I have a Swift Shell with the furnace that I picked up in January and finally got the chance to use it last weekend. I ran into an issue when what was supposed to be a mid 40 degree night turned to high 20's, My furnace wouldn't light, which it did when I tried it in the driveway after bringing the camper home.
What I'm finding is if the voltage is 12.8v or lower the furnace won't light (I haven't seen it at 12.9, but it doesn't light at 12.8 and but does at 13.0). The fan comes on and clicks, every once and a while you can hear the gas ignite then immediately shut off. If the voltage is 13v or high the furnace just starts like it should.
Before I go through the FWC furnace troubleshooting video, has anyone see this exact behavior before and have what fixed yours? I'm assuming either a sail switch being weird and not beeing triggered when the fan is moving fast enough, or maybe the circuit board that controls things (I've seen a weird number of odd issues where that was the problem). I'm assuming since it turns on correctly at voltage the thermostat probably isn't the issue.
My expectations are that fan speed issues due to voltage should start to happen around 12.3-4 volts since it's getting pretty low at that point. Let me know if that assumption is off with it.