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I am changing a Lennox control board old part # 97L4801 to a…

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I am changing a Lennox...
I am changing a Lennox control board old part # 97L4801 to a replacement board Emerson/white-Rodgers 21D83M-843.
The old board has 1 green wire and 1 yellow wire going to 24 VAC Return Pins and 1 blue going to a 24 VAC hot pin but the new board doesn't have these pins but has these color wires coming from one of the adapter wire harnesses. I see that I can plug the new yellow right to the XFMR and the blue to a circuit breaker that then continues to the XFMR but the green comes off the old board and splits going to a screw next to the transformer (looks like a ground) and the other half goes up into the furnace. The new green wire doesn't split and just has a connector that would fit onto a pin. How do I connect this?? Thanks
Submitted: 6 months ago.Category: HVAC
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Answered in 4 minutes by:
1/9/2018
HVAC Technician: Phil, Mechanical Engineer replied 6 months ago
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,726
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified

Hello. as the broad rule. you must replace the Lennox control boards with an *identical replacement... any time you use a board with different wire connections the odds of it working drop radically. Something like taking the printed circuit board from a very very nice toaster and trying to use it as the mother board in your PC....

.

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Customer reply replied 6 months ago
They don't make the old one any more. This is the replacement that you get when you order the old part number
HVAC Technician: Phil, Mechanical Engineer replied 6 months ago

Hello again, it seems that *you understand the situation rather well, it is a bit hard to parse your narrative however into something that I can grasp as clearly on this end.

.

so we can discuss it a bit at least.

You understand that there are two poles on the 24volt transformer, and if you are sure the yellow and green shown connected in your first photo, go to one side of the transformer, and that the blue goes to the other side of the transformer, you can most likely connect that way...

.

if you try that, buy 2 each 2 amp automotive type fuse and fuse holder for it and wire it them series with the blue wire, and yellow wire connections ...that might save your new board if we are incorrect in our assumptions.

.

Let me know if this makes sense on your end or not... we can go from there.

.

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HVAC Technician: Phil, Mechanical Engineer replied 6 months ago

Ooops, my edit needs fixing

if you try that, buy 2 each 2 amp automotive type fuses and fuse holders for it, and wire them in series with the blue wire, and yellow wire connections ...that might save your new board if we are incorrect in our assumptions.

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Customer reply replied 6 months ago
Its actually yellow and blue which go to either side of the transformer. It looks like green is just a ground. Green and yellow were connected on the same pin then green went to a grounding screw and yellow went to transformer. Yellow and blue both fit on new board Its just the green ground that doesnt have a place to go
HVAC Technician: Phil, Mechanical Engineer replied 6 months ago

Go ahead and try it...

.

The 2 amp fuses however are too small to support the normal amperage draw of that board... but for test purposes, if they blow instantly we know for a fact that its wired wrong... if they take a few seconds or a minute to blow, then the wiring is likely correct and you can remove the fuses and fuse holders and rely on the 3 amp fuse that is most likely fit into the new board

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HVAC Technician: Phil, Mechanical Engineer replied 6 months ago

If all that works

Its time to rate my input for the day, that gets me paid, locate the "finish and rate your expert" link near the top of the page and click on it. I will appreciate that very much.

I am in every day from 9 am to 11 pm west coast USA time to do any follow up needed.

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