Atwood 8500 Furnace

Sorry for the very late update. We went to pull the furnace out of the rv to work on it and for the sake to prove that it was still broke we tried it. To the dismay of th owner it lite right up. I think the flame sensor is done. The outside warmer temp was all it need to get it to sense the flame(heat).
He wants to leave it but I think come fall he will have the same issue.
Thoughts?

NOT the Sail Switch
1. The Spark Electrode could be bad or incorrectly gapped (enough gap to light the burner but too close to the burner to correctly sense the flame to send the signal back to the Ignitor module) VERY RARE!
2. The WIRE from the Ignitor board to the Spark Electrode could be bad and "leaking" the return AC flame sense circuit.
3. The Control Board is bad. Doug

You obviously have an older model because the newer models shut the fan OFF after the 3rd attempt. IF the Control module is bad, you can install the newer board and rewire to the newer style to kill the motor after 3 attempts.

Also try pulling the connections off the ignitor board and re-inserting them a few times to clean the contacts. It doesn't take much corrosion to kill the milli-volt flame sensor signal.

Flame sensor needs cleaning or adjusted.

Thanks for the Help. I will let you know how we make out.

If the sail switch was the problem it would have never lit. The problem soumds like there is no flame recognition so it shuts down. Find the flame sensor and take some fine grit sand paper to it and clean it up. Just as in residential gas furnaces if the flame sensor is rusted up it cannot sense the millivolt signal carried from the flame. Sounds weird but that's how it works. Shine it up like a new dime and see what happens. There is a sequence of events that have to happen to get the gas valve to open. If it was the sail switch then it would have sensed no air flow and would have stopped the ignition process. Sand down the flame sensor, its just a metal rod. Now your spark ignitor may also double as the flame sensor. If that's the case, sand that down but becarefull not to increase the gap between the electrodes

Ours did that - after 3 tries gas lockout because ignition didn't take place. It was the circuit board. Replaced with a board from dinosaur electronics which is supposed to be a better board.

Try cleaning the thermocouple too. With even a small amount of carbon on it the output will be reduced greatly. Use either a light brass brush of a dollar bill to clean it - carburetor cleaner does a good job as well. Don't use anything scratchy.

newman fulltimer wrote:
try cleaning the sail switch first

That was my first thought but I my reading lead me to believe that the furnace wouldnt even light if the sail switch was the issue?

try cleaning the sail switch first