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Open Roads Forum  >  Tech Issues

 > SF 42 furnace madness

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Lt46

Central Florida

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Posted: 02/24/20 06:15am Link  |  Quote  |  Print  |  Notify Moderator

So, turned on heat and got fan but no ignition. Pulled furnace and cleaned, while also doing a tune-up. New sail switch, new overtemp switch, new ignitor and new burner installed. Bench tested 12v + and 12v- connected, jumpered the 2 blue wires and got fan, gas valve click and ignitor sparking. All looks good so far. Re-installed, connected gas line, checked for 12v+ and 12v- at connection and is good, jumpered the 2 blue wires again and the cycle began as expected. Fan operated, click from gas valve and ignition of gas producing heat. Yeah, still good so far. Shut it down and went inside, set thermostat to 90*, set mode on T-stat to HEAT and NOTHING, no fan, no click, no spark, no nothing. What the heck is going on? Further investigation reveals my t-stat will not turn on the AC fan or AC cooling either, no nothing. This is beginning to look like a t-stat failure??? BTW, the 2 amp fuse in the t-sat checked and OK but I put new one in, just to rule it out. Anyone else have any ideas?
Thanks


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Chris Bryant

Arden, North Carolina

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Posted: 02/24/20 06:33am Link  |  Quote  |  Print  |  Notify Moderator

Try swapping the blue thermostat wires- one supplies 12 volts to the thermostat.


-- Chris Bryant

Lt46

Central Florida

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Posted: 02/24/20 06:39am Link  |  Quote  |  Print  |  Notify Moderator

Thanks Chris. I have 8 wires coming out of the t-stat with only one blue wire. I'm not understanding what you mean about swapping the blue thermostat wires?

Lt46

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Posted: 02/24/20 06:51am Link  |  Quote  |  Print  |  Notify Moderator

Thanks Chris. I have 8 wires coming out of the t-stat with only one blue wire. I'm not understanding what you mean about swapping the blue thermostat wires?

prichardson

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Posted: 02/24/20 07:14am Link  |  Quote  |  Print  |  Notify Moderator

Swap the two blue wires where you made the connection at the furnace.

Chris Bryant

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Posted: 02/24/20 07:25am Link  |  Quote  |  Print  |  Notify Moderator

What he said- swap the wires at the furnace, as one of those supplies power to the thermostat. If you swapped them when you reconnected them, nothing will work.

dougrainer

Carrolton, Texas

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Posted: 02/24/20 08:03am Link  |  Quote  |  Print  |  Notify Moderator

SWAP the blue wires at the furnace connection. Your system probably uses the 12 volt supply from the Furnace to power the wall Tstat and roof AC system. You have the feed side to the tstat on the RETURN voltage side. Doug

Lt46

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Posted: 02/24/20 08:44am Link  |  Quote  |  Print  |  Notify Moderator

Aha, that makes perfect sense and something I have not considered. Thanks very much for the assist.

mchero

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Posted: 02/24/20 10:49am Link  |  Quote  |  Print  |  Notify Moderator

My furnace depends on both the thermostat AND the control board up in the AC intake duct. You might need to check for power to that board. Also read where exercising the dip switches on that board, meaning if a switch is off flip it on then back to off, if on, flip to off then back on.

What brand of tstat? Might need a reset.


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Lt46

Central Florida

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Posted: 02/24/20 11:43am Link  |  Quote  |  Print  |  Notify Moderator

It's the OEM Coleman analog t-stat with furnace, fan and AC operations. Built in time delay relay is printed on the cover.
I followed my blue wires from the furnace harness to the on/off switch on the circuit board. Switching their position on the switch, had no effect on the operation of the furnace. Yet still, if I jumper these 2 wires, the furnce blower runs, sail switch closes, gas valve clicks and ignitor sparks. Once I remove the jumper to the blue wires nothing happens. T-stat set to HEAT, and TEMP set to 90* and nothing happens.Only with the jumper in place on the furnace harness can I get the unit to run. On top of this, setting T-stat to Fan/High or Low and/or COOL Fan/High or Low nothing happens. I'm pretty much pulling hairs out of my head!!!

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