Suburban SF-35F won't light

[ZagiFlyer]

Hey All,

I still have my Jazz after all these years and it's been fantastically reliable -- until now.

Recently the furnace, a Suburban SF-35F, won't light. Here is what I know thus far:

1. Voltage at battery is 12v.
2. Propane tanks are full and stove (immediately above furnace) runs steady.
3. Thermostat seems to be sending "call for heat", as I can hear a relay clicking at the furnace when it's supposed to turn on.
4. Fuse to furnace is fine.

To be fair, I haven't done ANYTHING to this furnace in terms of periodic maintenance since I purchased the rig new -- there is NO access to anything regarding the furnace.

I would ordinarily check the sail switch (etc.), but I can't reach it without pulling the furnace out of the cabinet, and I don't have a bench set-up to test the furnace out of the cabinet.

What should I check next and do I really need to pull this wretched thing out of the cabinet to troubleshoot it?

Thanks!

[ZagiFlyer]

Hey All,

1) Thanks for all your input and advise. As always, I really appreciate the combined expertise of the forum.

2) Did you know that, even though DC fuses are designed to blow apart so that you can visually see the break in the plastic window, sometimes they don't?

Here's what happened:
After checking all the fuses and connections, finding nothing and then posting, I pulled the furnace out of the cabinet to check wiring, etc. Nothing wrong there. I went through the vendor documentation for troubleshooting and nothing seemed to fit.

I then took my multi-meter and retested the fuses -- and the furnace fuse was faulty! Not blown through mind you so that you could see it. The filament was cracked so it kind of made a connection, but not reliably. When I bent the fuse just a little bit, it would show as open (blown) in the meter. But when I just checked it as it lay on the bench, sometimes it read fine.

So I replaced the stupid fuse and everything runs fine now.

And it only took me eight hours. 😞

[LM3929]

I had the same problem with the furnace in our Winnebago.

it ended up that the propane supply tube to the burner jet was corroded and the rust would flake off and plug the jet.

I ended up putting rust inhibitor in the tube and worked great after that.

LM3929

[64thunderbolt]

sail switch stops working around 10.5v No fan no power.

[milo]

Zag... have same problem with our Suburban SF-35F. It was running great and shut off and never come back on (first thought it was just cycling). Same symptoms, can hear it click when thermostat sends signal for heat...then nothing (battery is at 12.6). I haven't had a chance to look at it yet so I will be interested in seeing what you find out.

Milo

[enblethen]

No fan would suggest that either the fan relay is bad or the circuit breaker is tripped in the furnace.

[ZagiFlyer]

Hello to All (again),

Thanks for your help so far! Based on your input, I'll need to:

a) get a better multimeter - mine is analog. I can either choose the 10v scale or the 50v scale. So I can't read THAT accurately.

b) Clean this thing by pulling it out of the cabinet.

To answer your question, the fan never comes on at all. The sequence I see/hear is:

1) Turn thermostat requested temp down so it sends call for heat.
2) "Click" from furnace.

Then nothing at all after that. No fan, no nothing.

I'll check voltage with a digital multimeter and I'll pull the unit out and clean it.

Thanks everyone!

[Harvard]

Just because the stove is OK does not in itself guarantee gas is getting to the furnace. Each appliance has its own copper line from the galvanized manifold located outside and under the RV. It is not uncommon to have p-traps filled with oil in these copper lines.

[wa8yxm]

There is a process involved in the furnace working, I will try to type it as best I recall.

T-stat calls for heat
1: Control board starts blower
2: Blower comes up to speed and moves enough air to
3: Close the sail switch
4: Control board opens gas valve and
5: Starts the ignighter sparking.
6: FLAME ON
7: Thermocouple heats up and starts generating about .480 volts
8: Motor board shuts down ignighter and looks for .480 volts from thermocouple
9: Operation continues or not depending on 8
IF NO voltage: go back to 1, twice, if still no signal from thermocople lock out.
If yes: Flame continues till either the OVER TEMP sensor opens, in which case it is cut off till the furnace cools some.. AIR flow is lost or The T-Stat says "ENOUGH already"
I have never done an over temp so I think it will re-start when it cools but not sure.

When the T-stat says "ENOUGH" the flame is cut off,, then after a bit the blower

Possible issues.

Low battery, bad sail switch, old tired blower motor, dirty blower or air passage, bad control board, (If it never lights) improper spark gap (I have seen this on every furnace I've worked on, both in my RV and my parent's sticks & Bricks). and more.

But we need to know how far down the process it goes before it fails.

[wolfe10]

Agree, if voltage is 12.0, that is LOW. If it is really that low, start by charging battery.

But, tell us how far into the heat cycle it gets:

Thermostat signal to PC board.

Fan starts. Sail switch closes.

Gas valve opens and igniter sparks.

Furnace runs.

[Old-Biscuit]

12V is LOW voltage......should be up around 12.4V at least.

When you hear the 'click' does the fan come on and blow air??

Ain't nothing going to happen until that occurs.

Get you battery voltage up.

[enblethen]

I would bet on it needing cleaning.
Need to access it and either blow it out good with compressor or use a strong vacuum.
Either dirty sail switch or burner electrode.
Does the fan run? If not tripped circuit breaker in furnace.

[workhardplayha1]

When I have anything 12 volt that is not working properly I always check the ground first.