Intermittent Low Voltage at A/C blower/furnace


Old 06-28-19, 01:21 PM
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Intermittent Low Voltage at A/C blower/furnace

Hi, I'm hoping to get some advice on an issue with my A/C unit. The unit works normally, but after completing a cooling cycle the unit will not come back on, unless...I remove and reconnect the power supply wires. Then it clicks on and completes the cycle (most of the time).

When i disconnect the power wires they read around 85V. Then after some reconnecting/disconnecting the voltage magically shoots up to 115V, I reconnect, and it triggers on, finished the cooling cycle, and I have to restart it again the next day after it switches off again.

Other interesting observations:

Occasionally, simply flipping the switch at the power junction box will do the trick.
The outlet at the power junction (its combination switch/outlet single gang) reads a constant ~115V and does not drop.
The last time i tried to restart I heard it triggering but then it switched off 3 seconds later. It took 3 or 4 tries of connecting/disconnecting to get the unit on.
Once or twice I read 115V entering the control board but only 85V out of the board.
I get steady 24-28V readings out of the transformer and at R,G,Y when there is a call for cooling.

Is it possible that there is a loose connection at the junction that is causing the voltage to drop? Is there some kind of low-voltage breaker that triggers at some point and resets after a time? Is there maybe an intermittent short to ground somewhere? is it possibly a control board problem?

any insight would be appreciated
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Old 06-28-19, 06:43 PM
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I have seen the field installed high voltage wire nut in the furnace melted a few dozen times.

Where are you disconnecting power?

I have also seen furnace door switches fail to consistently provide power.
Old 06-28-19, 06:53 PM
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Very strange. It doesn't make sense that the 24vAC is not affected. The 120v supply, the service switch and the blower door switch will all affect the transformer supply too.
Old 06-29-19, 09:48 AM
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Thanks for the replies. Out of desperation I unplugged a separate room air filter that was on a wall plug. I think they must have wired that outlet to the same supply as the blower because it seems to have fixed the issue. AC cycles normally now. I'm not overly confident in that solution though, seems odd that it would start causing problems after months of working fine. Anyway, thanks again and I'll repost if it acts up again.
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