The contactor closes and opens very rapidly, sometimes it works fine, I replaced the contactor and the arking destroy the new one, what can cause that problem?

There could be any of several causes including, but not limited to: Incorrect refrigerant charge causing a pressure switch to break & make the control circuit; bad defrost board; faulty control circuit (could be anywhere).

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Originally Posted by Grady White

There could be any of several causes including, but not limited to: Incorrect refrigerant charge causing a pressure switch to break & make the control circuit; bad defrost board; faulty control circuit (could be anywhere).

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Thanks for your help, I replaced the defrost board, the contactor and install a new thermostat; Incorrect refrigerant charge I did not think about it but maybe the low pressure switch is doing it, what do you think about a weak transformer?

Quote:

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Originally Posted by Grady White

There could be any of several causes including, but not limited to: Incorrect refrigerant charge causing a pressure switch to break & make the control circuit; bad defrost board; faulty control circuit (could be anywhere).

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Maybe the fan contral board?

A weak transformer would certainly do it. So could the fan control board if it controls more than just the fan as it probably does. You would likely need a fast acting voltmeter or an analog meter to catch the transformer. Transformers rarely act like that. Usually if they are weak, they are weak all the time. I'd be inclinded to suspect the board.

Quote:

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Originally Posted by Grady White

A weak transformer would certainly do it. So could the fan control board if it controls more than just the fan as it probably does. You would likely need a fast acting voltmeter or an analog meter to catch the transformer. Transformers rarely act like that. Usually if they are weak, they are weak all the time. I'd be inclinded to suspect the board.

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Thanks again man, I will try both.