how ot test an ac contactor



Contactors are an essential component of an air conditioning system as they control the flow of electricity to the compressor and condenser fan motor. Over time, these contactors may wear out or become faulty, resulting in a malfunctioning AC unit. Therefore, regular testing of the AC contactor is crucial to ensure optimal performance and prevent any potential issues. In this article, we will delve into the process of testing an AC contactor to diagnose any problems and ensure its proper functioning. So, let's dive in and explore the various steps involved in testing an AC contactor.

Understanding the AC Contactor:

Before we proceed with the testing process, it is important to have a basic understanding of what an AC contactor is and how it works. An AC contactor is an electromechanical switching device that allows or interrupts the flow of electrical current to the compressor and condenser fan motor. It consists of three main components: a coil, contacts, and a set of springs.

When the AC unit is turned on, an electrical current passes through the coil, creating a magnetic field that pulls the contacts together, closing the circuit and allowing electricity to flow to the motor. When the AC unit is turned off, the coil is no longer energized, and the contacts spring back to their original position, interrupting the electrical flow. This simple yet critical mechanism helps in controlling the operation of the AC unit.

Tools Needed for Testing:

Before we begin testing the AC contactor, it is crucial to gather the necessary tools to ensure a seamless and efficient testing process. Here are the tools you will need:

1. Multimeter: A multimeter is an essential tool for testing electrical circuits and measuring voltage, current, and resistance. It allows you to check the continuity of the contactor's coil and contacts, ensuring they are functioning properly.

2. Insulated Screwdriver: An insulated screwdriver is necessary for accessing the contactor and making any necessary adjustments during the testing process. It helps prevent electrical shocks and ensures your safety while working with electrical components.

3. Safety Gloves and Goggles: Safety should always be a top priority when working with electrical components. Wearing gloves and goggles will protect you from potential accidents and electrical hazards.

Now that we have the required tools, let's move on to the step-by-step process of testing an AC contactor.

Testing the AC Contactor:

Step 1: Safety First!

Before you start testing the AC contactor, make sure to turn off the power supply to the air conditioning system. This will help prevent any electric shock or accidental damage to the AC unit. Use caution and follow all necessary safety precautions throughout the testing process.

Step 2: Accessing the Contactor:

To begin the testing process, you need to locate the AC contactor within your air conditioning unit. Usually, the contactor is situated in a metal box near the condenser unit. Remove the access panel to gain access to the contactor. Be careful while removing the panel, as there might be sharp edges.

Step 3: Inspecting the Contactor:

Now that you have accessed the contactor, a visual inspection is the first step to identify any visible signs of damage or wear. Look for burnt marks, pitted contacts, loose wires, or any other irregularities that may indicate a faulty contactor. If you notice any significant damage, it is recommended to replace the contactor with a new one.

Step 4: Testing the Coil:

Using the multimeter, set it to the resistance (ohms) mode. Locate the contactor's coil terminals. These terminals are usually labeled as "C" (common), "NO" (normally open), and "NC" (normally closed). Touch the multimeter probes to the coil terminals and check the resistance reading displayed on the multimeter. An infinite resistance reading indicates an open coil, requiring a replacement. If the resistance reading is within the specified range mentioned in the manufacturer's instructions, the coil is functioning correctly.

Step 5: Testing the Contacts:

To test the contacts, set the multimeter to the continuity mode. Gently separate the contacts by prying them apart using an insulated screwdriver. Place one multimeter probe on the stationary contact and the other probe on the movable contact. If the multimeter beeps or indicates continuity, it means the contacts are functioning correctly. However, if there is no continuity, it indicates a faulty contactor, requiring a replacement.

Step 6: Observing the Coil Activation:

With the power supply still turned off, you can perform an additional test to observe the contactor's coil activation. Turn on the AC system and observe the contactor's coil. If the coil is energized, it should create a strong magnetic field, causing the contacts to close and the unit to start. Conversely, if the coil is not energized, the contacts will remain open, and the unit will not start. This test ensures that the coil is receiving power and functioning properly.


Testing an AC contactor is an essential maintenance step to ensure the optimal functioning of your air conditioning system. By following the step-by-step process discussed in this article, you can identify any issues with the contactor's coil or contacts and take necessary corrective measures. Regular testing of the AC contactor can prevent potential malfunctions and extend the lifespan of your AC unit.

Remember always to prioritize safety while working with electrical components and follow the manufacturer's instructions. If you are unsure or uncomfortable performing the testing yourself, it is recommended to seek professional assistance from a qualified HVAC technician. By maintaining and testing your AC contactor regularly, you can enjoy the comfort of a well-functioning air conditioning system all year round.


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