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Old 07-19-2012, 05:28 PM   #1
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AC won't turn off based on temp, only if I switch from COOL to OFF


OK bit of an odd issue with the AC in my house.

A few days ago we found that the AC unit was not shutting off even when the temperature is below what is set on the thermostat ( Thermostat set to 75 House temp at 70). I found that I could cut the AC off by swithcing the thermostat from COOL to OFF. When doing this both the air handler in the basement, and the compressor outside power down. Figuring it was an issuing with the thermostat I replaced it, but the issue still remains, the AC won't shut off based on the thermostat temp, only if I switch from COOL to OFF.

In my googling I found a site that said I could troubleshoot whether it was the Air Handler control board or the thermostat by disconnecting the Yellow thermostat wire and seeing if the unit stayed on or off. When I disconnect the Yellow wire both the air handler and the outside compressor remain ON. Does this confirm the issue is in the control board? Is there any other checking I can do?

On a side note I saw that the most common AC problem is the contactor, I checked this on the outside unit and it functions as expected, it does not stick shut.

Any help or suggestions would be appreciated.

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Old 07-19-2012, 05:46 PM   #2
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AC won't turn off based on temp, only if I switch from COOL to OFF


More likely wire problem. Shorted somewhere.

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Old 07-19-2012, 06:06 PM   #3
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AC won't turn off based on temp, only if I switch from COOL to OFF


Easiest way to check that? Can I hook the thermostat straight up to the control board?
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Old 07-19-2012, 06:51 PM   #4
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AC won't turn off based on temp, only if I switch from COOL to OFF


while doing the testing shut the main disconnect out by the condenser...so only the contactor pulls in and out and your not short cycling the compressor.so you disconnected the yellow off the board and both stayed ON? with the stat off the subbase nothing should be running...check that.disconnect that yellow again at the board and if it is still running.... go out to the condenser access one of the low voltage wires and break that the contactor should drop out.keeping that disconnect OFF..touch the disconnected wire contactor should close open close...as you touch it and break it.with that Y disconnected there should be no action outside
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Old 07-19-2012, 06:56 PM   #5
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AC won't turn off based on temp, only if I switch from COOL to OFF


For clarification, I disconnected the yellow wire, wrapped it in some electrical tape, then hooked the thermostat back up so it was on the subase...everything turned ON and stayed ON.
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Old 07-19-2012, 08:01 PM   #6
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AC won't turn off based on temp, only if I switch from COOL to OFF


in the furnace disconnect one of the low voltage wires going out to the condenser should shut off outside they down have to be any specific color just a pair goingout onto the condenser

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