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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Blower motor on my furnace has stopped working. Help!

Customer Question

I have a blower motor on my furnace that decided to stop working today. I am using it with my AC.
Furnace is an Armstrong ultra 80 - GUJ125014-1B
I can see that the thermostat is calling for the fan to run, so the thermostat is good. I noticed that the compressor outside was running, and the fan in the furnace was not. Then I heard it try to start for about 2 seconds, but then it stopped.
Submitted: 1 year ago.
Category: HVAC
Customer: replied 1 year ago.
I can tell the thermostat is working; I tested the signal. Thermo is good. I noticed this when I heard the compressor outside running, but the fan in the furnace was not running. I opened up the door to the fan compartment and it was really cold but no fan running. I suspected the motor capacitor, so I pulled it off and tested it using my ohmeter. I don't have a capacity tester. I THINK the capacitor is OK but not sure. Using my cheesy ohmeter I see the resistance climb from 0 to 2000 in about three seconds. Should I replace it anyways and see what happens? Also, I tried to spin the motor manually and it feels fine. Thinking it is possibly the fan control or blower relay by digging around, but I do not know what those look like. The control board is: ST9120A2004
Customer: replied 1 year ago.
If you need to see some pics, I just sent the tag on the furnace, the capacitor, the control board SN, and the motor. For some reason I think the board, but not sure how to tell. I could measure the voltage going to the fan itself to see if it is getting any juice when the fan should be on, but not sure which of the 4 cables to test. There is a Black(H),White(C), Blue(M), and Red(L). WHere should I see voltage (AC)?
Customer: replied 1 year ago.
ANOTHER hint: Earlier in the day, I heard the fan motor kick in for about 2-3 seconds, but then it stopped. It did this twice that I know of. Now when I call for the fan (only the fan) to kick in, it does nothing. Also, I may be imagining it, but the transformer may not be as loud as it used to be. Perhaps because the door is open and it echoes less.
Customer: replied 1 year ago.
Heading to bed. Will check in the AM.
Customer: replied 1 year ago.
I just tested the transformer and it looks good. Shows 27V AC. I am thinking it is either the motor or the circuit board. Next step will be to try to check for voltage heading from the board to the motor. I am thinking it is the blue and black wire pair looking at the board. Will check that. As in, when I turn on the FAN, do I see voltage appear across the BLUE / BLACK pair of wires headed for the motor. I will disconnect the wires as I test that.
Customer: replied 1 year ago.
It's the board. When I call for the fan, I see no voltage across any of the WHITE, BLUE, RED, BLACK. Any combination. No light, no voltage. Then I bypassed the board and hooked the fan directly to 120V (via the WHITE and BLUE(Cool)), and the fan runs. I will go get a board. Can I have a refund since I didn't really need you? Thanks for your patience!

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