how to check an ac contactor with an ohmeter


How to Check an AC Contactor with an Ohmmeter


AC contactors are crucial components in the functioning of air conditioning systems. They control the flow of electricity to the compressor and ensure smooth operation. However, over time, contactors can experience wear and tear, leading to potential faults and issues. It is important to regularly inspect and test the contactor to ensure its efficiency. One such method to verify the contactor's health is by using an ohmmeter, a handy tool to measure electrical resistance. In this article, we will guide you through the process of checking an AC contactor with an ohmmeter to help you maintain the optimal performance of your air conditioning system.

Understanding AC Contactors

An AC contactor is an electrical switch that controls the flow of electricity to various parts of an air conditioning system. It works in conjunction with a thermostat, receiving a signal when cooling is required. The contactor then connects the power supply to the compressor and fan motors, allowing them to operate. Since contactors are frequently subjected to high voltage and electric arcs, they undergo significant wear and can develop faults over time.

When a contactor becomes faulty, it may cause the air conditioning system to malfunction, resulting in reduced cooling or even a complete system failure. Therefore, regular inspection and testing of the contactor are essential to ensure its optimal functioning. Using an ohmmeter can help diagnose any potential issues and determine if the contactor requires replacement.

Gathering the Necessary Tools

Before diving into the process of checking the AC contactor, you will need a few tools. Here's a list of what you'll require:

1. Ohmmeter: An ohmmeter is a device used to measure electrical resistance. It is available at hardware stores and online retailers.

2. Gloves: Since contactors are delicate and have electrical connections, it's advisable to wear gloves to protect yourself during testing.

3. Safety goggles: Safety goggles protect your eyes in case of any sparks or debris when accessing the contactor.

4. Screwdriver: A screwdriver will be necessary to open the electrical panel and access the contactor.

Step-by-Step Process to Check an AC Contactor

Now, let's dive into the step-by-step process of checking an AC contactor using an ohmmeter.

1. Preparing for the Test

Before beginning any maintenance or testing, it is crucial to ensure your safety. Remember to turn off the power supply to the air conditioning unit. This can usually be done by flipping the corresponding breaker switch in the electrical panel.

Once the power is turned off, put on your safety goggles and gloves to protect yourself. Safety should always be a priority during any electrical inspection or repair.

2. Accessing the Contactor

The contactor is located inside the electrical panel of the air conditioning system. Using a screwdriver, carefully remove the screws securing the panel cover. Gently lift the cover, exposing the internal components of the unit.

Note: If you are unsure about accessing the electrical panel or dealing with electrical wiring, it is highly recommended to consult a professional technician.

Once the panel cover is removed, locate the contactor. It is a box-shaped component with multiple electrical connections.

3. Identifying the Contactor's Terminals

To properly check the contactor, it is essential to identify its terminals. Terminals are the metal connectors where wires are attached.

Typically, contactors consist of three main terminals: L1, L2, and T. L1 and L2 are the power input terminals, while T is the terminal connected to the thermostat. These terminals can vary depending on the make and model of your air conditioning system.

4. Testing Continuity using an Ohmmeter

Continuity testing with an ohmmeter can help determine whether the contactor is functioning properly or if there is a fault. In this test, we will be checking for electrical resistance.

Set your ohmmeter to the lowest resistance setting (usually labeled as "Rx1" or "Continuity"). Touch the ohmmeter leads together to ensure that there is a signal, resulting in a resistance reading close to zero.

Next, using the screwdriver, gently disconnect the wires from the contactor terminals. Take note of the location and connection of each wire to ensure proper reconnection later.

With the ohmmeter still set to continuity, touch one ohmmeter lead to one of the power input terminals (L1 or L2) and the other lead to the thermostat terminal (T). The ohmmeter should display a resistance reading.

A low or zero resistance reading indicates the contactor is faulty, suggesting a short circuit. On the other hand, an infinite resistance reading (displayed as "OL" or "Open") implies an open circuit and calls for a replacement.

Repeat the process by touching each power terminal with the thermostat terminal to check for consistent resistance readings. If any resistance readings deviate significantly, it may indicate a problem with the contactor.

5. Inspecting and Replacing a Faulty Contactor

If you have identified a faulty contactor during the continuity test, it is crucial to replace it with a new one. Contactors are manufacturer-specific and can be obtained from HVAC suppliers or online retailers.

To replace the contactor, first, ensure the power supply to the air conditioning unit is switched off. Then, remove the faulty contactor by detaching the wires from its terminals. Take note of the wire connections or take a picture to assist with reinstallation.

Install the new contactor by connecting the wires to their respective terminals as noted or photographed earlier. Once all connections are secure, place the contactor back into its designated location within the electrical panel. Finally, reattach the panel cover using the screws.


Regularly checking the AC contactor using an ohmmeter is a vital maintenance task to ensure the uninterrupted operation of your air conditioning system. By following the step-by-step process outlined in this article, you can identify and resolve any issues with your contactor promptly. Remember to prioritize safety at all times and consult a professional technician if you are unsure about any aspect of the testing process. By maintaining a healthy contactor, you can enjoy efficient cooling comfort throughout the year.


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