Circulation fan not getting power


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Old 07-20-08, 07:43 PM
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Circulation fan not getting power

Thanks to these boards, I saved a bundle last month when the capacitor on my condenser fan went out.

Now it's a new problem. Yesterday, the circulation fan in my furnace quit working. Upon investigation, the drain line from the coil inside had plugged and a pretty significant amount of water had dripped inside the furnace (5 year old system). Everything had iced up, so I shut off the system, unplugged the drain, and thawed and dried it all out overnight.

The furnace has an ignition control unit; I put my multimeter on it and found there is no power to the fan. The compressor outside runs normally, but the circulation fan inside never kicks on, in ac or heat mode. Nor will it come on when I move the fan switch on the thermostat from auto to on. I have 120 vac on the wire coming from the access panel switch that plugs into the "Line" spot on the control unit, but have not measured any voltage on any other of the "Hot" lugs of the control unit.

For temporary relief from the heat, I have run power directly to the fan from a nearby outlet. It started right up and runs.

I have checked the capacitor on the fan motor, it tested fine. I tested voltage at the thermostat and jumped from red to green with no result.

The White Rodgers ignition control unit has a LED diagnostic indicator that gives me one flash of the LED when power is applied to the control. One flash is supposed to mean "external lockout." Does that suggest that there is a sensor of some kind that is signaling the unit to lock out?

Any other ideas? Replacing the control unit is about $275, which I have no problem doing if that is the culprit. I just would rather not drop that kind of money if there is something else that I haven't thought to test.
 
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Old 07-21-08, 06:37 AM
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Double post. Computer acting up lately.
 
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Old 07-21-08, 06:42 AM
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That ignition control module would not be affecting the blower fan. That powers up the ignitor and then powers up to open the gas valve for furnace.

If your blower runs direct off outlet, that means blower and capacitor are both good.

You have some other issue where power is not making it through the 24 volt pathway, or, at a 24 volt to 120 volt relay is bad, be my guess. You sound like you have soem basic sense of what you are doing. You just have to trace your problem back, to see where your blower wires go. Do you have another board, besides the White Rogers ignition control that has all kinds of resistors, diodes and relays on it, like down in the blower compartment, hanging there? Sometimes parts are nto evn replaceble onthose type boards, although there have ben poster where in the past that have managed to solder in new parts. But this is the exception. Normally people replace such boards for like $85-150 or so.

I'm the first to respond. Others here may have other or more specific ideas.

Another idea crossed my mind after I submitted: IF the module has anything to do with it, it be because your 24 volts shorted out from the water, and your transformer is knocked out [but, you did say you checked at thermostat, and I presume you found 24 volts in yourr testing?], or maybe if you have a 3 amp fuse on a board, it maybe went out. Check to see if you have 24 volts (if you do the transformer is working), and if you lose it somewhere.
 
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Old 07-21-08, 09:53 PM
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Thanks for the suggestions. I'm now on a business trip for three days and won't be able to look at the unit again until Wed. evening.

I did have 24 vac at thermostat. But after finding that, I didn't check any more of the 24 volt side.

Also, there is no other board, the ignition control unit appears to handle it all.

I'll have to dig in a little deeper with the schematic in hand when I get home.

thanks again.
 
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Old 07-22-08, 06:51 AM
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Did you check inside the blower compartemnt door? Look in there. Let us know what you are finding out when your testing resumes Wednesday or Thursday.
 
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Old 07-24-08, 12:28 PM
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Issue Resolved!

Pulled control unit this morning. Opened it up, quickly found a bank of 4 resistors in parallel that had obviously gotten quite warm, even melting the solder on the board. Resoldered those.
Also found a diode that had failed. 99 cents at Radio Shack replaced it.

Put it back together and everything works well. I even earned a high five from teenage son.

Thanks to all for the info on this board that encouraged me to keep searching for solution myself. I'd bet I saved a $500 service call.
 
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Old 07-24-08, 04:36 PM
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Originally Posted by lkshep
Pulled control unit this morning.
Ignition control module you spoke of a few posts down? Or did you find another control?

IF it was the igniton control module, you must have a more complex one, than the typical White Rogers ones I have dealt with, with more terminals.
 
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Old 07-24-08, 08:41 PM
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Only the one control. Everything in one unit. Very similar to this:


I found more evidence that water had gotten inside the control, I suspect that led to the problem. I'm just really glad it was an inexpensive fix. Thanks again for the help.
 
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Old 07-25-08, 05:45 AM
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Thanks for the pic. That explains it. A lot more to this particular module than just being a plain simple ignition module.
 
 

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