Air Conditioning Contactor



An air conditioning contactor is an electrical switch that delivers line voltage to the compressor.

Sometimes power is supplied to both the compressor and the condenser fan motor by the same contactor.


When control power is applied to the contactor holding coil, the contacts should pull in, and the compressor should run.


Here are the steps I take when I think I have an air conditioning contactor problem.


If the contactor doesn't pull in when the thermostat is turned on:

First, verify that there is correct control voltage at the coil terminals.

Measure the voltage with a voltage tester, and read the coil voltage specs on the contactor.


If you're getting correct control voltage at the contactor and it doesn't pull in, turn off power to the unit, pull the leads off the contactor coil, and check the resistance of the coil.

You should get a readable resistance if the contactor is good.

A reading of infinity means the coil is open, and in that case the coil or contactor must be replaced.


If you get a readable resistance and the contactor isn't pulling in, push in on the armature (the set of contacts that move in and out).

If it's stuck and doesn't move in and out, the next step depends on the size of the contactor and the circumstances.


If it's a small air conditioning contactor, it's probably more cost effective to replace it on the spot.

If it's a large contactor and replacement parts are readily available, then it might make sense to disassemble and repair the contactor.


Remember that the customer is paying for your time as well as the part.


If the contactor pulls in and the compressor and condenser fan don't run, the first thing you have to do is determine whether the load device is actually getting correct voltage, and has not failed.


After you've verified that the load is not getting correct voltage, and has not failed,
verify that there is correct voltage on both the line side and the load side of the contactor.


Turn off power, and remove all leads from the load side of the contactor, then turn on power and energize the unit so the contactor will pull in.


Check voltage between terminals, and from each terminal to ground.

If you have a bad set of contacts, this should identify them.

Clean or replace the bad contacts, or replace the contactor.


If all readings are good, you may have a set of contacts that are failing when a load is applied accross them.


To check for this, turn off power, re-connect all leads to the load side of the contactor, turn on power, and energize the system so the contactor will pull in.


Check voltage between the load side terminals, check voltage from the terminals to ground, and check voltage from the line side terminals to the load side terminals on each leg.

This will identify a bad set of contacts, or maybe 2 or 3 bad sets.

Clean or replace the bad contacts if possible, or replace the contactor.


If you have good voltage on all load side terminals, your air conditioning contactor has not failed, and the problem is in the load side circuit, not the contactor.


Is your air conditioning contactor noisy?

I have heard contactors that made loud humming noises, and that the customer didn't want to replace until it failed.

Those contactors worked fine, and I never had to replace them.


Chattering is different. It's when the contacts open and close at machine gun speed.

It makes a loud hammering noise, and is harmful to the contacts and the load device.


If you're working on an air conditioning contactor that is chattering:

Verify that you have full, steady control voltage at the contactor coil.

I have seen 24 volt contactors chatter when receiving only 21 volts.

To raise the voltage, change the tap connections on the control transformer.


I have also seen contactors chatter when they were wired in series with certain types of solid state delay on make timers.

This was in a circuit with several parallel contactor/timer circuits controlled by a multi-stage temperature control, and supplied by one step down transformer.

The solution was to install a step down transformer for each leg, and control each leg with a relay energized by the temperature control.

The temperature control energized the relay, and the relay conducted 24 volts to the timer and contactor.

The chattering stopped.


When replacing an air conditioning 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,

And always follow safety precautions.


I hope these air conditioning contactor tips will help you with your troubleshooting; and please, feel free to contact us with any specific HVAC questions you might have, including questions about air conditioning on Guam, and refrigeration on Guam.

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