Furnace issues..but weird

[Matlck333]

Hey guys....I’m hoping someone can perhaps help me with an issue I’m having with my fa-7920 furnace in a bit of an older motor home. I’m pretty handy with fixing things....but this issue stumps me (thinking maybe motherboard...but I don’t know the finer details about what or how it controls things).
My issue: with the proper voltage, the electric ignition and furnace work perfectly. I can set a temperature on the thermostat and it’ll warm the space to the desired setting. For the first few cycles it works perfectly.....but then, after sometimes 2, sometimes 6 cycles....when the furnace enters the no heat and cool down portion of the cycle, the blower remains engaged until the battery is depleted. Even if the temperature drops below what the thermostat is set for, it won’t reengage unless I switch it off, let it shut down, and then switch it back on. I’m then good to maybe get 2 to 6 cycles again.
It’s driving me crazy, so any help would be incredibly appreciated.
Thanks.
Ps, if you’re correct, and you’re in Banff any time soon....I’ll buy you a beer...or 6 (it is Banff ��) ��

[RPR]

Is there a separate relay that the mother board activates to turn the blower fan on that may be sticking. Some circuit boards operate off a low voltage and use relay to send a higher voltage to fan. Sensor that monitors temperature on cool down cycle may be faulty. And of course the good old mother board.

[Matlck333]

Thanks for the suggestion. I have a wiring diagram, but don’t see said sensors. Thoughts on diagnosis/process of elimination? And legit, thanks for any advice or thoughts

[RPR]

May not have a sensor. May be a programmed timer in board.

[ALJO]

There's always a high limit sensor and a sail switch for the blower. Also the ignition probe has a function.
High limit and sail switch has a closed contact for ignition to occur. The fan blows first for +/- 15 seconds to clean the chamber before ignition.

[ALJO]

You might have a faulty board, but first eliminate the other possibilities.

[curver900]

a volt meter is probably your only friend.

but for a cheap amount of money I would replace the thermostat maybe go digital and see if that doesn't solve it.. if it does then that was easy...
if not then it is a relay that when you reset the thermostat it goes back too its proper state. When it gets too hot it goes into an "on" state.. replacement is the only option (you can test it buy shielding it from the heat while you are there and watching it)

Trace the wires from the board to the fan and the ones to the thermostat... are there any relays in that circuit? is there a relay on the fan that sends a signal back to the furnace or sends a signal to the thermostat? if so those need to be checked.. if not then you are down to the board itself... without the wiring diagram it would be hard to diagnose...

if you can share the wiring diagram that would be useful

we will be heading by Banff in August... mmmmm beer......

[Matlck333]

Turns out it was the thermostat. A $20.00 replacement and an easy install. Love cheap fixes. There was a wire fried going to the old thermostats Anticipator.
Thanks again for all the suggestions. ����

[ALJO]

If a wire is fried on the TH, then the question is still why and you should find the cause of that problem. From the furnace to the TH it might touch another live wire, for example by vibration during travel. Wires don't get hot without a reason.

[Matlck333]

@ALJO.....I understand this....BUT, this particular thermostat is just on the south side of 35 years old. Honestly...it’s been working ever since the replacement...and on older rigs like this.....one repair at a time. Thank you for your concern, though. 😁

[curver900]

Originally Posted by Matlck333

Turns out it was the thermostat. A $20.00 replacement and an easy install. Love cheap fixes. There was a wire fried going to the old thermostats Anticipator.
Thanks again for all the suggestions. ����

let me know where you are in Banff we can swing by for those beers around August 4th....