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Old 10-30-2010, 05:02 PM   #1
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Another blower thread...Please help!!!

I've spent the last two days reading threads that have been posted about blower fans not turning off and I have not yet been able to figure out what is going on with mine.
I have an old Sears Homart furnace. Last week we started noticing a burning electric smell. Upon further investigation I found the motor was on the way out. So I got a new replacement a new capacitor while I was at it (figured it was cheap enough and wanted something to match up with the motor). I got everything wired up fine. Furnace turns on fine...furnace turns off fine. Blower motor, however, will just keep running. I had been noticing that the old blower motor would go for a long time before shutting off, however it would eventually shut off. The new replacement motor, however, will run all night unless I flip the breaker to the furnace. In trying to diagnose this problem I have noticed that if I flip the breaker off and flip it back on in a minute the motor turns back on and continues to run. However if I flip the breaker off and keep it off for say 10-15 minutes, when I flip the break back on the motor stays off. It is only when I turn the thermostat up and the furnace cycles again that the motor does not turn off.
I pulled out the fan/limit switches and did notice some dirt/dust buildup in them. I vacuumed them out and they look fine. Put them back in and the fan continues to run.
The wiring on the new motor was slightly different than the wiring on the old motor. I've included the wiring diagrams of both the furnace and the new motor. I wired up the new motor as per the motor diagram. Really, the only difference is the old motor ran the black wire through the capacitor, while the new motor actually had two wires specifically for the capacitor while the black motor wire just hooks up to the black lead.
I've read about the "auto" shut off mode on some furnace limit/off switches, but these don't seem to have this. I can't figure out what is actually supposed to turn the motor off when the furnace cools down and why it is not working. The only thing I can think of is that it MUST be the fan/limit switch is malfunctioning. Any suggestions would be VERY welcome...I'm pulling my hair out trying to figure this out, and considering how simple the wiring is on this furnace, there really can't be too many possibilities.


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Old 10-30-2010, 06:02 PM   #2
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Test the wires comming out of the fan switch. If the wire to the fan (load wire) is still powered when the fan should be off, I would say the fan switch part of the fan/ limit switch is sticking keeping power to the fan. Try tapping on that switch when the fan is on & see it it shuts off..

That's an old furnace. Have it checked for cracked heat exchanger


Last edited by kenmac; 10-30-2010 at 08:16 PM.
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Old 11-01-2010, 04:50 AM   #3
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Fan off temp is set pretty low, looks like 85, set it to 110, and see if that helps. If not. then the fan control is worn out.
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