furnace fan running constantly


I have a 25 year old carrier upflow furnace that the fan runs constantly.I replaced the old blower motor along with the capacitor. The old one froze up. After installing the new one , it runs even the new themostat if in the "off" position. I am getting 110 volts into and coming out of the transformer which does seem right. Also, there is no voltage at the "r" and "c" terminals.Also the A/C has power and does not kick on. Any ideas? Thanks -------------------------------------
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probuilt1 wrote:

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responding to http://www.homeownershub.com/maintenance/furnace-fan-running-constantly-519585-.htm probuilt1 wrote:
Tony Hwang wrote:

-------------------------------------
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probuilt1 wrote:

http://tinyurl.com/287k68b
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On Mon, 20 Sep 2010 13:13:51 -0500, Dean Hoffman

I'll second that !!!! They get stuck and they need to be replaced. A couple screws, a few wires and $20 should do ya....
Shut off the power, disconnect the wires, and put a multimeter across the fan limit switch, If it's ON (registers on the meter), and does not go off, its defective. Replace it. No big deal!
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25 yrs old ,I think you need a new furnace.

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My gas furnace is 50 years old and still works great. Years ago I filled in the old gas nozzle and then drilled out a new hole much smaller. My gas bill is now 2/3 of what it used to be as I am not putting so much heat up the chimney. Still never run full time even in -20F temperatures we get once a winter or so. Still have sufficient air/gas flow that the flame does not back up toward the gas nozzle. A couple of carbon monoxide detectors protect us if/when the fire box gets a hole. The furnace plenum switches are the likely problem, the low temperature relay may be sticking, causing the motor to run even when the plenum temperature goes below whatever temp it is set to turn off at.
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wrote:

At his era I think a lot of the carriers had a simple timer circuit to keep the blower running for cool down.
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On Sep 20, 12:58pm, commbuilder40_at_aol_dot snipped-for-privacy@foo.com (probuilt1) wrote:

There's probably a relay stuck on the control board. When the first motor froze it may have welded the relay contacts closed. You'll have to trace the motor connections back on the board to see where the relay is.
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*110 volts on each side of the transformer does not seem right. Normally one side would be 110 and the other side would be something like 12 or 24 volts.
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responding to http://www.homeownershub.com/maintenance/furnace-fan-running-constantly-519585-.htm probuilt1 wrote:
John Grabowski wrote:

Someone suggested that the relay may have welded itself together when the motor froze. The relays do not appear to be easily removed. I think the board needs to replaced which probably means its time for a new furnace Thanks for the help. -------------------------------------
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On Sep 21, 12:27am, commbuilder40_at_aol_dot snipped-for-privacy@foo.com (probuilt1) wrote:

Well first I would confirm with an ohm meter and the power off to the unit. As to replacing them it's not all that difficult. Problem is finding matches.
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mine you can have the thermostat turned off, but there is a separate control for the fan. During the cooling season I leave the fan on continuously, which would give the same result you're reporting.
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Just take the wires off the thermostat. Did the fan quit running? If it did, you need a new thermostat. If not, it's the fan limit switch, or control board with relay. If the relay got welded together from an overload of the former siezed up fan, you might be able to separate the contacts with a screwdriver (with POWER OFF). Just play with the relay and see if the contacts are stuck. If you do separate them, use a piece of sandpaper to clean them, or very thin file. I would find a replacement ASAP.
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