how to check contactor on ac unit


How to Check Contactor on AC Unit

Is your air conditioning unit not functioning properly? Does it fail to cool your home or office as it once did? Before calling a professional technician and incurring costly repair bills, it may be worth checking the contactor on your AC unit. The contactor is an essential component that allows the flow of electricity to the compressor and condenser fan motor. Over time, contactors can become worn or damaged, leading to a breakdown in the operation of your AC unit. In this article, we will explore the steps to check the contactor on your AC unit, ensuring that your system is in optimal condition and eliminating the need for potentially unnecessary repairs.

Understanding the Contactor

A contactor is an electrical switch that is responsible for controlling the flow of electricity to various components of your AC unit. It consists of a coil and metal contacts that connect and disconnect the power supply. The coil is energized by a low voltage signal from the thermostat, and when it receives this signal, it creates a magnetic field that then closes or opens the contacts. When the contacts close, electricity flows to the compressor and condenser fan motor, allowing them to function. Conversely, when the contacts open, power supply to these components is interrupted, causing them to stop running. By understanding the role of the contactor, you can evaluate its performance and identify any issues that may be present.

Signs of a Faulty Contactor

Before delving into the process of checking the contactor, it is important to be aware of the signs that may indicate a faulty contactor. By recognizing these signs early on, you can address the issue promptly, preventing any further damage to your AC unit. Some common signs of a faulty contactor include:

1. Intermittent Cooling: If you notice that your AC unit is providing intermittent cooling, whereby it sporadically cools and then fails to cool, this could be indicative of a faulty contactor. When the contacts in the contactor become worn or damaged, they may fail to make a solid connection, resulting in intermittent power flow to the compressor and condenser fan motor.

2. Loud Clicking Sound: A clicking sound when your AC unit starts or stops can be a sign of a damaged contactor. As the contacts wear down, they may fail to operate smoothly, causing a distinct clicking sound when they close or open.

3. AC Unit Not Turning On: If your AC unit fails to turn on altogether, it is possible that the contactor is the culprit. A faulty contactor may prevent electricity from reaching the compressor and condenser fan motor, resulting in a complete system failure.

4. Stuck Contacts: In some cases, the contacts in the contactor can become stuck or welded together due to electrical arcing. This can result in a constant power supply to the compressor and condenser fan motor, causing the unit to continuously run without shutting off.

5. Burnt or Pitted Contacts: Upon visual inspection, if you notice that the contacts in the contactor appear burnt or pitted, this indicates significant wear and tear. These damaged contacts will hinder the flow of electricity and require immediate attention.

How to Check the Contactor

Now that we have identified the signs of a faulty contactor, let's delve into the process of checking the contactor on your AC unit. Please note that performing these steps requires basic knowledge of electrical systems and safety precautions. If you are not confident in your abilities, it is always best to consult a professional technician.

1. Turn Off the Power:

The first step in checking the contactor is to ensure that the power supply to your AC unit is turned off. Locate the electrical disconnect box, typically situated near the outdoor unit, and open the cover. Inside, you will find a switch or circuit breaker dedicated to the AC unit. Switch it to the "Off" position or turn off the dedicated circuit breaker.

2. Remove the Access Panel:

Next, remove the access panel on the side of the outdoor unit. Depending on the unit's make and model, you may need to unscrew the panel or release any latches holding it in place. Once removed, set the panel aside in a safe location for reassembly later.

3. Locate the Contactor:

With the panel removed, you will now have a clear view of the components inside the AC unit. Locate the contactor, which is typically a black or gray box-shaped component with multiple terminals and metal contacts.

4. Inspect the Contactor:

Perform a visual inspection of the contactor, paying close attention to the contacts and the overall condition of the component. Look out for signs of burning, pitting, or any visible damage to the contacts. Additionally, check for any loose or disconnected wires.

5. Test the Contactor:

To assess the contactor's functionality, use a multimeter, which is a device that measures electrical voltage, resistance, and current. Set the multimeter to the Ohms setting and proceed with the following steps:

a. Disconnect Wires:

Carefully disconnect the wires connected to the contactor's coil terminals. Take note of the colors and their respective locations to ensure correct reconnection later.

b. Measure Resistance:

With the multimeter probes, touch them to the coil terminals where the wires were previously connected. The multimeter should display a resistance value within the expected range. Consult the manufacturer's specifications or the AC unit's manual for the appropriate resistance value.

c. Check Contacts:

Next, use the multimeter probes to test the state of the contact terminals. While wearing appropriate safety gloves, carefully touch one probe to each of the contact terminals within the contactor. The multimeter should indicate no continuity or infinite resistance, indicating that the contacts are open. If the multimeter displays any other value, it suggests that the contacts are closed and stuck together.

6. Replacement and Reassembly:

If the contactor is faulty, it is recommended to replace it with a new one of the same specifications. Once you have the new contactor, follow the steps in reverse order to reassemble the AC unit. Ensure that all wires are securely connected and finalize by switching the power back on through the electrical disconnect box.


Regularly checking and maintaining the contactor on your AC unit is vital to ensure its optimal performance and longevity. By recognizing the signs of a faulty contactor and following the steps outlined in this article, you can confidently assess the contactor's condition and take appropriate measures to address any issues. Remember, electrical systems can be dangerous, so always exercise caution and consider seeking professional assistance if you are unsure about performing any troubleshooting or repairs.


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