how to test an ac contactor



An AC contactor is a vital component in air conditioning systems that controls the flow of electricity to the compressor and motor. Over time, contactors can wear out or malfunction, leading to various issues like failure to start or stop the compressor, intermittent cooling, and inefficient operation. Testing an AC contactor can help identify any problems and ensure that it is functioning optimally. In this article, we will delve into the process of testing an AC contactor to diagnose and troubleshoot any issues that may arise.

What is an AC Contactor?

An AC contactor is an electrical component that is responsible for initiating and breaking the flow of electricity to the compressor and motor of an air conditioning unit. It comprises an electromagnet and a set of contacts that are mechanically linked. When the coil of the electromagnet is energized, it generates a magnetic field that pulls the contacts together, allowing electricity to flow and activating the compressor. When the coil is de-energized, the contacts spring apart, breaking the electrical circuit and shutting down the compressor.

Why Test an AC Contactor?

Testing an AC contactor is crucial to ensure proper operation of your air conditioning system. Over time, contactors can accumulate dirt, dust, and debris, leading to electrical resistance and poor connectivity. This can result in several issues such as the compressor not turning on or off, intermittent cooling, or inefficient cooling. By testing the contactor, you can identify and rectify any problems before they escalate, ultimately improving the performance and lifespan of your air conditioning unit.

Tools Required for Testing an AC Contactor:

Before proceeding with the testing procedure, it's essential to gather the necessary tools. Here are the tools required for testing an AC contactor:

1. Multimeter: A multimeter is a versatile tool used to measure voltage, continuity, and resistance. It is essential for testing the electrical parameters of the contactor.

2. Screwdriver: A screwdriver with insulated handles is necessary for removing the cover of the contactor and accessing its internal components.

3. Safety Gloves: Safety gloves should be used to protect your hands from any potential electrical hazards during the testing process.

4. Safety Glasses: Safety glasses provide eye protection against any electrical sparks or debris that may occur while handling the contactor.

Testing Procedure:

Now that you have the necessary tools, let's dive into the step-by-step procedure for testing an AC contactor:

Step 1: Turn Off the Power:

Before starting the testing process, it is crucial to turn off the power supply to the air conditioning unit. This prevents any electrical mishaps and ensures your safety during the procedure. Locate the circuit breaker or disconnect switch that controls the power supply to the AC unit and switch it off.

Step 2: Access the Contactor:

Next, you need to access the contactor by removing the cover or access panel of the air conditioning unit. Typically, the contactor is located near the compressor and can be accessed by removing a few screws. Before proceeding, make sure to wear safety gloves and glasses for added protection.

Step 3: Inspect for Physical Damage:

Once you have accessed the contactor, visually inspect it for any signs of physical damage or wear. Look out for burnt or melted components, loose connections, or signs of overheating. If you notice any significant damage, it may be necessary to replace the entire contactor rather than attempting to repair it.

Step 4: Test for Voltage:

To test the contactor for voltage, set your multimeter to the AC voltage setting. Carefully touch the probes of the multimeter to the input terminals of the contactor. The input terminals are where the power supply wires are connected. Ensure that the probes make a secure connection with the terminals.

Step 5: Check for Proper Voltage:

With the multimeter probes securely connected to the input terminals, turn on the power supply to the air conditioning unit. The multimeter will display the voltage readings. Confirm that the voltage matches the specifications mentioned on the contactor. If the voltage is significantly lower or higher than the recommended range, it indicates a problem with the power supply or contactor.

Step 6: Test for Continuity:

Continuity testing helps identify any breaks or open circuits within the contactor. Set your multimeter to the continuity or ohms setting and touch the probes to the contactor's output terminals. The output terminals are connected to the compressor and motor. Ensure the probes establish a secure connection with the terminals.

Step 7: Check for Continuity:

With the multimeter probes securely connected to the output terminals, activate the contactor by turning on the power supply to the air conditioning unit. The multimeter will display a reading indicating continuity if the contactor is functioning correctly.

Step 8: Inspect the Contacts:

While the contactor is activated, visually inspect the contacts for signs of arcing, pitting, or any other damage. Additionally, observe if the contacts are closing and opening smoothly without any hesitation. If you notice any irregularities, it may indicate a need for contactor replacement or servicing.


Testing an AC contactor is a vital step in maintaining the optimal performance of your air conditioning system. By following the step-by-step procedure outlined in this article, you can identify any issues with the contactor and undertake the necessary repairs or replacements. Regular testing and maintenance of the contactor will not only ensure a comfortable indoor environment but also extend the lifespan of your air conditioning unit. Remember to exercise caution and prioritize safety throughout the testing process.


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