Air conditioning is a fundamental need for a comfortable and healthy indoor atmosphere. Over time, the wear and tear of an AC unit can lead to certain problems, and one of them is a damaged contactor. An AC contactor is an essential part of the heating and cooling system that controls the high-voltage power that enters the unit’s compressor and adjusts the refrigerant flow. It is vital to check the contactor’s condition to ensure your AC system’s efficient functioning. In this article, we will discuss different steps to test AC contactors to determine their working capacity and the problems that may arise.

1. Understanding the Role of AC Contactors in HVAC Systems

In any HVAC system, the AC contactor plays a vital role in ensuring that the compressor and fan motor are powered up correctly. These electrical switches are responsible for providing the electrical connection between the control circuit and the high voltage parts of the system. They essentially take the incoming AC voltage and use it to power up the compressor and fan motor.

The AC contactor consists of three parts: the coil, contacts, and auxiliary contacts. The coil is responsible for pulling the contacts together and switching over the power supply to the high voltage parts of the system. Once the connections are made, the auxiliary contacts come into play, providing a low voltage signal to the control circuit, indicating that the system is running properly.

Without AC contactors, HVAC systems simply wouldn’t work. They are essential for providing power to the compressor and fan motor, ensuring that the system runs smoothly and efficiently. However, like any other component in an HVAC system, AC contactors can also wear and tear over time, leading to potential issues with system performance. Understanding their role is crucial in troubleshooting these issues and ensuring their optimal performance.

2. Signs of a Faulty AC Contactor: What to Look For

One of the essential components of an HVAC system is the AC contactor. It is responsible for controlling the flow of electricity from the main power source to the compressor and the condenser fan motor. When the AC contactor starts to fail, it can cause serious problems to your HVAC system, leading to expensive repairs if not fixed promptly. For this reason, it is crucial to know the signs of a faulty AC contactor to address the issue right away.

Buzzing or Humming Sound

If you hear a buzzing or humming sound coming from your HVAC system when you turn it on, it could be a sign of a damaged AC contactor. The sound is an indication that the contactor is not functioning correctly, and it might need replacement.

AC Unit Fails to Turn On or Off

Another sign of a faulty AC contactor is when the AC unit fails to turn on or off. When the AC contactor is damaged, it can cause a delay in the system’s functions, making it difficult for your AC unit to turn on or off. It may also cause the system to shut down unexpectedly, leading to discomfort and inconvenience.

Loud Clicking Noises

A loud clicking noise coming from your air conditioning unit can indicate that the contactor is wearing out and needs replacing. The sound is often an indication that the contactor is struggling to make a connection and may cause significant damage to your unit if not addressed promptly.

If you notice any of these signs, it is essential to troubleshoot the problem immediately. A faulty AC contactor can cause poor performance in your AC unit and reduce its lifespan if left unaddressed. In the next section, we’ll discuss the equipment you need to test your AC contactor effectively and safely.

3. Equipment Needed for Testing AC Contactors: A Comprehensive Checklist

Before you start testing an AC contactor, it is essential to have the right tools and equipment by your side. This is to ensure that you are safe and that the testing process is carried out efficiently. In this section, we will provide you with a comprehensive checklist of the equipment you will need for testing AC contactors.

1. Safety Equipment: Your safety should always come first, especially when dealing with electrical components. You will need to have safety equipment such as non-conductive gloves, safety glasses, and insulated screwdrivers.

2. Digital Multimeter: A digital multimeter is an essential tool for testing AC contactors. It is used to measure the voltage, resistance, and continuity of electrical circuits.

3. Voltage Tester: A voltage tester is used to determine whether a circuit is live or not. It is essential when testing AC contactors to ensure that the power is turned off before testing.

4. Insulated Screwdrivers: Insulated screwdrivers are necessary for making adjustments or testing AC contactors. The insulation prevents any electrical shock.

5. Wiring Diagram: A wiring diagram is a schematic representation of the electrical circuits in your HVAC system. It is essential when testing AC contactors to know which wires you are testing.

6. Ohmmeter: An ohmmeter is used to measure the resistance of a circuit. It is necessary when testing the continuity of the contactor.

7. Crocodile Clips: Crocodile clips are used to connect the multimeter to the electrical components that you are testing.

8. Access to Power Supply: You need access to the power supply of the HVAC system that you are testing. This is important for testing the AC contactor when it is under load.

In conclusion, having the right equipment for testing AC contactors is essential to ensure that you accomplish the task safely and effectively. Without these tools, you risk damaging the components or worse, injuring yourself. Make sure that you have everything listed on this checklist before testing AC contactors.

4. Step-by-Step Guide on How to Test AC Contactors Safely and Effectively


In this section, we will discuss a comprehensive step-by-step guide on how to test AC contactors. It is vital to follow these steps diligently to ensure accurate testing and prevent damage or personal injury. Let’s begin:

Step 1: Turn Off Power Supply

The first step is to turn off the power supply to the HVAC unit. This ensures your safety during testing and prevents damage to the equipment. Use a voltage detector to confirm that there is no electrical current flowing through the unit.

Step 2: Open the Control Panel

Access the control panel of the HVAC unit and locate the contactor. It is usually a black or silver-colored box with wires connected to it.

Step 3: Inspect the Contact Points

Inspect the contact points of the contactor for any signs of damage, melting, or burning. If there are any visible issues with the contact points, replace the contactor immediately.

Step 4: Test for Continuity

With a multimeter set on the continuity test setting, test the contactor’s terminals for continuity. Place one probe on a terminal, and the other on the adjacent terminal. There should be continuity, indicating that the contactor is functioning correctly.

