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coldinindy 10-09-2012 09:22 AM

Furnace Blower just hums
The blower motor won't move when it should. If I open the furnace and move the fan the motor works for a while and slows then stops. There is a hum coming from the section the fan is in when the motor should be running. Same is true for either in furnace mode or just fan set to 'on'.
There's a schematic on the inside panel and a spec sheet but neither of them tell me who made the furnace.
Thanks for any help.

Wuzzat? 10-09-2012 10:52 AM


Originally Posted by coldinindy (Post 78102)
If I open the furnace and move the fan the motor works for a while and slows then stops.

Run/start motor capacitor has changed value? You can measure it without a C meter in several different ways.

The data printed on the capacitor may be covered by its clamp.

coldinindy 10-09-2012 06:02 PM

Thanks for the reply. I'm not sure how to measure the capacitor. I have a VOM, will that work?
Thanks again.

Wuzzat? 10-09-2012 07:31 PM

Post a link to your meter
Pick one that you're comfortable with -

if you have a digital meter with 10 megohm or 20 megohm input resistance, discharge the cap with a one megohm resistor from Radio Shack across the cap
charge this combination with a 9v battery
disconnect the battery and time how long the cap/resistor takes to get to 3.3v (actually, 0.368 of the initial cap voltage, which may be as high as 9.6v)
the time in seconds is the capacity in uF
wire the cap across the 10v to 24v output of a doorbell transformer
measure the voltage V across and the current I through the cap
then uF = I x 2653/V

For this last one, the more I, the more capacity. For 1 uF and 12v I get an I of 4.5 mA. It also assumes you have 60 Hz power and not 50 Hz power.

Daryl in Nanoose 10-14-2012 10:33 AM

WOW wouldn't it be easier just to replace the Motor?

asbestos 10-14-2012 11:46 AM


Originally Posted by Daryl in Nanoose (Post 78331)
WOW wouldn't it be easier just to replace the Motor?

call me lazy, and part of the throw it out generation but I would think about replacing the cap or the motor.

Older Capacitors (1980's and prior) can have PCB's in them :(

Wuzzat? 10-14-2012 02:06 PM

You are less likely to be chasing your tail and going in circles when troubleshooting if you know the value of the old cap.
And the new cap, if you buy one. Sometimes new parts get instantly destroyed.

Information has value and this kind of info is a bargain at almost any price.

notmrjohn 10-29-2012 11:01 AM

My truck stopped running so I traded it in on a new one. Dealer showed up with tow truck to haul my old one away. He poured some gas into my old truck and drove it away.;)

If you put a new capacitor on a bad motor you can burn the capicitor up in seconds.If you put a new motor on a bad capacitor it won't run. In both cases you've wasted time and money. New motor can run a couple of hundred bucks.And often is not returnable, especially if you damaged it by trying to run it of bad cap.

The problem may be neither, without checking each part you could wind up will he nill he replacing every part in the unit. A pro will check the parts, he may even try a heavy duty jump start capacitor.

The damage to parts may be caused by another problem, that could just damage your new parts. But in this case my bet is on bad capacitor, usually the case, new none pcb caps just don't last as long, heat gets to them, especially if blower is after heat source. Replacing parts is not fixing anything,
your just a part replacer, where's the glory in that? This is a DIY forum after all.

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