A/C condenser issues.

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Old 03-19-16, 02:28 PM
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A/C condenser issues.

I'm stuck. Neither condenser fan or compressor will turn on. Voltage across low voltage line (measured at wire nuts just inside condenser panel) is 24VAC. Voltage across L1/L2 at contactor is 240VAC. I assumed I had a bad contacter, but new one does not solve the problem. New contactor coil measures 17ohm, so seems ok.

With 24vac signal to condenser, I measured voltage across coil directly. Note that contactor is wide open. Basically measure zero volts. If I measure left low voltage coil terminal to ground, I see 24vac. If I measure right low voltage coil terminal to ground, I also see 24vac. If I short either low voltage terminal to ground, the contacter closes.

any ideas? Appreciate the help
 
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Old 03-19-16, 03:04 PM
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A little confused. If you short the coil to ground, it should pop the fuse in the furnace.
 
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Old 03-19-16, 03:20 PM
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I agree, but 3amp fuse survived the brief experiments. Question is why is the low side coil terminal also at 24vac? Any ideas.
 
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Old 03-19-16, 03:39 PM
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Welcome to the forums.

You posted in the A/C section so we can assume this is an A/C system and not a heat pump.... correct ?

If you have 24vac at the wirenut connections where the t'stat wiring connects to the condensor, with the condensor connected, then the problem is in the condensor.

If you have the voltage at the wirenuts but not at the contactor then it appears you have a safety switch open. Could be a re-settable high pressure switch or a low pressure switch trip indicating a low refrigerant charge.
 
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Old 03-20-16, 02:09 PM
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Pete, you called it. Pressure switch is locking out the contactor. Did not know there was a pressure switch until now. As best I can tell, switch can't be removed without releasing refrigerant. Is that typically the case, or is there something like a schrader valve in the fitting to allow leak free replacement. If not, guess it is time to call a pro. Thanks very much for the help
 
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Old 03-20-16, 02:12 PM
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By the way, switch not resettable, so perhaps indicator of low refrigerant. Switch part number is Texas Instruments 20ps08-0018.
 
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Old 03-20-16, 02:14 PM
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While it's possible that the pressure switch is defective, a more likely cause for it being open is that either the pressure is too low (low pressure switch), or the switch needs resetting (high pressure switch). If there's no reset button on the switch, it's likely a low pressure switch and the system is likely low on refrigerant. You'll need to call an HVAC tech to come and check pressures and temperatures to determine if the system is low on refrigerant. If it's not, then the pressure switch is bad. Either way, you'll need an HVAC tech to proceed any further.
 
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Old 03-20-16, 02:29 PM
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I agree with Bob.


Highly unlikely to be a defective low pressure switch. More likely to be low on refrigerant.
Some switches are easy to change with no refrigerant loss and some need a system evacuation.
 
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Old 03-20-16, 04:46 PM
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Pete and Bob, thanks for the guidance. Time to call the pros. Now off to our leaky washing machine.
 
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