what does a ac contactor do


What Does an AC Contactor Do?

As summer approaches, many of us rely on our air conditioning systems to keep our homes cool and comfortable. Behind the scenes, there are several components that work together to ensure the smooth functioning of an AC unit. One essential component is the AC contactor, which plays a crucial role in controlling the flow of electricity to the air conditioner's compressor and condenser fan motor. In this article, we will explore in detail what an AC contactor is, how it works, and why it is such a vital part of your air conditioning system.

Understanding the Basics: What is an AC Contactor?

An AC contactor is an electrical switch that allows or disconnects the flow of electricity to certain parts of an air conditioning system. It acts as a relay between the power source and the compressor and condenser fan motor. When the thermostat calls for cooling, it sends a low-voltage signal to the AC contactor, energizing it and allowing the high-voltage current to flow through. These contactors are usually installed near the compressor inside the outdoor unit of the AC system.

How Does an AC Contactor Work?

When the voltage from the thermostat reaches the AC contactor, it magnetizes the contactor coil, causing the movable contacts inside to close. The closing of these contacts establishes an electrical connection between the power source and the compressor and condenser fan motor, allowing them to operate. Similarly, when the thermostat signals the end of the cooling cycle, the contactor coil is de-energized, causing the contacts to open, thus cutting off the power supply to the compressor and fan motor.

The contacts within an AC contactor play a vital role in managing the electrical load. Compressors and fan motors require a significant amount of power to function, and the contactor must be able to handle this load while ensuring the safety and efficient operation of the system. High-quality contactors are designed to withstand this high electrical load and frequent use, ensuring smooth operation and longevity of the air conditioning system.

The Importance of a Functional AC Contactor

The AC contactor is a critical component of an air conditioning system, and its proper functioning is essential for the overall performance and efficiency of the unit. A faulty or malfunctioning contactor can lead to various problems, including:

1. No Cooling: If the contactor fails, it will not allow electricity to reach the compressor and fan motor, resulting in no cooling or airflow from the system.

2. Frequent Cycling: A worn-out contactor may cause the compressor and fan motor to turn on and off rapidly. This frequent cycling can cause unnecessary stress on the system and increase energy consumption.

3. High Energy Bills: An inefficient contactor can prevent the compressor and fan motor from running at optimal levels, leading to higher energy consumption and increased utility bills.

4. Component Damage: In some cases, a malfunctioning contactor may cause the compressor or fan motor to receive incorrect voltage or experience voltage fluctuations, which can lead to premature component failure.

Signs of a Failing AC Contactor

Over time, an AC contactor may start showing signs of wear and tear. It is crucial to be aware of these signs and address them promptly to prevent further damage to the system. Some common signs of a failing AC contactor include:

1. Clicking Sound: If you hear a clicking sound coming from the outdoor unit without the system starting, it may indicate a faulty contactor that is not allowing the power to reach the compressor and fan motor.

2. Constant Running: On the other hand, if the contactor gets stuck in the closed position, the compressor and fan motor might keep running continuously even when the desired temperature has been reached.

3. Burnt or Pitted Contacts: Inspect the contacts inside the contactor for burn marks, pitting, or signs of damage. If you observe any of these signs, it likely means that the contactor needs to be replaced.

4. System Not Turning On: If your air conditioning system is completely unresponsive and does not turn on despite the thermostat settings, it is possible that a faulty contactor is to blame.

Replacing an AC Contactor

Replacing an AC contactor is a task that should be carried out by a qualified HVAC professional. The process involves several steps to ensure safe installation and proper functioning of the new contactor. Here is a brief overview of the replacement process:

1. Turn Off the Power: Before starting any work, turn off the power supply to the air conditioning system from the main electrical panel.

2. Remove the Cover: Open the access panel on the outdoor unit to gain access to the contactor.

3. Take Note of the Wiring: Take careful note of the wiring connections on the existing contactor. It is essential to replicate these connections accurately on the new contactor.

4. Disconnect Wires: Using a screwdriver or pliers, carefully disconnect the wires from the old contactor. Take note of the wire colors and where they were connected.

5. Remove the Old Contactor: Unscrew the contactor from its mounting plate and take it out of the unit.

6. Install the New Contactor: Position the new contactor in place and secure it to the mounting plate using the provided screws.

7. Reconnect Wires: Attach the wires to the appropriate terminals on the new contactor, following the notes made during the disconnection.

8. Replace the Cover: Close the access panel and ensure it is securely fastened.

9. Restore Power: Finally, turn the power supply back on from the main electrical panel.


An AC contactor is a vital component of an air conditioning system, responsible for controlling the flow of electricity to the compressor and condenser fan motor. Understanding its role and ensuring its proper functioning is crucial for the efficient operation and longevity of your AC unit. Regular maintenance and timely replacement of a faulty contactor are essential to avoid costly repairs and ensure a cool and comfortable home during those hot summer days. If you notice any signs of a failing contactor, it is recommended to consult with an HVAC professional to assess the situation and carry out any necessary repairs or replacements. Remember, a properly functioning contactor is the key to a well-performing air conditioning system.


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