how to test and replace an ac contactor



Replacing an AC contactor is an essential maintenance task that every homeowner should be familiar with. The AC contactor is a critical component of your air conditioning system, responsible for controlling the flow of electrical power to the compressor and fan motors. Over time, contactors can wear out or become damaged, resulting in improper functioning of your AC unit. In this article, we will guide you through the process of testing and replacing an AC contactor to ensure the efficient operation of your cooling system.

Symptoms of a Faulty AC Contactor:

Symptom 1: AC System Fails to Start

When your AC system fails to start altogether, a faulty contactor is a potential culprit. Contactors can become stuck in the open position, preventing power from flowing to the compressor and fan motor. However, it's essential to rule out other possibilities such as a tripped circuit breaker or a blown fuse before confirming a faulty contactor.

Symptom 2: AC System Constantly Cycles On and Off

If you notice that your AC system continuously turns on and off without providing adequate cooling, it may indicate a problem with the contactor. A worn-out contactor can cause the compressor and fan motor to intermittently lose power, leading to frequent cycling of the AC system.

Symptom 3: Buzzing or Clicking Noises from the AC Unit

Unusual buzzing or clicking sounds originating from your AC unit can suggest a faulty contactor. When the electrical contacts inside the contactor become worn or damaged, they may create abnormal sounds as they struggle to make proper connections.

Symptom 4: AC System Runs But Provides Insufficient Cooling

Inadequate cooling despite your AC system running could be a sign of a faulty contactor. A contactor with worn or pitted contacts may not provide a consistent and reliable flow of electrical power, resulting in reduced cooling performance.

Symptom 5: Burning Smell or Visual Damage

If you detect a burning smell or notice visual damage, such as melted or blackened components around the contactor, it indicates a severe issue with the electrical connections. In such cases, it's crucial to replace the contactor immediately to prevent further damage or potential hazards.

Testing the AC Contactor:

Before you proceed with replacing the contactor, it's essential to perform some tests to confirm its functionality. Follow the steps below to test the AC contactor:

Step 1: Safety First!

Before conducting any electrical tests or replacing components, it's crucial to prioritize safety. Begin by turning off the power to the AC unit at the main electrical panel. This ensures that you won't encounter any electric shock hazard during the testing process.

Step 2: Locating the Contactor

The AC contactor is generally located in the condensing unit of your air conditioning system. The condensing unit is situated outside your home and contains the compressor, fan, and other vital components. You can identify the contactor by its box-like appearance with multiple electrical connections.

Step 3: Inspecting the Contactor

Carefully inspect the contactor for any visible signs of damage or burning. Look for melted plastic, blackened areas, or loose connections. Note down any observations, as this will help you determine if the contactor needs replacement.

Step 4: Testing for Voltage

To test for voltage, you will need a multimeter. Set the meter to the AC voltage setting and attach the leads to the line contacts of the contactor. Make sure to keep your hands and the leads clear of any exposed wires or terminals to avoid any mishaps.

Step 5: Measuring Voltage

Once the multimeter is properly connected, turn the power back on at the main electrical panel. Carefully observe the multimeter reading. A reading of 220-240 volts indicates that the contactor is receiving the correct voltage supply. However, if the reading is zero or significantly lower, it suggests that the contactor is defective and needs replacement.

Replacing the AC Contactor:

Now that you have confirmed a faulty contactor, it's time to replace it. Follow these steps to ensure a safe and successful replacement:

Step 1: Safety Precautions

Similar to the testing process, ensure that the power to the AC unit is turned off at the main electrical panel before proceeding with the replacement. Safety should always be the top priority when working with any electrical components.

Step 2: Documenting Wiring Connections

Before disconnecting any wires from the existing contactor, take clear photographs or label the wires with tape to document their exact positions. This will prevent confusion when connecting the new contactor.

Step 3: Removing the Old Contactor

Using a screwdriver, carefully remove the screws securing the contactor to the condensing unit. Gently pull the contactor out, taking note of the placement and location of any additional components or wires connected to it.

Step 4: Disconnecting Wires

Using a screwdriver or pliers, detach the wires from the contactor. Pay close attention to the wire positions and connective points. Double-check that each wire is labeled correctly, ensuring a seamless installation of the new contactor.

Step 5: Installing the New Contactor

Place the new contactor in the same location from where you removed the old one. Take care to align the mounting holes properly. Gradually reconnect the wires to their respective terminals, ensuring a secure and snug connection.

Step 6: Securing the New Contactor

Once the wiring is complete, use the screws you previously removed to secure the new contactor to the condensing unit. Ensure that it is firmly in place and that all components are properly aligned.


In conclusion, testing and replacing an AC contactor are vital steps when troubleshooting and maintaining your air conditioning system. By recognizing the symptoms of a faulty contactor and following proper testing procedures, you can diagnose the issue accurately. Additionally, replacing the contactor correctly and safely will restore the efficient operation of your AC unit. Remember, safety should always be a priority when working with electrical components, and if you're unsure about any step, it's best to consult a professional HVAC technician to avoid any potential mistakes or injuries. By staying knowledgeable and proactive, you can ensure that your AC system continues to keep you cool and comfortable throughout the summer months.


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