside the evaporator cooling fins. The sensing bulb is a sealed tube containing a small amount of refrigerant. The changes in temperature of the evaporator cause the refrigerant inside the sensing bulb to expand or contract. The action of the internal pressure of the sensing bulb controls the amount of refrigerant that flows through the expansion valve by varying the size of the orifice.

Expansion valves are used more often in air conditioning systems equipped in imported and older domestic vehicles. The systems can be identified by use of a receiver drier in the high pressure line of the air conditioning system, between the condenser and the expansion valve. Refer to an auto repair manual for accurate diagnostic flow charts covering your automobile’s A/C system.