When liquid refrigerant is forced through a restricted opening, it changes from a liquid to a gas. This change of state causes the refrigerant to become cold, and as it passes through the system, it absorbs heat from the passenger compartment. The vehicle's air-conditioning relies on this change of state to operate, and the expansion valve provides the restriction used to control the volume and pressure flowing through the system. Diagnoses of expansion valve malfunctions are done with an A/C gauge set.
Attach the gauges to the A/C system by locating the low-pressure and high-pressure service ports. The low-pressure port can be found on the low side line between the accumulator (sometimes on the accumulator) and the compressor. The accumulator is the cylindrical part attached to the system near the passenger side of the firewall. The high-pressure port is located on the high side line between the condenser and evaporator. The condenser is the radiator-like part located in the front of the vehicle, and the evaporator is located under the dash. Make sure the valves are closed on the gauge assembly, and attach the quick-disconnect hoses to the service ports.
Start the engine and set the A/C controls to max A/C, medium blower speed and recirculating air. Allow the system to run for five minutes for pressures to stabilise.
Normal gauge readings will be between 30 and 45 psi on the low side, and 200 to 350 psi on the high side. The expansion valve can fail in one of two ways. When stuck open, it allows two much refrigerant flow, and a drop in high side pressure. A gauge reading of less than 200 psi on the high side will be seen when this happens. When stuck closed, a higher than normal high side reading will be seen. Typically, this will be over 400 psi and climb until the high-pressure switch shuts down the system. Very low or very high readings on the high-side gauge indicate a faulty expansion valve.