Strange Atwood furnace problem

[Lee]

Atwood 8531-IV-DCLP furnace.

TT is parked under a pole barn and is plugged into 115VAC power year round. Batteries are at 12.9V when unplugged from AC. Propane tanks are full and all other propane appliances work.

Furnace has operated just fine for many years until recently.

When a call for heat is performed at the thermostat, the blower will come on for ~5 seconds and then stop. After it stops a "buzzing" sound is heard from the furnace control board and a 1-blink error code LED. This blink translates to an air flow/ sail switch problem.

I have partially disassembled furnace and have continuity thru the sail switch when manually activated. Also have continuity thru the Hi-Temp sensor. Thinking maybe the sail is "stiff" and the blower can't actuate it,... I manually activated it when the furnace attempts to start. No change.

I discovered that the "buzzing" sound is coming from the large relay on the control board that activates the blower. The armature in the relay must be being commanded on/off at a high rate of speed.

Ordered a replacement control board and installed....... exact same failure symptoms!!!!!!!!

Have reseated every spade lug connection in the furnace... no change.

At a loss,.... anyone have any ideas?

[ChuckS]

The buzzing is most likely the TDR and its not getting enough holding current to stay activated .. hence the buzzing sound...

Have you looked at the fuse for the furnace and if it is the typical black auto reset fuse it may be a contributor

You've got a new control board installed, and have reseated all the connections to the new board..

But are you certain that your 12 volt supply leads to the furnace control board are good?

I thought about maybe a dragging furnace blower motor but the fact that it initially spins up kind of rules that out

I would .. if nothing else wire a direct 12 volt wire from battery to control board 12 volt incoming connection and a good ground and then try furnace...

If it then fires up you know you've got an issue with the source supply leads to the control board...

I am also not thinking about the two blue signal wires being an issue since the furnace does initially try to run with the motor spinning up

[jsb5717]

Also, sail switches are cheap. If it were mine I would likely by one to try it. It's possible that yours is failing and, perhaps, intermittent. I know they are a pretty simple/basic switch so perhaps it's a low percentage guess. But, if that's not it then I have a backup. At least I would know as I'm working down my troubleshooting list.