How to Test an AC Contactor with a Multimeter

Electricity is one of a few different power sources that can be used to fuel an air conditioner or furnace. Electrical parts of an AC unit usually include the heating pump, which regulates temperature, and a blower fan, which helps distribute hot or cold air. Often times, when something like a blower stops working, it’s the result of a smaller electrical component failure—such as a contactor. 

AC contactors regulate the flow of electricity throughout your unit. They work in conjunction with the compressor and condenser unit in your AC to form the electrical power unit. When you turn your unit on, the contactor receives a low voltage signal that creates a magnetic field. Like a bridge, this field closes the circuit within your unit, allowing higher voltage connections to take place. This is what powers the air conditioner’s fan and compression motors. 

There are two types of contactors. A single pole that contains a single magnetic coil, allowing for one circuit connection, and a double pole that has two coils to make two circuit connections. 

Signs of a failing contactor can present themselves as either mechanical failures or physical signs of damage. These are the most common.

Continuous running

The AC runs continuously, even when the unit is shut off.

Clicking

Clicking is likely the result of an electric issue, although it could mean a problem with the thermostat, compressor, or capacitor. 

Unexpected physical deterioration

Noticeable to the eye and also known as pitting, this might occur as a result of extreme temperature damage. Can also be the result of pests, debris, or other environmental factors.

Expected deterioration 

Also noticeable to the eye, this would be categorized as physical signs of wire deterioration as a result of time and usage.

Of course, not all AC issues are a result of a failing contactor. If you notice any of these other signs, it’s best not to turn your unit on before you’ve made the necessary repairs.

Squealing

This may be the result of a broken or worn down fan belt and could prevent your AC from circulating cold air.

Rattling

If the sound isn’t present when the unit is on, a loose fan or obstruction to the fan might be the cause.

Grinding

A grinding sound could be the result of worn motor bearings that need to be replaced.

There are usually two contactors to each switch, marked as a line and terminal. The line (L) contains the voltage feed while the terminal (T) is connected to the electrical device being controlled. To test a contactor with a multimeter or voltmeter, use the following steps as a guide.

Step 1. Remove wires from the line.

Remove electrical power to the L side of the contactor, then use a screwdriver to pull the wires from the line-side screws. Mark wires to keep track (L1, L2, L3…).

Step 2. Remove wires from the terminal.

Repeat the same process on the terminal side of the contactor. Don’t leave any wires behind, or you could get a false reading. 

Step 3. Turn on the control switch.

Put the control switch of the contactor in the "on" position and wait until you hear an audible click, followed by a humming sound from the contactor.

Step 4. Connect the multimeter.

Attach the red lead to the ohm connector and the black lead to the common. Turn on the multimeter. Touch the two leads together; the meter should read 0 ohms.

Step 5. Test each line.

Test each set of L1-to-T1 contacts by placing the red lead to your L1 and the black lead to your T1. If any set fails to read 0 ohms, there is likely an issue with the contactor.

Step 6. Check for a faulty coil connection.

Try cycling the control switch for the coil and check for an audible click. If you don’t hear one, it’s time to check the voltage of the coil. 

Step 7. Check voltage.

First, check the specifications of the circuit to find the appropriate voltage information. Switch the meter to volts and move the red lead over to the volts connector on the meter, then apply power to the coil. Touch each lead of the meter to each connector on the coil to check for the supplied voltage. 

Step 8. Check ohms.

Remove the wires to the coil with a screwdriver and switch the meter to ohms. Touch each lead to the coil connectors and look for a reading between 10 to 100 ohms. If you do not get this reading, you have a bad coil.

If your contactor or coil is malfunctioning, you can purchase a new one separately to repair your AC unit. If your tests conclude that you don’t have an issue with your contactor, consider some of the other failure signs listed above.

The product statements contained in this guide are intended for general informational purposes only. Such product statements do not constitute a product recommendation or representation as to the appropriateness, accuracy, completeness, correctness or currentness of the information provided. Information provided in this guide does not replace the use by you of any manufacturer instructions, technical product manual, or other professional resource or adviser available to you. Always read, understand and follow all manufacturer instructions.