Detailed information on how to replace a contactor in your air conditioning unit.
One of the more simple repairs you can make on an air conditioning (AC) unit is replacing an electrical component called a contactor. A contactor is basically a switch that is controlled by your thermostat that energizes the compressor and fan on your exterior condensing unit. When you set your thermostat to turn on the AC unit, it sends a 24 volt signal to a small magnetic coil. This signal energizes the coil and pulls in a set of metal contacts that complete the circuit for the compressor and fan.
Over time the contactor surfaces get coated in carbon from the frequent switching and arcing to complete the circuit. When the carbon gets too thick the contactor will make either a very loud humming noise or chatter when the unit first comes on. Sometimes the magnetic coil shorts out, but it is simpler to just replace the entire contactor.
If you choose to perform this repair, care must be taken to make sure the unit is completely shut off as you will have to remove wires with as much as 240 volts going through them. Condensing units are required to have a service disconnect within 3 feet of the unit and if there is not one installed, stop and have a qualified electrician install one for you.
This repair should be performed if your unit does not come on, but it has power.
Tools and Supplies
Multimeter or Voltmeter
The contactor is typically located in the area where the electrical connection to the unit is made. The contactor is a small dark brown or black cube with Sta-kon disconnects (a company that makes electrical connectors; “Stay Connected”) on the sides and terminals on the top and bottom.
Condensing Unit with cover removed (Contactor on the left, capacitor is silver component on the right.) Blue and yellow wires are low voltage, black and brown on bottom are high voltage in, two blacks and two reds are high voltage out.
This figure explains the components you need to know about when replacing a contactor. The label will be on the front or side.
• Blue Arrow – Sta-Kon disconnect terminal to energize the magnetic coil. Only low-voltage wires go on the terminal on the sides. At least one wire will be on each side.
• Green Arrow – Screw to remove the cover to access the contacts and test button.
• Orange Arrow – Line-voltage wires coming from the electrical disconnect. The terminals at the top go to the fan and compressor.
• Red Arrow – Mounting screw hole to fasten the contactor to the metal casing of the unit.
Sta-Kon Disconnect Terminal
Since the unit is not running, it is best to disconnect the power and remove the old contactor and take it to your nearest electrical distributor or refrigeration supply house to make sure you get the correct replacement. The important information you will need is the coil voltage (Typically 24 volts), Amperage (30, 40, etc.), Voltage (220, 240).
Here is an old contactor. The rectangular button on the center is a button that when pressed in with an insulated screwdriver or a test probe from your voltmeter will complete the connection and let you see if everything else is working in your unit. If you press it in and the unit starts as normal, you can be fairly certain that the problem is with your contactor.
Turn the power off to the unit and use caution when removing the access cover. Turn the power back on to the condensing unit.
Plug type Service Disconnect
Set your thermostat to cool and turn down the temperature setting so that the unit will come on. The fan on the air handler, the indoor unit, should come on. If it does, then you know that the thermostat is sending a signal to both units.
With your voltmeter, test the side terminals to see if you have at least 22 volts. Below this and the contactor may not have enough power to completely pull in the contact. You may have to locate and replace the transformer first.
If you have around 24v, then carefully press the test button in on the contactor and see if the fan and compressor runs.
If the contactor is energized and pulled in, then you may have a problem with the start/run capacitor on the unit, but that is another article.
NOTE: When removing the wires from the contactor, use caution as there will still be voltage present from the capacitor. The capacitor is used to control the voltage going to the fan and compressor to extend the life of the equipment. It is basically a battery and will still retain voltage after the power is shut off.
Turn off power to the unit and have the correct replacement contactor in hand.
1. Make a detailed diagram of the wires going into the bottom and out of the top of the contactor. Note the colors as sometimes they can be numbered or have a white stripe on the side. For a typical residential unit this should not be the case.
2. Remove the mounting screw on the contactor. (Since you already have the contactor removed when you took it to the distributor to purchase an equal replacement, I should omit this step, but I can’t.)
3. Fasten the new contactor in place and connect the low voltage wires to the sides.
4. Reconnect the wires from the disconnect to the bottom of the contactor. You may want to cut back the wires or clean off any oxidation.
5. Turn power back on and set the thermostat to cool and check to see if the contactor pulls in. If it does, turn power back off.
6. Test the terminals with your voltmeter to ensure that there is no power. Connect the wires into the top of the contactor to the fan and compressor. You may need to use needle-nose pliers to get a good grip. Check for any worn out connectors and replace as needed.
7. Reinstall cover and turn disconnect back on.
8. Monitor the unit and make sure that the temperature is maintained.
When purchasing a replacement contactor:
Be sure the control voltage rating is correct.
Be sure the load capacity (amperage) is correct.
Try to get a contactor with the same number of poles.
Try to get a contactor with terminals that match the original.
You may want to inspect your unit annually and spray the contactor with contact cleaner every Spring.
See my articles regarding AC maintenance at:
Spring Cleaning and Start Up for Your Air Conditioner
Correcting Common Problems in Your Air Conditioning Ductwork