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Old 06-23-2010, 06:41 PM   #1
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New control unit, but AC still not working

After a nasty storm over the weekend, our AC would no longer kick on and neither would the blower. I bought a new White Rodgers 50A55-843 control unit and installed it today. All went well except one ugly part. I accidentally connected the Line-H to the EAC-H terminal. The blower started running right away when I flipped the power switch on the side of the furnace. When I saw that the AC compressor wasn't running, I turned things off and rechecked, then found my mistake and switched the power line coming from the cover safety switch back to the Line-H terminal.
In this position I am not getting any blower and no AC.

I checked the 24 volt transformer and the control board is not supplying power to it. Did I blow my new $100 control unit by switching up the wires? I checked the fuse as well by the way and it is ok. I don't have any status light flashing errors either.

The house is 85 degrees so until I get it figured out, I ran a separate 24 volt AC transformer to the AC contactor to get the AC unit running, and I switched the hot line back to EAC to run the blower.

Any ideas?
Eric (hot and sweaty in Michigan..)


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Old 06-23-2010, 06:51 PM   #2
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You may have damaged the board. The EAC terminal gets energized when the board energizes the fan for heating or cooling. You backfed voltage to a relay which was not supposed to happen and after that damage occured.


"Cut it twice and it is still too short".
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Old 06-23-2010, 07:51 PM   #3
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Guess you never checked to see if the thermostat was sending 24 volts to the Y and G terminal of the board.
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