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obscenic's avatar
obscenic
Explorer
Aug 24, 2014

Atwood 7900 series furnace troubleshooting

I have a brand new 7916-II installed in my small coach. Thermostat is brand new, t-stat wiring is brand new.

The furnace gets most of the way through the ignition cycle before failing.

1. T-stat closes correctly
2. Fan motor spins up
3. Sail switch closes
4. Gas valve opens
5. Igniter sparks for 6-7 seconds
6. No ignition
7. Igniter waits, then sparks again.
8. No ignition
9. Igniter waits, then sparks again.
10. No ignition
11. Blower stops. Red LED blinks 3 times indicating Ignition Lockout Fault.

About ONE in ten times it works. No rhyme or reason. Sometimes it fires up first try.

Options ruled out:
-All components are new
-Vent/intake are not blocked in any way
-Power supply is a steady 12.2vdc
-Propane supply hasn't been tested yet, but not sure that it would ever work if the pressure were too low. Propane tank is newly filled.
  • ScottG's avatar
    ScottG
    Nomad
    Aug 24, 2014
    Sounds like it's not getting enough gas. Crack the line going to it to bleed air out. Does the stove get a nice strong flame?
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    Aug 24, 2014
    Spark electrode not properly positioned....

    Needs to be directly over the burner slots and the gap needs to be 1/8" (tip to rod).

    Also burner slots need to be clear of any rust/debris etc. that would disrupt gas flow.
  • jetcare's avatar
    jetcare
    Explorer
    Aug 25, 2014
    Loosen the propane line going to the furnace just a bit and see if you get propane flow. If you do, retighten propane line and wait 10 minutes. Stand outside by the furnace and have someone operate the thermostat. Stick you nose up near the exhaust pipe but keep your face away from the pipe. (Do you smell propane during and right after each ignition attempt). If you do, you have an igniter placement problem or an igniter lead wire that is shorted somewhere allowing the spark to jump to ground and not through the igniter.

    If you don't smell propane,(which is usually what I find) remove the burner assembly and inspect the LP orifice. I normally find a bug inside the orifice.

    All this is assuming you described your problem correctly and you have no LP ignition, not even 2-5 seconds worth.
  • obscenic's avatar
    obscenic
    Explorer
    Aug 25, 2014
    With so many brilliant minds here pointing towards the propane flow, I built a manometer, and to my dismay, I'm only getting about 6" W.C. at the valve! Guess I need to track that down before I try anything else.
  • obscenic's avatar
    obscenic
    Explorer
    Aug 25, 2014
    So I have 11 WC" at the regulator, so that's not the problem. I'm measuring 6 at the valve outlet, but perhaps that is correct, since half of the pressure would be going into the burner head?
  • obscenic's avatar
    obscenic
    Explorer
    Dec 19, 2016
    I forgot to post my results from when I actually traced the issue. It was resolved on page 4 of this thread:

    http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27915352/srt/pa/pging/1/page/3.cfm

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