how to test a ac contactor



As an essential component in an air conditioning system, the AC contactor is responsible for controlling the flow of electricity to the compressor and fan motors. Over time, these contactors can wear out or become faulty, leading to issues with your AC unit. Regular testing of the AC contactor is crucial to ensure its proper functioning and prevent any potential malfunctions. In this article, we will guide you through the process of testing an AC contactor, step by step, to help you diagnose any problems and potentially save on costly repairs.

Checking for Power:

One of the first steps in testing an AC contactor is to check for power. Begin by switching off the power supply to the air conditioning unit from the breaker panel. It is essential to ensure your safety by disconnecting power before conducting any tests. Once the power is off, locate the contactor, which is typically a black box with several electrical wires connected to it.

Inspecting for Damage:

After verifying that there is no power running to the unit, the next step is to inspect the contactor for any visible damage. Carefully examine the contactor's housing, terminals, and electrical connections for signs of burning, melted plastic, or loose wires. These indications could imply an underlying issue with the contactor, which may require further investigation or even replacement. If you notice any obvious damage, it is advisable to consult a professional technician for assistance.

Testing for Continuity:

Continuity testing is a crucial step in determining if an AC contactor is functioning correctly. To conduct this test, begin by using a multimeter set to the continuity or resistance mode. With the power still turned off, remove the wires connected to the contactor terminals, making sure to label or take a picture of their placement for reinstallation. Once the wires are disconnected, place the multimeter probes on each terminal of the contactor, one probe on the common terminal and the other on each of the load terminals. If the multimeter beeps or shows zero resistance, it indicates that the contactor has continuity and is functioning properly.

Testing the Coil:

The coil is a vital part of the AC contactor, controlling the electromagnetic function of the device. To test the coil, set the multimeter to the appropriate voltage setting, usually 24V AC. Carefully touch the multimeter probes to the coil terminals, ensuring they make good contact. If the multimeter displays the correct voltage or there is a reasonably close reading, it signifies that the coil is working correctly. However, if there is no voltage or a significantly different reading, the coil may be faulty, and further inspection or replacement may be necessary.

Testing the Contacts:

The contacts within the AC contactor can wear out over time due to frequent use or environmental factors. To test the contacts, you will need to remove the cover of the contactor, which is usually held in place by screws or clips. Once the cover is removed, visually inspect the contacts for any signs of pitting, burning, or excessive wear. If the contacts appear damaged, they will likely need to be replaced.

To perform a more accurate test on the contacts, set the multimeter to the resistance mode and touch one probe to the stationary contact and the other to the moving contact. The multimeter should display a low resistance or continuity reading, indicating that the contacts are making proper electrical connection. If there is no continuity or a high resistance reading, it suggests that the contacts are worn out and need replacement.


In conclusion, regularly testing the AC contactor is crucial for ensuring the smooth operation of your air conditioning system. By following the steps outlined in this article, you can effectively diagnose potential issues and address them promptly. Remember to exercise caution and turn off the power supply before conducting any tests to ensure your safety. If you encounter difficulties or are unsure about the testing process, it is recommended to seek professional assistance. Regular maintenance and periodic contactor testing will contribute to the longevity and performance of your air conditioning system, ultimately providing you with a cool and comfortable living environment.


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