80 volts to blower motor?

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Old 05-19-11, 11:40 AM
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80 volts to blower motor?

Hi,

I'm having a problem with my American Standard Freedom 90 single stage natural gas furnace. I believe I have it diagnosed to a bad main board, or the blower motor. The unit fires up and seems to run fine. After about 45 seconds of it burning properly, it gives the signal for the blower motor to kick on and start circulating air, but the blower never starts. I measured the leads going to the blower motor. At first it's of course at 0. When it clicks to start the blower motor I'm reading about 80 volts. This should be 115-120, yes? At 80 volts, would the blower motor turn at all? Mine isn't even budging. Would it be safe to disconnect all leads coming from the blower to the main board, and then "hot wire" the blower with 115v to test it? Or if I'm showing 80 volts going to the blower is the problem definitely my board? Thanks!
 
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Old 05-19-11, 01:52 PM
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you very well may have a bad control, but lets test first. what is the incoming voltage to the furnace? Turn your thermostat to off and select fan on... does the blower come on? if it does, then skip the rest. Turn the stat off and fan to auto, Next remove the black wire from x-fmr terminal on control board...then install black wire from the blower motor on the x-fmr terminal.... push in the door switch.... this will provide the motor with the same voltage as whatever is coming into the furnace. Does the motor start?
 
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Old 05-19-11, 02:11 PM
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If you are using a digital meter, you could be seeing a phantom voltage.
 
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Old 05-19-11, 03:16 PM
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Originally Posted by hvactechfw View Post
you very well may have a bad control, but lets test first. what is the incoming voltage to the furnace? Turn your thermostat to off and select fan on... does the blower come on? if it does, then skip the rest. Turn the stat off and fan to auto, Next remove the black wire from x-fmr terminal on control board...then install black wire from the blower motor on the x-fmr terminal.... push in the door switch.... this will provide the motor with the same voltage as whatever is coming into the furnace. Does the motor start?
Unfortunately it's an old honeywell dial type thermostat without independent fan switches. Just red and white coming from the thermostat. With this type of thermostat, if I remove the black wire from the x-fmr terminal and install the black wire from the from the blower motor (and push the door switch), would that be an appropriate test?
 
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Old 05-19-11, 03:23 PM
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that would be an appropriate test, and or you can jumper R to G at the control board which will bring on the fan for cooling speed.
 
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Old 05-19-11, 03:56 PM
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most likely the control board is burnt out on the back at the heating terminal.
 
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Old 05-20-11, 09:23 AM
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Originally Posted by hvactechfw View Post
most likely the control board is burnt out on the back at the heating terminal.
Sounds like you nailed it. I had a similar problem on a Carrier, at times it would go off on limit, voltage read showed low voltage to the heat speed( I've forgotten how low), pulled board and found bad solder joint on back side.
 
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