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Old 06-28-2012, 10:54 PM   #1
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Buzzing relay in A.C. unit


I have a (10 Yr. old?) Trane whole-house A.C. compressor unit that came with the house. This year, I noticed a buzzing sound from the unit when it runs. Upon inspection, the relay (motor contactor #3100-20Q1542, 2-pole 30-Amp, w/24-Volt coil) seems to have the contacts intermittently buzzing when the compressor runs.

Tapping the contactor can start it buzzing when it is not, or quiet it when it is buzzing. The motor itself seems to run smoothly regardless whether the contacts buzz or not. The contacts can be seen vibrating when it is buzzing.

The coil voltage reads approx. 24VAC with an analog multimeter.

Question: Should I simply replace the device with a new or equivalent contactor as a first step, or is this a sign of more serious problems requiring an expensive service call immediately. The noise resonating in the metal housing is tolerable while I decide what to do.

Thanks all.

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Old 06-28-2012, 11:33 PM   #2
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Buzzing relay in A.C. unit


Probably just worn contacts that are pitted and not enough surface area in contact when the contactor is asked to close. I'd just replace it. That way you get a new 24V coil and spring also.

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Old 06-29-2012, 12:46 AM   #3
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Buzzing relay in A.C. unit


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Originally Posted by stubie View Post
Probably just worn contacts that are pitted and not enough surface area in contact when the contactor is asked to close. I'd just replace it. That way you get a new 24V coil and spring also.
Agree.

Those relays are not all that expensive anyway.

A buzzing relay is not a sign of anything else going wrong, it's just worn out.

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Old 06-29-2012, 09:06 AM   #4
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Buzzing relay in A.C. unit


Replace it soon so it does not harm the motor.
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Old 06-29-2012, 09:42 AM   #5
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Buzzing relay in A.C. unit


shut the disconnect off outside with the stat calling for cooling slight spray of WD40/with red tube extension... on the magnet area metal bars might soften it up on the sound.its a combo of rust/dust or even bugs looking for a warm place....break one of the low voltage wires and quick cycle the contactor as you try the WD40 (thats why i wanted the condenser disconnect off)control voltage comes from the air handler....or just replace it local Grainger #5B131...easy fix for DIYr.are the contacts on the contactor arc'd or black

Last edited by biggles; 06-29-2012 at 09:44 AM.
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