SF 42 furnace madness

[Lt46]

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

[mchero]

Good News! Great that we have Doug on this site!

[Lt46]

Doug, Thankyou, thankyou, thankyou for insisting I chase the 12v on the blue wire. I pulled the furnace once again and chased the blue wire. Low and behold I find a butt connector tucked in and under the circuit board. Of course it had separated and once redone SUCCESS!!!! Thanks again for your guidance. All systems are running perfectly.

[Lt46]

mchero, yes, I have both.

Doug I will probe the furnace harness and check for power on the blue wires. I have already confirmed that I have 12v+ @ the furnace but never checked for 12v+ on the blue wire. Yes, you are correct I have no power from t-stat to rooftop coleman. Probing the Y, GH, GL and Ground on the AC unit shows nothing. FAN alone does not work either. I'll get to that later this morning and post my results. Thanks

[dougrainer]

Lt46 wrote:
Thanks Doug. The furnace harness has the red, yellow, and 2 blue wires which terminate into a molded plug that connects to Winnebago's house wiring. The 2 blue wires I followed from there and they terminate into the mini on/off switch near the circuit board. I see no other blue wires. BTW, I never altered any Winnie wires nor the furnace wires. Molded connectors at each ends of the Winnie wires.

Is it possible you swapped the blue wires inside the furnace when you did all the work? The MAIN thing, you should have 12 volts positive on 1 of the blue wires exiting the furnace. Report back on this. Yes, most RVP and Dometic systems have a relay, but you state the wall tstat is dead. Which means you have no power to the tstat or upper control board. Doug

[mchero]

Do you have a test light or volt meter?

[Lt46]

Doug, I remember reading a post of yours to another camper with a similiar problem (2010). You asked him if he had a Dometic AC and if so there would be a relay up there worth looking at. Does that also apply to Coleman 7333C886 units?

[Lt46]

Thanks Doug. The furnace harness has the red, yellow, and 2 blue wires which terminate into a molded plug that connects to Winnebago's house wiring. The 2 blue wires I followed from there and they terminate into the mini on/off switch near the circuit board. I see no other blue wires. BTW, I never altered any Winnie wires nor the furnace wires. Molded connectors at each ends of the Winnie wires.

[dougrainer]

You are missing where the wires we are talking about are located. There is NO switch at all. You should have a RED/YELLOW and those 2 blue wires that exit the furnace case and go to the interior of the RV. Switch the 2 blue wires where they are next to that Red/Yellow. Doug

[Lt46]

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!!!

[mchero]

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.

[Lt46]

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

[dougrainer]

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

[Chris_Bryant]

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.

[prichardson]

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