how to test contactor on ac


How to Test Contactor on AC


When it comes to troubleshooting an air conditioning system, one of the key components that requires attention is the contactor. A contactor is an electrical relay that controls the flow of electricity to different parts of the AC unit. Over time, contactors can become worn or damaged, resulting in problems with the operation of the system. To diagnose any issues with the contactor, proper testing is necessary. In this article, we will guide you through the process of testing the contactor on your AC system, ensuring that it is functioning optimally.

The importance of the contactor

The contactor plays a crucial role in the operation of an air conditioning system. It is responsible for controlling the flow of electricity to the compressor, condenser fan motor, and other components. When the thermostat calls for cooling, the contactor engages, allowing the necessary current to flow through the system. Without a properly functioning contactor, the AC unit may fail to turn on, or it may not be able to cycle on and off correctly. Therefore, it is crucial to regularly test and maintain the contactor to ensure the efficient functioning of your AC system.

Signs of a faulty contactor

Before jumping into the testing process, it is essential to identify the signs that may indicate a faulty contactor. By recognizing these symptoms, you can confirm whether the contactor is the culprit behind the AC system's malfunction. Here are some common indicators of a faulty contactor:

1. AC not turning on

If your air conditioner fails to turn on even after adjusting the thermostat, there is a possibility that the contactor is malfunctioning. The contactor may have become stuck or worn out over time, preventing the necessary electrical current from reaching the system.

2. AC not turning off

On the other hand, if the AC unit does not turn off even when the desired temperature has been reached, it may be due to a faulty contactor. In this case, the contactor may have welded contacts, which keep the electrical circuit closed, causing the system to run continuously.

3. Strange noises

A faulty contactor can also produce audible signs of malfunction. If you hear buzzing, humming, or clicking noises coming from the AC unit, it could indicate arcing within the contactor. Arcing can cause damage to the contact surfaces and may eventually lead to a complete failure of the contactor.

4. Intermittent cooling

If your AC is blowing cool air inconsistently or the cooling cycles are irregular, it could be an indicator of contactor issues. The contactor's improper functioning may disrupt the electrical flow to the compressor and condenser fan motor, resulting in fluctuating cooling performance.

5. Burn marks or corrosion

During a visual inspection of the contactor, if you notice any burn marks or signs of corrosion, it is a clear indication that the contactor needs testing. Burn marks may suggest damage to the contacts due to arcing or overheating, while corrosion can cause poor electrical connectivity.

If you experience any of these symptoms, it is crucial to test the contactor to identify the root cause of the problem and take corrective measures accordingly.

Testing the contactor

Now that we have discussed the importance of the contactor and the signs of a faulty one, let's dive into the process of testing it. Proper testing will help you determine whether the contactor is the cause of the AC system's issues, or if the problem lies elsewhere. Follow these steps to test the contactor on your AC system:

1. Ensure safety precautions

Before beginning any electrical testing, it is crucial to prioritize safety. Make sure to turn off the AC unit and disconnect it from the power source. Double-check that the power is indeed disconnected by attempting to turn on the AC. Additionally, wear safety gloves and goggles to protect yourself from any potential harm.

2. Locate the contactor

The contactor is typically located inside the outdoor condensing unit. It can be identified as a rectangular-shaped relay with multiple terminals and wires connected to it. In some cases, the contactor may be found inside the air handler unit, depending on the specific setup of your AC system.

3. Check for power

Using a multimeter set to the voltage range, carefully test the contactor for the presence of electrical power. The multimeter should be set to an appropriate voltage range, usually 240V for residential AC units. Touch the probes of the multimeter to the line-side terminals of the contactor while keeping a safe distance from any exposed wires or terminals.

When there is no power supply, the multimeter reading should show zero volts. However, if voltage is present, it indicates that the contactor is receiving electrical power. This reading implies that the contactor is not the cause of the AC system's failure to start.

4. Inspect for contactor coil continuity

The next step involves testing the electrical continuity of the contactor's coil. This testing determines whether the coil winding is intact, ensuring that the magnetic field required to close the contacts is functioning correctly. Set your multimeter to the continuity mode and touch the probes to the coil terminals of the contactor.

A proper functioning contactor should show continuity, indicating that the coil is not damaged or open. If the multimeter does not detect continuity, it signifies a faulty contactor requiring replacement.

5. Verify contactor switching

Finally, it is essential to test whether the contactor is effectively switching on and off when electrical power is applied. This test ensures that the contacts are making proper connections and transmitting electricity to the necessary components. With the power reconnected, set your multimeter to measure continuity and touch one probe to one of the line-side terminals while touching the other probe to the load-side terminal of the contactor.

When the thermostat calls for cooling, the contactor should engage, closing the contacts and allowing current to flow. The multimeter should display continuity, indicating that the contacts have successfully closed. Similarly, when the thermostat is satisfied and the system needs to turn off, the contacts should open, interrupting the current flow. The multimeter reading should show no continuity in this case.


In conclusion, properly testing the contactor on your AC system is vital to ensure the efficient functioning of the unit. By recognizing the signs of a faulty contactor and following the testing steps outlined in this article, you can accurately diagnose any contactor issues and take appropriate measures, such as contactor replacement, if necessary. Remember to always prioritize safety and consult a professional if you are unsure or uncomfortable performing the testing process yourself. Regular maintenance and testing of the contactor will contribute to the longevity and reliability of your air conditioning system, ultimately providing you with a comfortable environment all year round.


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