Atwood 8535 IV-DCLP Mind Boggling

[dalyfr]

I am experiencing similar problems with my Atwood 8535-IV-DCLP as MichaelD123 had stated in his post of Feb. 1, 2016: furnace fires up - but only runs for about 2 seconds, shuts the gas valve, flame goes out, then it re-ignites, burns 2 seconds, goes out, and repeats this cycle without stopping the fan. It will cycle 3-4 cycles then the blower will stop, then within 10 seconds the whole process will start over. I have tested everything I can test: replaced anything that might be a possible problem (?). The last thing I tried was to pull the gas valve/ignition “module” (for lack of a better term) from the unit: attached extension wires to facilitate moving the “module” a few inches from the furnace: connected 12v to the furnace, flipped the reset switch, and the burner fired up and burned perfectly! I ran it for 30 seconds or so before I shut it down, and it seemed to burn flawlessly. Put the “module” back in the furnace, powered it up, and it started the 2 second cycle again.

All this is being done “on the bench”, not in the RV.

Please, any suggestions would be greatly appreciated.

 

[Gary RV_Wizard]

The "module" is the furnace controller circuit board, right? AFAIK, that's the only electronic "module" in an Atwood 85xx IV furnace and it does everything, i.e. fan, gas valve, ignition, and flame/heat monitoring. At a guess, your added extension wire solved an issue with the connections to the board. Either by eliminating a short somewhere (ground, maybe?), or by making a more reliable connection to other components. Needs further investigation & diagnosis to figure out just what is happening when you add the extension wire(s).

 

[dalyfr]

The "module" is the furnace controller circuit board, right? AFAIK, that's the only electronic "module" in an Atwood 85xx IV furnace and it does everything, i.e. fan, gas valve, ignition, and flame/heat monitoring. At a guess, your added extension wire solved an issue with the connections to the board. Either by eliminating a short somewhere (ground, maybe?), or by making a more reliable connection to other components. Needs further investigation & diagnosis to figure out just what is happening when you add the extension wire(s).

No, that was a poor choice of words, I should have said “assembly” (gas valve and igniter/flame sensor).

I think I resolved the problem: the exhaust was too “open” so there was not enough static pressure to maintain the burn. I had restricted the exhaust similar to the restriction of the exhaust pipe using foil tape, but evidently that restriction was not sufficient. I removed the exhaust piping from the RV and inserted it into the furnace on the bench and flipped on the reset switch and the furnace ignited correctly and seems to be operating correctly now.

 

[Gary RV_Wizard]

These gas furnaces are extremely sensitive to air/fuel mixture and even a slight disruption in the air intake or exhaust tubes can cause a no-start or flame-out condition. A few rust flakes, a spider web, etc can be the difference between run or not. I guess the same applies to a modified intake & exhaust.

 

[Ray-IN]

I agree Gary, RV LP furnaces are not known for their cutting-edge technology.

 

[Old_Crow]

These gas furnaces are extremely sensitive to air/fuel mixture and even a slight disruption in the air intake or exhaust tubes can cause a no-start or flame-out condition. A few rust flakes, a spider web, etc can be the difference between run or not. I guess the same applies to a modified intake & exhaust.

I've discovered on both of my 8500 series furnaces that they won't run without the exhaust stack in place. I've also learned to coat the stack with anti-sieze on the part where it slides into the rest of the furnace. I ended up replacing a furnace because everything was so rusted that I couldn't get it apart without destroying it.