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Old 10-15-2013, 07:14 PM   #1
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Goodman GMPN100-4 indoor blower only runs at high speed


Furnace runs great with one exception, the indoor blower motor only runs at high speed. I determined that the t-stat does pull in the IDBR1 relay on the B18099-13 control board (as indicated on the attached Goodman schematic) making the heat contact live. I have swapped the motor leads but all three leads cause the motor to run at high speed. I rang out each motor lead to neutral and found 2.7, 3.3 and 4.4 ohms so the motor appears OK. The capacitor brings the motor up and keeps it running without fail. The t-stat and furnace is only wired for heat. Any suggestions?
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Old 10-15-2013, 08:30 PM   #2
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Goodman GMPN100-4 indoor blower only runs at high speed


You really want it to be on Med-High, not High. It may have been set for high, due to undersized duct work, undersized unit, older home that leaks as if the windows are not closed, etc.. A lot of factors why it was set that way originally.

Med-Hi on the motor is Yellow. Post a picture showing how the motor is hooked up right now.

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Old 10-15-2013, 08:47 PM   #3
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Goodman GMPN100-4 indoor blower only runs at high speed


Often there is only a 50 RPM difference between high and med, and between med and low. So it can be hard to tell if the motor slowed down.
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Old 10-15-2013, 08:56 PM   #4
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Goodman GMPN100-4 indoor blower only runs at high speed


They should be able to do it with a timing strobe. About the only way to figure out the rpm of a motor.
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Old 10-16-2013, 04:28 AM   #5
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Goodman GMPN100-4 indoor blower only runs at high speed


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Originally Posted by gregzoll View Post
They should be able to do it with a timing strobe. About the only way to figure out the rpm of a motor.
Won't be accurate on a PSC, VS, or X13 blower motor.
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Old 10-16-2013, 04:34 AM   #6
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Goodman GMPN100-4 indoor blower only runs at high speed


Thanks for your posts! I was wondering what the difference in RPM's was between each of the high, medium, and low motor speeds and without a strobe I would never be able to pick that up. Homeowner is complaining about heat blowing cold air. This is a (OVERSIZED) 100 K BTU/Hr furnace that was installed 20 years ago in an 800 square foot bungalow. I found the fan off delay set for 150 seconds, so I can change that to 90 seconds. Based on your replies, if that adjustment doesn't make the homeowner happy, I have been looking installing an AC motor speed rheostat inside the furnace cabinet in series with the motor low speed (RED) leg.
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Old 10-16-2013, 04:46 AM   #7
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Goodman GMPN100-4 indoor blower only runs at high speed


Blowing cold air may be from under sized duct work. The high limit may be shutting down the burners.
Or, it could be that the original installers lower the manifold pressure to prevent high limmit trips. but lowered it too much for the fan speeds of that furnace.

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