Suburban NT Furnace Problem

[ATL]

I have an '83 Winnebago Brave with a Suburban NT-34S furnace.  I had it repaired in April and they replaced the limit switch.  It worked great for a 2 week trip in June, then I parked it until Oct.  Now it won't fire.  I've read the other related post, and I have some avenues to explore, but I have a few specifics that I'm hoping to get some info on.

The furnace blower does kick in when I turn it on.  As far as I can tell the gas is not turning on at all (no click or gas noise), and I'm not hearing any spark.  I did read in the other posts that it could be a few different things.  First, the sail switch: Does anyone know where to find this?  Second, the actual voltage at the module board: I'll get a VOM and test this (any pointers?).  Third, I've been trying repeatedly to start it so it could be the lockout switch: Is this the same as the limit switch? (I could not find it in the scematic)  If it is not the same, does anyone know where it is or how to reset or test it?

How likely is it a different part would fail so soon after a repair?  The dealer said it is under warranty, but I have a feeling it won't be the limit switch again and they'll charge another $500 to fix the new problem.  That's practically a new furnace.  Any help would be greatly appreciated.  I'm in Alaska and it's getting cold up here!

 

[Gary RV_Wizard]

The sail switch is usually located almost directly in front of the blower outlet.  It's typically a paddle-shaped lever attached to a little switch.

The actual voltage throughout your RV should be between 12.6 (battery only) and 13.7  (when the conveter/charger is operating). But the furnace board and fan should run fine down as low as 12.0 (or even 11.6).

 

[Karl]

The limit switch, if present, will prevent the furnace itself from overheating and possibly causing other damage. It is normally closed, but if it is open, that would prevent the furnace from lighting at all. Sorry, can't help with the specific location, but it should be mounted directly on the heat exchanger.

 

[gonepostal]

If your rig has been parked for quite a while I would suspect a power problem.  The unit is designed to run the fan for a couple minutes before it tries to light to clear out any residual exhaust gases.  If the battery is at all low, the fan won't build up enough speed to activate the sail switch.  That would prevent your gas valve from opening and the ignitor from sparking.  If your battery tests okay without a load, it just may not have enough juice when it's under load.  Check the power at the board while the fan is running.  The new limit switch could be bad, but most likely not, and you don't have a lockout switch for multiple tries. 

Sarah

 

[dpickard]

WOW!!!

That lady sounds like she knows her stuff!!!

Hey Sarah, know anything about Jake brakes?

Dianne

 

[Karl]

Low voltage at the controller board and/or blockage in the air flow (sail switch not closing) will definitely cause problems. Check for leaves, nests, dead mice (ugh!) or other blockage. The lockout switch is not really a switch at all but part of the controller circuitry. It electronically prevents the unit from going beyond 3 tries; then says "Go fix something first before I let you do some damage". Turning the furnace completely off (not just the thermostat) will reset it. Check voltage coming into the controller board and the ignitor board. If below about 11.5 volts, operation may be erratic or not operational at all. Gas valves may be sticking or not operational. There are two of them for each furnace; if either one fials, gas will not flow. Could be crud buildup in them, but check the other things first.

 

[Jim Dick]

WOW!!!

That lady sounds like she knows her stuff!!!

Hey Sarah, know anything about Jake brakes?

Dianne

Dianne,

She sure does. Those of us who knew her when are real proud.  Number one in her RV tech class and the prettiest tech I've ever come across!  

Are you asking about a real Jake brake or the exhaust brake most diesel pushers use?

 

[gonepostal]

Dianne,
Jim's reply notwithstanding, I don't know a thing about Jake brakes except some people have them and some don't!   I used to work on RV's but mostly on the appliances, very little on the engines and then under protest.  I've been away from RVs for several years - don't even own one anymore - and am pretty rusty on a lot of stuff, especially details, but some things I did so often I can remember the general things.
Sarah

 

[BernieD]

Sarah

But you did a marvelous job on turn signals. The type that magnetically attach to the top of the towed ;D ;D ;D

 

[Jim Dick]

Dianne,
Jim's reply notwithstanding, I don't know a thing about Jake brakes except some people have them and some don't!   I used to work on RV's but mostly on the appliances, very little on the engines and then under protest.  I've been away from RVs for several years - don't even own one anymore - and am pretty rusty on a lot of stuff, especially details, but some things I did so often I can remember the general things.
Sarah

Sarah,

You are still my favorite RV tech and always will be. My reference was to the appliance side knowing that the chassis is a completely different world. Sure do miss seeiing you at rallies. Hope we can make it to MN again sometime!!!