Suburban NT-12SE furnace only lights with cover off - why?

[ShrimpBurrito]

Hi -

I'm hoping I posted this in the right place. We have a 2004 Casita trailer with a gas-fired furnace -- a Suburban NT-12SE -- that does not consistently light, except when I remove the front cover, in which case it lights all the time (so far, anyway).

The start-up sequence is normal -- the fan starts, the gas valve opens (because I can small gas outside the trailer by the furnace vent), and then I hear a clicking sound, which I presume to be the ignitor firing. It tries to ignite 3 times, and when it doesn't light, the fan shuts down. When it does light (not very often), the furnace operates normally.

Looking through the sight glass, I can see when the spark should be, and when it lights, including when the cover is off, I can see spark. When it fails to light, I cannot see spark in that same place -- so that seems to be the source of the problem.

The intake and exhaust vents are both perfectly clear.

I checked the ignition wire (at least the part that i can get to), and it appears fine -- insulation shows no sign of degradation or cracking. But just for heck of it, I slid a thick piece of insulative sleeving over the wire, but that did not change anything.

Any suggestions as to what might be the problem? It's cold at night where we camp!

Many thanks,
Dave

[ShrimpBurrito]

Yes, the gas valve is opening when the cover is on because I can smell it coming out the exhaust vent outside.

True, the ignitor wouldn't work with the cover removed if that were the culprit, but I'm just operating under the assumption that there is some defect in the wire that is not visible to the eye. Perhaps there is a small crack in the insulation that is shorting to the cover when it's on. I doubt it, but it's cheap and easy to replace, and plus, I don't know what else to check. No debris on the burner.

I called Suburban, and they said they don't even make the NT-12SE any more because it had problems lighting. But it sounded like that was more due to an inadequate amount of gas, as the NT-16 is the same exact unit except it has a higher BTU burner.

Dave

[Ex-Tech]

Having the cover off allows more air to flow from the fan.
Are you sure that the gas valve is opening when the cover is on?

If the ignitor was defective, it wouldn't work even with the cover removed.

[ShrimpBurrito]

Thanks for the suggestions, Ex-Tech. Those are good ideas, but I don't know if any of them would explain why the furnace lights reliably when the cover is off.

The battery voltage is ok, but aside from that, I've been doing this testing with the trailer hooked up to AC power. So the furnace has been running off the converter DC output.

If the sail switch or fan motor were defective, the propane valve wouldn't open, correct? (and the fan blows out air, so that seems fine)

The furnace goes through a normal start-up procedure: the fan starts up, the propane valve opens, and the ignitor attempts to spark. The only thing that's not happening is that a spark is not actually being generated -- when the cover is on.

I think I'm going to replace the ignitor. It's only $10 and is easy to replace. I will also check the burner for mud daubers and other debris -- I hadn't thought of that.

Any other thoughts?

Many thanks,
Dave

[Ex-Tech]

Could be the following:
Low battery voltage not allowing fan to reach enough RPMs to activate sail switch.

Defective sail switch.

Dirt around the pivot point on the sail switch not always allowing full closure of the switch.

Defective fan motor.

Mud daubers or Derise on the burner.