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 24v to control board but not to AC Contactor
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Posted - 09/12/2019 :  1:40:22 PM  Show Profile  Reply with Quote
I have a Lennox 80MGF3-75-A1 Furnace with Lennox 10AC30-3P Outdoor compressor unit. I believe I have damaged my control board.

Recently came home to an 83-degree house. When I checked the outdoor unit; the compressor fan motor was hot, humming and not turning, so I pulled the cutoff. After allowing the motor to cool - reinstalled the cutoff and pushed the fan with a stick. It slowly gained speed and seemed to run OK so I ordered a replacement capacitor.

By the time the capacitor arrived the next evening the house was warm again and the compressor fan not turning. I disconnected the motor and checked the windings with an ohm meter. Both windings now showed an open line. I replaced compressor fan motor and capacitor. (The inside of the fan motor was a mess blackened windings and stator damage.)

Because it would take several minutes to get from the thermostat to the outdoor unit I had disconnected one of the 24 volt lines from the 10AC contactor so that I could be at the outdoor unit to observe the fan motor startup process. When I pressed the 24-volt line to its connector, the fan and compressor started, the coolant line got cold and cool air was reported from the indoor vents after a couple of minutes.

When I released the 24-volt connector I had been holding against the terminal on one side of the relay coil, it swung cross and briefly touched the (uninsulated) connector the opposite side of the coil creating a spark. After this there was no voltage on the 24-volt lines at the outdoor unit. I verified (looped) continuity through both wires leading from the indoor to the outdoor unit. The indoor blower was still running as expected and the control board (20J8001) had 24 volts across most of the expected places, but there were 0 V across the two points on the board that lead to the contactor coil on the outdoor unit (labelled C and Y on the board)

It does not appear that this control board has a 3-amp fuse like more modern boards. The single led on the board is blinking slowly (normal operation). There are no visible signs of heat damage to the front or back of the board or to any of the components, but everything points to a bad board.

1) Am I missing another possibility?

2) I realize and HVAC tech would not waste time replacing components on a control board, but with no signs of damage, I'm guessing one of the components (i.e. a relay) on the control board is damaged. Would it be worth the trouble to try replacing the component before replacing the board?

3) Several threads on this site mentioned that for control boards that do not have a fuse integrated with the 24 volt circuit there are typically an upgraded board available with the same functionality AND a fuse to prevent damage in the event of a momentary short like the one I experienced. Is there and upgraded board for the Lennox 20J8001 with a fuse in the 24-volt circuit? Part number?

Thanks for taking the time to read this! Please let me know if I can supply additional info.
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