Can contactor fail with short on 24v side.


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Old 07-17-17, 09:23 PM
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Can contactor fail with short on 24v side.

Traced failure of AC to only seeing 2 ohms on roundtrip of low voltage. So its either a short somewhere on the LV line to compressor or short at contactor.

So is contactor coil failing as short a possibility? Not my house otherwise I could just open it up and test. I will in a few days, but looking for input
 
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Old 07-17-17, 09:54 PM
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Yes..... very possible and quite common. At 2 ohms that contactors coil is shorted.
 
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Old 07-17-17, 10:14 PM
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Cool. Ill check that. Hoping to not re-run the wires. Not an easy job.
 
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Old 07-18-17, 05:29 AM
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Do you have a transformer tripping? You can isolate the contactor from the circuit and verify if that is your cause. Contactor coils shorting out are not uncommon. If it is in fact the coil, you only have to replace the contactor and not the wiring. Good news is contactors are typically inexpensive (for contractors anyways).
 
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Old 07-19-17, 10:07 PM
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I have not had chance to follow up. transformer had burned out. Then home warranty scheduled tech, who showed up and refused to touch it because I had put hands on it, despite the fact I had done half his job for him. Refused to even read the note I had left explaining what I had done and fixed. So closed it all up and waiting to hear about second visit. If nothing by this weekend, I will open compressor and check contactor. Yes, will remove wires to isolate and check impedance. Crossing my fingers its that and not wiring (rats) issue.
 
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Old 07-20-17, 09:06 AM
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A coil could certainly short out; some contactors have exposed coils, others are protected. Contactors have a simple 25 volt input (usually a pair of leads on one side, or one lead on each side), and they have multiple "Poles". (The large screw-lugs at the "top and bottom" of the contactor) If you have three large lugs across the top and bottom, you have 3-phase power; otherwise you will have usually have two large lugs at the top & bottom. Usually the mains power comes in at the bottom, and is taken off at the top to feed the compressor and fan.

Avoid touching any exposed wire while your unit has power - especially near the capacitor

When the unit is off you should get your normal mains power across the bottom lugs, and nothing across the top lugs. (And nothing across the side terminals)

When the unit is in cooling mode, you should get 25 volts AC across the side terminals (relay power), and across the top you should get the same mains power that you read at the bottom (120/240).

I am not sure how many ohms that coil will show normally - testing for the voltage should give you a better picture of if the Contactor is working correctly, and if your thermostat is sending out proper voltage. If you get no mains power out the top, you have an issue with the thermostat or contactor. You could always:
1 - shut off power to the unit (double check with meter)
2 - carefully pull the two low-voltage leads off the terminals on the side of the relay (the 25VAC lines)
3 - tape your meter to the low voltage leads, and carefully power the unit on and set it to cooling mode at the thermostat.
4 - do you get 25 volts from the thermostat (with no "shorted" contactor coil attached?)

If you have a wiring setup with more than a pair of relay wires, keep the wires that go to each side together when testing.
 
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Old 07-20-17, 09:24 AM
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I feel your pain about tech not working on A/C, My company would not work on anything customer had took apart for any reason. Usually because it took longer to fix what was taken apart than do repair. Got my a** chewed because I took 3 hours to do a 30 minute job because somebody took it apart wrong.
 
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Old 07-20-17, 09:31 AM
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When the unit is off you should get your normal mains power across the bottom lugs, and nothing across the top lugs. (And nothing across the side terminals)
There is no "top" or "bottom" to a contactor. There are L and T terminals.
The power can come into the top or the bottom of the contactor depending on its design.

When checking for a short it is better to use an ohmmeter. A short on a 24vac line will either blow a fuse or smoke the transformer.
 
 

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