how to check a contactor on a ac unit


How to Check a Contactor on an AC Unit

Is your AC unit not working properly? Have you noticed that it's not turning on or off at the right times? If so, there's a good chance that the contactor on your AC unit is faulty. The contactor is a crucial component of the AC system that controls the flow of electricity to the different parts of the unit. In this article, we will guide you through the process of checking the contactor on your AC unit to determine if it needs to be replaced or repaired.

Understanding the Contactor

Before we delve into the process of checking the contactor, it's important to have a basic understanding of what it is and how it works. The contactor is an electrical switch that is responsible for controlling the flow of electricity between the AC unit's power supply and its components. It consists of a coil and a set of contacts, which are usually made of copper.

When the temperature inside your home rises and triggers the thermostat to activate the AC unit, the contactor receives an electrical signal. The coil in the contactor then creates a magnetic field, which attracts the contacts and closes the circuit, allowing electricity to flow to the compressor and fan motors. This initiates the cooling process.

Substandard Contactors: Signs and Symptoms

Over time, contactors can wear out or become damaged, leading to a range of issues with your AC unit. The following are common signs and symptoms that may indicate a faulty or malfunctioning contactor:

1. Unit Not Turning On: If your AC unit is not turning on at all, it may be due to a faulty contactor. A worn-out contactor may not be able to complete the circuit, preventing the flow of electricity to the various components of the unit.

2. Unit Not Turning Off: On the other hand, if your AC unit is constantly running and not turning off, the contactor may be stuck in the closed position. This means that the electrical circuit remains connected even when it should be disconnected.

3. Buzzing or Clicking Sounds: A buzzing or clicking sound coming from the AC unit can be an indicator of a faulty contactor. These sounds may occur when the contactor is struggling to make a connection or when it's trying to break a connection.

4. Visible Damage: It's essential to physically inspect the contactor for any visible damage. Signs of damage may include burnt or melted contacts, loose wires, or a dislodged coil. These issues can affect the contactor's ability to function properly.

5. Tripped Circuit Breaker: If your AC unit repeatedly trips the circuit breaker, it could be a sign of a failing contactor. When the contactor is faulty, it may result in an excessive electrical load that exceeds the capacity of the circuit, causing it to trip.

Checking the Contactor: Step-by-Step Guide

Now that you know the signs of a faulty contactor, let's explore the step-by-step process of checking it to confirm whether it needs to be replaced or repaired.

1. Safety First: Before starting any work on your AC unit, it's crucial to prioritize your safety. Turn off the power to your AC unit by switching off the circuit breaker that controls it. This will prevent any accidental electric shocks during the inspection.

2. Accessing the Contactor: The contactor is typically located inside the outdoor unit of your AC system. Remove the access panel by unscrewing or lifting it, following the manufacturer's instructions. This will provide you with clear access to the contactor.

3. Inspecting the Contacts: With the access panel removed, visually examine the contacts on the contactor. Look for any signs of burning, melting, or visible damage. If you notice any damage, it's a clear indication that the contactor needs to be replaced.

4. Testing for Continuity: Using a multimeter set to measure continuity, you can determine if the contacts are making a proper connection. Start by disconnecting the wires connected to the coil terminals of the contactor. Then, place the multimeter leads on each of the open coil terminals. If the multimeter shows a reading of infinity, it means that the contacts are open and not making a connection.

5. Coil Resistance Test: If the contactor passes the continuity test, you can proceed with testing the coil resistance. Set your multimeter to the appropriate resistance range and measure the resistance across the coil terminals. Compare the reading with the manufacturer's specifications. If the reading is significantly different, it may indicate a faulty coil.

6. Replacing the Contactor: If you've confirmed that the contactor is faulty, it's time to replace it. Start by disconnecting the remaining wires connected to the contactor, ensuring that you note down their exact positions. Then, unscrew the contactor from its mounting bracket and carefully remove it. Install the new contactor by following the reverse steps.


A properly functioning contactor is essential to ensure the efficient operation of your AC unit. By understanding the signs of a faulty contactor and following the step-by-step guide to check it, you can determine if it needs to be repaired or replaced. Remember to prioritize safety throughout the process and consult a professional if you're unsure or uncomfortable with handling electrical components. By addressing any issues with the contactor, you can restore the optimal performance of your AC unit and enjoy a cool and comfortable environment.


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