Surburban Furnace Not Igniting

[brisath]

Some other threads have helped answer some questions (here, here, and here), but here is where I am at with the furnace on my 1996 Winnebago Vectra:

1. Turn thermostat up past room temperature
2. Blower comes on and seems to run good
3. Light on switchboard never comes on.
4. No noise of an attempt to light or fire furnace
5. Only cool air blows through vents

Other info
-2 new batteries
-doesn't work under shore plugin or idling under motor
-I've also gone through the instruction manual steps for getting it to fire, which includes 5 minutes with propane turned off, then turn propane on and repeat.
-Propane is at 2/3 level
-The switch on the furnace itself is ON
-5 amp fuse on the circuit board looks good
-water heater pilot works just fine and comes on regularly. Refrigerator works when unplugged, stove burners work fine.

If it is a sail switch, how do I check that?
As a first step, would it be good to fill the propane tank completely?

Here is a picture of the furnace:


and circuit board

[fischer]

If you go to
bryantrv.com/docs.html
scroll down to you model furnace many have pictures of the parts and troubleshooting charts.

[brisath]

Today I was replacing the roof fan and a circuit breaker blew. I looked at it and it says "Furnace/ceiling fan" on the breaker that popped out. This makes me wonder if somehow the faulty ceiling vent/fan is causing the problem. We're replacing it with a different fan that doesn't have an automatic raise/lower switch so we have less wire to connect. I put the breaker back and got the fan working correctly but there are 2 loose purple wires in the picture below. Could these go to the furnace? Any ideas how to wire it?


It appears that the limit switch may be on the back of the furnace, which I'll have to pull out to get to.

[brisath]

I'm not seeing it. Would it be between the thermostat and furnace or maybe over near the breakers? I'm also not sure what it looks like.

[Chris_Bryant]

Look inside the coach- there is an access door on the inside, and the limit switch will be right there. A lot of times the limit switch terminals are just corroded, and the unit can be fixed with a pencil eraser.

[Old-Biscuit]

If I remember right.....limit switch is on backside of combustion chamber.
Have to remove from cabinet to get to limit switch.

Check for power at connector on module.....should be a blue wire from t-stat

[brisath]

Circuit diagram shows limit switch before sail switch but I'm having trouble finding it. Any advice for what it looks like or where to look?

[Old-Biscuit]

Check for 12V at/thru 'high limit switch'

The sail switch and high limit switch are in series.......power goes to one then other before going back to module.
Sometimes sail switch is first sometimes high limit switch is first in circuit.

[brisath]

Did that while fan was working and no reading on either terminal of sail switch. Also, on that multimeter I had to plugin the red to the right side for it to work and confirmed that on the battery itself and also got 12 volt reading on the lead the fan on the furnace which is operating.

[Old-Biscuit]

Meter set to 'DCV'
Red probe plugged into 'DC' (left side)
Black probe plugged into 'com' (center)

Then red probe touching one terminal on sail switch, black probe touching a good clean ground

Turn furnace ON
Should read 12V DC on terminal...then check other terminal. It should also read 12V DC if contacts are making up

[brisath]

I had a multimeter trying to check the sail switch today but I guess I don't know how to use it because nothing was happening on the readout that I could make sense of. For starters, I'm not sure if I'm placing the wands correctly. Can anyone provide explicit instruction on how I test that. The device I have is http://www.equus.com/Product/3320/Auto-Ranging-DMM

[brisath]

Unfortunately I forgot my voltmeter when checking it today, but I did confirm the switch is activated with the fan. The light on the circuit board doesn't come on though and no ignition. I'll check with voltmeter next. Thanks for the help so far.

[Old-Biscuit]

Check for 12V power to sail switch and then with fan running check for power out of switch........two terminals on switch.....one power in the other power out.

The sail switch is just a simple make/break type switch.
The 'paddle' from switch extends into fan blower air flow.
With fan up to speed (actually at 75% speed) the air flow moves paddle making up the contacts in switch

[brisath]

Great info! How can I tail if the sail switch isn't working. Is it visible or audible? What should I look for there? I didn't find that answer in the guide.

[Old-Biscuit]

Middle section of picture.......
The white box just above cross brace is the 'sail switch'
Above it is the 'timed delay relay'

Here is a genric rv furnace troubleshooting guide. Good info.
LINK