Hi, Joel--
I just ran onto another troubleshooting manual for the Atwood furnace which may seem more readable than the other one I linked to. I'm going to call it the
Gary Brinck troubleshooting manual. Both manuals have the sequence-of-operations info Russ mentioned above.
Other thoughts.....
- Several places in the manuals refer to the importance of adequate voltage (as others have mentioned). I'd check voltage at the switch or circuit breaker, hoping to see something in the 12s to the mid-13s. I believe the furnace will start up with very low voltage-- 10.5 or so-- but the motor may not turn fast enough for sustained operation (because of insufficient air flow). Brinck says in practice the furnace may not run below 11.5 v. If you've been testing only on batteries, you'll want to also test on shorepower.
- Both manuals mention high-limit cycling, i.e., low voltage causing the motor to run slow, resulting in overheating and the high-temperature limit switch tripping. The service manual tells us (in the trouble-shooting section) that in rare cases a limit switch can be overly sensitive.
- I'd be curious to know whether there's sooting in the exhaust pipe of your furnace (which Brinck says may indicate insufficient combustion air flow)
- You mention you had an air-flow/limit indication at least once. Did that happen more than once? If so, what were the circumstances?
- You noted that your furnace shut down (after running successfully for a while, I presume) about ten minutes after you put the duct-blank in place. You then removed the blank and the furnace ran successfully. Was there an indication on the control board LED when the furnace stopped (with the blank in place)? And are you sure the thermostat circuit was still calling for heat at that point? (and of course if you still had the thermostat wires twisted together you know it was).
- Any discussion of air flow typically gets into the role of the sail switch. It's role in the startup sequence is to close when the airflow past its vane indicates the motor is up to at least 75% speed. I can't think how it might be involved in the duct-blank situation but you did get at least one fault possibly involving the sail switch. That switch is located behind the motor enclosure half and is relatively easy to access and test (a few continuity tests while you manipulate it should work). You'd also be looking for a bent vane or any kind of fluff hanging off it. And of course you'd be getting a clearer look at the blower wheel while you have that cover off. You can see the sail switch at the 4:45 mark in this video of a similar model. Also, the photo of the 85xx-IV in the Brinck manual shows the sail switch is there.
Edited 9/25 to add:
- you might want to call around to local RV shops to see if they'll test your control board (if the need arises). My local RV shop keeps a tester at the parts counter and tests boards for free. But I'm sure policies vary. Before going in, you might want to research replacement board cost (I'm thinking Amazon and Dinosaur Electronics).