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Old 09-08-2011, 10:06 AM   #1
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ICM280 Board problem


I have a ICM280 on a gas furnance. The Tstat is not connected. Yet when I power up the unit the red light comes on, a relay clicks then both the vent motor and idf start and the red light turns off. I know that when I disconnect the roll out circuit the vent motor turns off but the idf stays on. I called ICM280 and they said check voltage between the r & c. Does that mean r from the ( to room tstat and common ) or what is it that I do check. Also I believe it is a open limit switch, which I think is behind the gas valve ( i did not have time to check it yesterday. Does that sound fimiliar, if the limit switch is open the vent motor and idf turn on without a call for?
Thank you in advance

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Old 09-08-2011, 11:56 AM   #2
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ICM280 Board problem


After a second chance today looking at the unit, I tried my theory on the limit switch and was correct. So for you that dont know, when the limit switch is open the vent motor and idf will turn on.

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Old 09-08-2011, 02:24 PM   #3
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ICM280 Board problem


That is a lockout mode which should flash a error code on the main board. If the limit is failed, you can test with a multimeter on "connectivity" setting (it should beep or show 0 ohms)

If that is the diagnosis please be careful, limit switches are like airbags in cars; they go off for a reason. This could indicate a bad heat exchanger, which is very dangerous.

Good luck!
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Old 09-08-2011, 08:40 PM   #4
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ICM280 Board problem


Thanks for your response.

It was the limit switch, afterward I tried to test multiple reasons why the idf and vent motor would turn on without calling for. The limit switch was the only reason why they do, however if the transformer 120/24v is wired in reverse ( either Hot to white and Neutral to Black or secondary side being whatever colors are flipped ) then it shows the same problem minus the blinking light. That however is not a failure problem but a faulty wiring which can not be over looked by a tech before he finishes the install because the unit wont work until fixed. So I'm just pointing that out that it can show very similar problems. But the limit switch is the only reason why the motors would be on without a tstat call.
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Old 09-08-2011, 08:59 PM   #5
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ICM280 Board problem


Perhaps. The reversing of polarity is a rare service call. It is usually due as you said to improper wiring from an initial install. It used to come up occasionally when "twinned" units were changed. Is there some diagnostic in the units schematic that led you to the decision?

It is also possible that the ECM is faulty, ot the wiring harness is not properly connecting if you can 100% eliminate the other elements through sequential elimination.

But again, a snap disc limit is a permanent failure for a reason. Verifying airflow, venting and testing the heat exchanger are essential if I replace one of those in good conscience.

Last edited by Dr.HVAC; 09-08-2011 at 09:03 PM.
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Old 09-08-2011, 09:19 PM   #6
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ICM280 Board problem


No, I'm a electrician by trade and I usually try random stuff after I fix a trouble this way in the future if I run into a problem I'm quick to know why it happened. Especially when it's 24v control wiring or just about any appliance. Furnances are easy and just about all wiring is but the first time I looked at the unit the day was coming to end and I knew I was going back so I decided to ask to save time. Then when I looked at the schematic after hours I realized I never checked the limit.

The board itself on that unit was completely ok and yes I did verify all aspects of the unit when I replaced the limit.

All good stuff to know, for anyone reading this thread, know the sumary ( If a vent motor and idf are running without a calling for the problem is most likely a open limit switch or faulty transformer wiring) . Pending the Board is wired right else where.

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