Step 5: Test for Voltage

With the multimeter set to the voltage test setting, test for voltage at the contactor’s terminals. Turn on the power supply for the HVAC unit and test each terminal. There should be a reading of 24 volts or more. If there is no voltage reading, it indicates a faulty contactor or a problem with the system’s wiring.

Following these steps will help you test your AC contactor safely and effectively. It is crucial to seek professional assistance if you encounter any complicated issues with your AC contactor or HVAC system.

5. Troubleshooting Common Issues and Solutions When Testing AC Contactors

Testing AC contactors is an essential part of HVAC maintenance. During the testing process, there are several common issues that technicians can encounter, and troubleshooting them will help to ensure the system operates smoothly.

Common Issues and Solutions

If the contactor fails to engage after testing it, it could be due to insufficient power supply or voltage issues. Check the voltage level at the circuit breaker, and if there’s no power supply, then call an electrician to fix it. On the other hand, if the voltage is low, a loose or corroded connection could be causing the issue, and tightening the connection or cleaning it should solve the problem.

Another common issue is a faulty coil in the contactor. The coil is responsible for generating the magnetic field that pulls the contactor down, and if it’s not working correctly, it won’t be able to engage. To fix this, you’ll need to replace the coil altogether to ensure the contactor works correctly.

Lastly, if the contactor is sticking, it could be due to debris or rust build-up on the surface. Cleaning the contactor with a soft-bristled brush and a cleaning solution should help dislodge any debris and restore the contactor’s functionality.

In conclusion

Testing AC contactors is a crucial aspect of HVAC maintenance, and troubleshooting common issues will ensure the system operates smoothly. Technicians must know how to address these problems when they arise to avoid system breakdowns. However, for complex issues, it’s always best to seek professional assistance to prevent further damage to the system.

6. Advantages of Regularly Testing AC Contactors for Optimal System Performance

Regular testing of AC contactors in HVAC systems can significantly improve the overall performance and durability of the unit. In this section, we will discuss the benefits of regular testing and how it can help prolong the life of your system.

Improved Energy Efficiency

AC contactors that are faulty or not working at their optimum level can cause the HVAC system to function below its expected energy efficiency. This can result in higher energy bills and reduced performance. Regular testing of the AC contactors can detect any issues and allow for quick, cost-effective repairs, ultimately saving energy and money in the long run.

Reduced Maintenance Costs

Regularly testing AC contactors can help detect any electrical issues before they become major problems. This can prevent costly repairs and replacements down the line, reducing the overall maintenance costs of the HVAC system. Early detection and repair of any faults help prolong the life of the contactors and reduce overall system downtime.

Improved Comfort and Air Quality

A faulty AC contactor can cause the HVAC system to malfunction, leading to inadequate cooling or heating of the building. This can cause discomfort to building occupants and impact indoor air quality. Regular testing of the AC contactors ensures that the HVAC system is functioning at optimum levels and can effectively cool or heat the building while improving the air quality.

In conclusion, regular testing of AC contactors is crucial for optimal system performance and functionality. It can help improve energy efficiency, reduce maintenance costs, and improve indoor comfort and air quality. It is recommended to seek professional assistance for complex AC contactor issues to ensure a safe and effective testing process.

7. Importance of Seeking Professional Assistance for Complex AC Contactor Issues

While testing AC contactors can be a straightforward process, there are instances when complex issues can arise, which may require professional assistance. Attempting to fix complicated problems without the proper knowledge and experience can result in significant damages, costly repairs, and safety hazards.

Identifying Complex AC Contactor Issues

Some signs of complex AC contactor problems may include abnormal noises, burning smells, frequent tripping, and voltage fluctuations. These issues may indicate wiring problems, issues with the control circuit board, or faulty capacitors, among others.

Why Seek Professional Assistance for Complex AC Contactor Problems?

Professional HVAC technicians have the expertise and tools that can help diagnose and fix complex AC contactor issues. These experts can provide prompt and efficient solutions that can save homeowners valuable time and money. Also, seeking professional assistance can prevent accidents and injuries that can occur during complex repairs.

When dealing with complex AC contactor problems, it’s better to seek professional assistance to avoid further damages to the system and ensure safe operation. Homeowners can reach out to licensed HVAC contractors who can provide reliable and efficient solutions for all their HVAC needs.

People Also Ask

How do you test an AC contactor with a multimeter?

To test an AC contactor with a multimeter, turn off the power supply and remove the wires that connect to the contactor. Set the multimeter to the resistance mode and touch the probes to the contactor’s terminals. If the reading is zero, then the contactor is in good condition.

How do you know if an AC contactor is bad?

To know if an AC contactor is bad, first, check for visible signs of damage such as melting around the contacts or burnt wires. If there are no visible signs, use a multimeter to check the resistance of the contactor. If the resistance reading is above the normal range, then the contactor is bad and needs to be replaced.

What causes an AC contactor to fail?

An AC contactor can fail due to several reasons, including overheating, electrical overload, corrosion, or worn-out contacts. Dirt and debris buildup can also cause malfunctioning of the contactor since it can prevent the contacts from closing correctly.

Can I replace an AC contactor myself?

Yes, You can replace an AC contactor yourself if you have the proper knowledge and tools. However, if you are not comfortable working with electrical components, it is recommended to hire a professional to do it for you.

How often should AC contactors be replaced?

After 10 years of use, the contactor needs to be replaced, but that could be earlier depending on how frequently they are used and if they are exposed to harsh weather conditions. Over time, the contacts can become pitted, corroded, or contaminated, causing them to stick together.


To test an AC contactor, you need to turn off the power supply, remove the wires connected to it, and use a multimeter to measure resistance. If the resistance is within the normal range, then the contactor is good; otherwise, it needs to be replaced. Replacing the contactor should only be done by a professional unless you have experience in dealing with electrical components. Regular maintenance can help extend the contactors’ life and avoid premature failure.