How to Test an AC Expansion Valve

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Thermostatic expansion valves rely on pressure to regulate the flow of refrigerant.

Thermostatic expansion valves regulate the flow of refrigerant to an evaporator inside a cooling system. TEVs or TXVs, as they are often shorthanded, consist of a sensing bulb, capillary tube, diaphragm and valve body. The thermal bulb is partly filled with refrigerant and connected to the diaphragm by the capillary tube. The diaphragm closes and opens over the valve body to allow refrigerant to pass. The vapor pressure of the bulb acts to open the diaphragm, while the evaporator pressure and the force of a spring inside the valve body counteract the vapor pressure to close the diaphragm. If your air conditioning unit is not working properly, there is a simple test that you can conduct yourself to determine if the expansion valve has failed.


Step 1

Make sure the AC unit is plugged in or, if it is in an automobile, turn the vehicle on. You will be testing the function of the expansion valve, so it will need a source of power. Access the cooling unit by removing any covers, if necessary.

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Step 2

Use the manual to locate the thermostatic expansion valve. Look for a small bulb joined to a thin capillary tube that is connected to a generally T-shaped valve body.


Step 3

Lift the sensing bulb from the suction bulb well. The method of removal may vary depending on the type of AC unit. Refer to the manual, if necessary.

Step 4

Hold the sensing bulb in your hands for two minutes or place it in warm water.

Step 5

The warmth should activate the refrigerant to start flowing into the evaporator. You will be able to recognize this change audibly. If this does not occur, it can be an indication that the powerhead of the expansion valve has failed. Replace it with one of an appropriate capacity.



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