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Furnace Control Board

posted Oct 26, 2020, 6:25 AM by Quoc-Anh Vuong   [ updated Oct 26, 2020, 6:59 AM ]
Once again, we have to fix the heater! 
The fan blower motor is overheated every 5min or so. After taking the motor out, cleaning the squirrel cage blades, oil the bearing, and reassembled everything back, same thing! It's overheated and shutdown, while the gas heater is still going full 
steam. I even opened up the condenser coil to clean it up, thinking that it's dirty and block the air flow, causing the motor to lack of air cooling it, which in turn overheating the motor. But the condenser coil is clean except for small weird looking blob of dirt.

After researching the internet, reading all about AC blower motor overheating issue, there is one guy mentioned about testing the control board, perhaps the motor draws too much current, which causes it to overheat. It was 11pm when I open up the furnace again (for the 15th time today), put my volt meter on the Heating tab make sure everything is working the way it should, then move the test probe on the Cool, there it is! Both Heating & Cooling tab have a solid 120V, pumping more current to the motor. In the Heating mode, only the Heating tab should have 120VAC, thus by providing additional 120VAC on the Cooling tab, the control board spins the motor at highest speed (Cooling fan speed), and also current now travels in parallel, providing additional current. 

So, just by moving the black wire (highest fan speed) to the M2, which is reserved/parking, the motor now runs smoothly, and no more overheating.

I also ordered new control board on Amazon, which should be here in few days. It's a White-Rodgers 50T35-730/743

White-Rodgers 50T35-743