Atwood Furnance Only Blows Hot for 3 seconds

[Wildstandog]

I recently invested in a 1992 fleetwood wilderness. Super clean, meticulous owner, always kept indoor, no bugs.



Fired up the furnance and it didnt blow heat.



Investigated it further and it will blow hot for 3-5 seconds then shut off.



Investigated that further and I presume it's either the sail switch or the hi limiter that causing it to kill the heater.



My questions are - how accurate is the analysis?



How could I determine if it's the sail switch or high limiter ?

>> once I know this I will change the part.



Thanks

[Dutch Master]

You could either use a volt meter or a jumper. If using a jumper - remove both leads, insert a wire between them. Start the furnace again and see if it keeps running. If it does, that component is faulty.
Regards
Dutch Master

[ga traveler]

If the high limit switch is open, it would not light. It sounds like the furnace is not recognizing it is lit and is shutting off the gas valve. It has a flame prover circuit that recognizes that the furnace is lit and keeps the gas valve open. If this fails, it does not recognize the flame and the gas valve closes.

[Old-Biscuit]

Sail switch...if not closing then no DC circuit for ignition
*it's closing cause you get ignition

Hi Limit Switch......opens when air temp thru heat exchanger exceeds 170*F/190*F (model dependent)
*takes longer then 3-5 seconds to achieve that temp rise


Spark Electrode....proves the high voltage park for ignition
BUT is also used as return path for the Flame Proving Signal (milliamp current generated by flame)
*dirty connection, soot/carbon on electrode etc will prohibit Milliamp return...circuit board shuts down heating

Suspect spark electrode is your issue.....pull it and clean up the electrode with emery cloth
Also check the spark gap...SB/ 1/8"