Blow motor dead

Problems related to residential installations.

Blow motor dead

Postby Cinnamon44 » Fri Jan 20, 2017 8:53 pm

Hello Freon,
I have a problem with Lennox model 80MGF 4/5-100-7 gas furnace. The blow motor did not on ether on cool and heat( i did put on fan mode it did not work neither). I see flame is on and the air condition mode is working. But when it call for blow motor did not respond. I did check voltage on terminal "Heat" on control board it dosen't have 120.0 volt there.( i hear the click on relay calling for blow).
Thanks.
Huuh
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- Blow motor dead

Postby Freon » Sat Jan 21, 2017 10:42 am

Turn off the power to the furnace. Write down what color motor wire connects to what terminal on the control board (Red to Heat etc). Next disconnect all motor wires from the control board. Turn on the power to the furnace.

At the thermostst set the FAN to ON. Now go to the control board and look at the connection places where the motor wires connected. Find the terminal on the control board labeled common or C. If you are unsure, you can use the metal frame where you can see metal and not paint. With your multimeter set to volts-AC, CAREFULLY place one probe at the Common or C terminal (or metal furnace frame) and the other meter probe on the COOL terminal at the control board. If you get zero volts, try the Heat terminal. If you get zero volts, stop.

Usually the FAN- ON setting uses the COOL blower speed. So you should have seen 120 volts AC between the COOL terminal on the control board and the Common (C) or metal frame. If you are not seeing any voltages at the terminals, it appears there's a problem with the control board. I would carefully remove it (turn off all power and mark all wire connections). Carefully examine the board for cold solder joints (loose connections) or signs of overheating (burnt area that's brown or black). Also put your nose close to the open end of the motor itself and see if you detect any burnt smell. I doubt you will.
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- Blow motor dead

Postby Cinnamon44 » Mon Jan 23, 2017 12:19 pm

Good morning Freon,
Thank you for reply. I did find out the board was bad at terminal supply 120 volt for blower fan. The relay was burn on that pin. It was RKA-5DG-24. I google but could not find any where. The Orvac electronic help deck said it could be replace by R22-5D16-24 relay. Could you give me your advise?(I was radio & TV repair back in 90's so I know how to de-solder and solder it back).
Regards.
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- Blow motor dead

Postby Freon » Mon Jan 23, 2017 8:19 pm

I can only tell you what you may need to consider if you decide to try and repair the control board with a new relay. However I would first make sure the specific relay (cool) is the true cause.

Do everything I said before about removing the blower motor connections from the control board. Now you will test the Heat blower motor relay. Turn on the furnace power, call for heat and then check the voltage between the control board Heat terminal and ground for 120 volts AC. If you see 120 volts, as you should (allow for the heat exchanger to heat up or programmed timing to run maybe 2 minutes). If you do see the 120 volts then your conclusion of a bad Cool relay seems good.

Here is what you need to consider. Will the replacement relay fit into the circuit board holes? Dimensions are critical however you can modify it since you know about soldering. Next question, are the voltages and current correct? Switching voltage 24 volts AC and the contactor voltage 120 volts at the correct amperage. Data from the motor may tell you the needed amperage.

Good luck.
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