how to check ac contactor coil


The Importance of Checking AC Contactor Coil

The air conditioning system is an essential part of every household, providing cool and comfortable indoor temperatures during hot summer months. To ensure the smooth functioning of the AC unit, regular maintenance and inspection are necessary. One crucial component that needs attention is the AC contactor coil. The contactor coil is responsible for regulating and controlling the flow of electrical currents within the system. It plays a vital role in the operation of the compressor and fan motors. Therefore, it is essential to regularly check the AC contactor coil to ensure its proper functioning. In this article, we will delve into the process of checking the AC contactor coil, its significance, and the necessary steps for inspection.

Understanding the AC Contactor Coil

Before we delve into the process of checking the AC contactor coil, it is essential to have a basic understanding of its functionality and purpose. The contactor coil is an electromagnet that operates as a switch in the AC system. When an electrical current passes through the coil, it produces a magnetic field that attracts a movable iron core. This action closes the contactor, allowing the electrical current to flow to the compressor and fan motors. When the coil is de-energized, the magnetic field dissipates, and the contactor opens, interrupting the electrical current flow.

Signs of a Faulty AC Contactor Coil

A malfunctioning contactor coil can lead to various issues within the AC system. It is crucial to be aware of the signs that indicate a faulty contactor coil. This knowledge allows for timely inspection and repair, preventing further damage to the unit. Here are a few common indicators of a faulty AC contactor coil:

1. Non-Functioning Compressor and Fan Motors

When the contactor coil fails, it may not provide the necessary voltage required to power the motors. As a result, the compressor and fan motors might not start or run efficiently. In some cases, the motors may not start at all, causing the AC unit to blow out warm or no air.

2. Frequent Breaker Trips

A faulty contactor coil can cause the circuit breaker to trip frequently. This occurs when the electromagnetic field generated by the coil becomes erratic or imbalanced, creating an overload in the circuit. If you notice the circuit breaker constantly tripping, it may indicate an issue with the contactor coil.

3. Unusual Buzzing or Clicking Sounds

When the AC contactor coil is malfunctioning, it may produce unusual sounds. These sounds can manifest as buzzing, clicking, or humming noises. If you hear any irregular sounds coming from the AC unit, it is crucial to investigate and check the contactor coil.

4. Improper Functioning of the Thermostat

A faulty contactor coil can also impact the functioning of the thermostat. If the AC unit fails to respond to temperature adjustments or doesn't reach the desired temperature, it could be a sign of a malfunctioning contactor coil.

5. Burnt or Pitted Contacts

Inspecting the contactor visually can provide valuable insights into its condition. If you notice burnt or pitted contacts, it indicates that the contactor coil could be failing. Pitting occurs due to arcing caused by the contactor's electrical current flow, and burnt contacts can occur due to excessive heat generated during contactor operation.

How to Check the AC Contactor Coil

Now that we understand the importance of a well-functioning contactor coil and the signs of a faulty coil, let's discuss how to check the AC contactor coil. It is essential to note that working with electrical components can be dangerous, and if you are not confident in performing the inspection, it is best to seek professional help. However, if you are experienced and knowledgeable in electrical work, here are the steps to check the AC contactor coil:

1. Ensure Safety Precautions

Before beginning any inspection, it is crucial to take proper safety precautions. Turn off the power to the AC unit at the main electrical panel to avoid any risk of electrical shock. It is also advisable to use appropriate personal protective equipment (PPE), such as insulated gloves and safety goggles, to minimize the risk of injury.

2. Access the Contactor

Locate the contactor within the AC system. The contactor is typically a black or gray box-like component with numerous wires connected to it. It is commonly situated near the compressor unit.

3. Inspect for Physical Damage

Visually inspect the contactor for any physical damage such as cracks, melted plastic, or loose connections. These signs may indicate a faulty or damaged contactor coil.

4. Check for Voltage

Using a multimeter, set it to measure voltage and carefully test the voltage across the contactor coil terminals. Ensure there is no power running to the unit before performing this step. If the reading indicates zero voltage or a significantly lower voltage than specified, it may indicate a faulty coil.

5. Test the Coil's Resistance

Set the multimeter to measure resistance and carefully test the resistance across the contactor coil terminals. The resistance value should match the manufacturer's specifications. If the reading deviates significantly from the specified range, it may indicate a faulty coil.

6. Inspect the Contacts

Examine the contacts within the contactor for any signs of damage, pitting, or corrosion. Using an emery cloth, gently clean the contacts if necessary, ensuring a smooth and clean surface for optimal performance.

7. Replace if Necessary

If any of the above steps indicate a faulty AC contactor coil, it is advisable to replace it with a new one. Remember to follow the manufacturer's instructions and recommended procedures while replacing the contactor coil.


Regular inspection and maintenance of the AC contactor coil are crucial for ensuring the smooth operation of the air conditioning system. By understanding the signs of a faulty contactor coil and following the necessary steps to check its condition, homeowners can detect issues early and prevent further damage. Remember, if you are not confident in performing the inspection, it is always best to seek professional assistance to avoid any risks or accidents. Taking proactive measures in maintaining the contactor coil will help extend the lifespan of the AC unit and ensure maximum comfort during hot summer months.


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