SF30-F furnace issue

[bandlgeorge]

Hi all. I have a 2010 322 QBS with a SF30-F furnace. Furnace quit operating the other day. Here’s what I know.
No blown fuses.
Plenty of gas.
Thermostat still clicks once when slid up to activate.
Thermostat still operates AC unit.

Pulled furnace and found:
Manual applied power to blower fan through leads and it runs strong.
Sail switch doesn’t appear stuck and has continuity.
Board was bench tested and passed with flying colors.
On/off switch is on.
Thermal cut off switch has continuity.

Sooooo. What I understand is that when the thermostat trips, the board starts the blower fan first, then sail switch, then igniter, then gas valve, and away you go.

Only thought I have left is maybe the thermostat wires lost continuity???

Any other suggestions or leads. I’m at a loss here and not sure which way to go.

Thanks again for any input.

[tech740]

I have same model rig. Last year I had to change a propane regulator out because it would allow flow but not enough flow. I know you said you have plenty of gas but do you mean at the tank or at the appliance?

[bandlgeorge]

Thanks for the reply. I have the rig attached to a 100 gallon tank. Seems like the blower should be kicking on before all the other safety mechanisms check off. But that’s my amateur understanding. Propane stove still works fine, although it probably doesn’t require the same flow as the furnace.

[chuckster57]

Your pretty much spot on on the order of events. I would test for voltage at the gas valve. If you have it then I would replace board.

[bandlgeorge]

Copy. Last thought. If I trigger the thermostat, shouldn’t I have 12 volts at the furnace end of the thermostat wires? I’ll probably try that tonight and if no luck, have the board bench tested tomorrow. Thanks again for the help.

[kokomimi]

I know you said no blown fuses however our 2014 Bullet we just bought in Feb had the same symptoms - AC works but furnace did not - nor would the power supply charge the battery. I found the two 40 amp fuses blown. Changed them out and furnace works fine.
Good luck,
Pat

[chuckster57]

I know you said no blown fuses however our 2014 Bullet we just bought in Feb had the same symptoms - AC works but furnace did not - nor would the power supply charge the battery. I found the two 40 amp fuses blown. Changed them out and furnace works fine.
Good luck,
Pat
AC operates on 110VAC, so you were plugged into shore power or a generator. Furnace operates on 12VDC, totally separate. The two fuses you mention are the “reverse polarity” fuses which will blow if you put the battery leads on backwards, or while plugged in, allow the positive battery lead to touch the frame. Once they blow you don’t have any 12V in the trailer.

[chuckster57]

Copy. Last thought. If I trigger the thermostat, shouldn’t I have 12 volts at the furnace end of the thermostat wires? I’ll probably try that tonight and if no luck, have the board bench tested tomorrow. Thanks again for the help.

We use a test light, not an Ohm meter, but yeah that sounds right. Probably 4 wires right? Power, ground, signal to and from thermostat. Thermostat wires usually blue. We bench test as follows:

Good battery, power and ground hooked up. When you join the thermostat wires, the sequence begins. High limit first, then sail. Then gas valve and sparker. Test light at each side of both switches. Then gas valve. Listen for igniter, DO NOT use test light or Ohm meter. If you don’t hear sparker then replace it first.

There is a board tester made by Dinasour, but it’s a bit pricey for the DIY’er.

[rhagfo]

You might listen at the vent, if you hear it fire, but not continue to fire you might try replacing your regulator. Mine bench tested fine, but failed in the rig. The stove worked fine, but the regulator could not supply enough to keep the furnace fired.

[bandlgeorge]

So sorry had a secondbench test done that came back intermittent operation. Possible bad relay that worked when tapped on. So I ordered the dinosaur brand replacement board and will install tomorrow and let you all know what happens. If not that, then I’ll check the regulator and gas valve. Thanks again!

[bandlgeorge]

Update. Put the new Dinosaur board in tonight. Now the blower works great, but no the burner doesn’t light. Gas is on and purged at the closest appliance. Sail switch appears fine and passes continuity test and does not have any debris or appear like it has been or could be getting stuck. Guessing the gas valve or Ignitor is out. Any tips on how to test each component or other components that could be bad?

[chuckster57]

If you can see the schematic, it will show the color of the wire to the gas valve. I use a test light that can pierce the insulation and then just start the furnace. As for the igniter, if you hear it clicking it’s probably working.

Another possibility is the burner is covered with a mud dawber nest.

[bandlgeorge]

——-FIXED——-
Thanks or all the help guys. Troubleshoot more today. Long story short, I don’t think it was the board, but the board did help with diagnostics (for me at least). After my issues of no ignition after installing the board, i contacted Dinosaur to understand their product. After learning what the LED indicators on the board meant, I was able to determine I had either a sail switch problem or gas valve, as mentioned in an earlier reply. I had already checked the sail switch once, but started there as the blower (first step) was working. Sail switch had continuity, with my pin point contacts on my multimeter. Removed sail switch to eliminate and made a jumper to complete the circuit and the furnace ran 100%. Took the sail switch under better light and noticed buildup on the hot side spade, although contact still seemed sufficient. Sanded the spade contacts cleaned, reinstalled, and I’m back in business. Not crazy to have a board that I didn’t actually need, but it is a better board and it did help with diagnostics.
Thanks again!!

[BigTexRex]

Great troubleshooting...glad you got it fixed.

[bspiek]

I believe the Sail switch has to close for ignition, but has to open again to allow any additional calls for heat. So without fan running their should be no continuity.

[bspiek]

Sorry didn't make it to second page.