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How AC expansion valve system works

Auto AC Expansion Valve Operation

How does an automotive AC expansion valve system work

Many car makers now use automotive AC expansion valve in their AC systems. An expansion valve is simply a refrigerant metering device to control the rate of flow of refrigerant into the evaporator located in your dash. In the illustration below, you’ll see that low pressure gas flows out of the evaporator coil and through the expansion valve. As it flows the refrigerant temperature is detected by the rod running through the port. The rod is attached to the diaphragm at the end of the valve. Based on the difference in temperature between a gas in the outer portion of the diaphragm and the temperature cooled by the rod, the diaphragm moves a center metering rod in or out of the high pressure port to meter the refrigerant into the evaporator. It’s really that simple.

Refrigerant flow through the system

When the AC compressor engages, it sucks low pressure refrigerant gas from the evaporator. The temperature of the gas changes the flow rate of high pressure liquid coming into the evaporator. The compressor then compresses the gas into a higher pressure and forces it into the condenser. During the compression cycle, the gas rises in temperature. Airflow across the condenser cools the high pressure gas, causing it to condense into a high pressure liquid.

The high pressure refrigerant passes through a receiver/dried to remove any moisture. It then flows to the metering port on the expansion valve. Once the high pressure liquid passes through the metering port, the liquid pressure drops and the liquid begins to fill the evaporator. Airflow across the evaporator causes the liquid refrigerant to boil and turn into a gas again. The amount of heat it takes to change the refrigerant from liquid to gas is what an AC system removes from your car.

AC expansion valve

How AC expansion valves work

What goes wrong with an expansion valve?

Expansion valves can fail in three ways:

    1. The metering rod gets clogged with AC sealant.
    2. The metering rod binds in the port
    3. The expansion valve diaphragm develops a leak, preventing the metering rod from moving.
clogged expansion valve

Clogged expansion valve

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Posted on by Rick Muscoplat

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