Furnace Doesn't Stay Lit - Montana Owners Club

Mopar_Mudder · 07-22-2021, 07:41 AM

Well I have been trouble shooting with no success. I have an AFMD35131 that is giving me issues. RV is new to me and the furnace didn't seem to give off alot of heat and now it doesn't give any at all. Exhaust fan kicks on as expected and then furnace will light for a second or two and go out. This process goes on forever, light and go out. It almost seems like a lack of gas or air so that the fire can't keep going. It does have a new burner and igniter.

I know everyone's first thought will be the gas valve is being shut off because of no signal from the flame sensor, but that is not what is happening. The gas valve does shut off but it is a few seconds after the flame goes out. I also verified this by jumping constant power to the valve so that it stayed open, flame still goes out.

I have blown compressed air through the flue and found nothing blocking it. Air flow is good coming out when the fan is on. In have cleaned the orifice in the valve, it was not dirty. I checked gas pressure tonight at the fitting that hooks to the furnace and got 10-3/4", I think it calls for 11". Water heater, fridge, stove all work fine.

I can't see where it can be anything electrical when the valve is open and gas is coming out it doesn't burn. Only thing I see is where the metal shroud of the gas valve hits the plastic shroud of the blower the plastic had melted a little. So their is not a perfect seal for the fan blowing into the burner, but the way it is designed I don't think their ever was.

MOODY · 07-22-2021, 08:06 AM

Outside temp?

rames14 · 07-22-2021, 08:14 AM

Could you give a little more info such as model of Montana, year, manufacturer of furnace. Most of us don’t deal in part numbers. There are two issues that come up continuously- sail switch and control board. We had a similar issue with our 2010 Big Sky. A mouse had shorted the circuit board (horizontal mount - mouse used as a bathroom). Two potential causes.

Mopar_Mudder · 07-22-2021, 08:18 AM

Quote:

Originally Posted by MOODY

Outside temp?

Outside air temp do you mean? It is in the 70's

Mopar_Mudder · 07-22-2021, 08:24 AM

Quote:

Originally Posted by rames14

Could you give a little more info such as model of Montana, year, manufacturer of furnace. Most of us don’t deal in part numbers. There are two issues that come up continuously- sail switch and control board. We had a similar issue with our 2010 Big Sky. A mouse had shorted the circuit board (horizontal mount - mouse used as a bathroom). Two potential causes.

2017 3790rd and the furnace is an Atwood.

Can't be a board or electric issue, I can hot wire the valve open and it still wont stay lit.

From my understanding if the sail switch is bad the exhaust fan doesn't run? My exhaust fan come on fine and blows hard. Unless it is suppose to be variable speed fan and it is blowing too hard for it to light?

MOODY · 07-22-2021, 08:37 AM

Check voltage at the circuit board. Should be 12 volts coming in and 12 volts to your sensors. Mine was putting only 3 volts with temps below 40.
Either electrical or fuel issue (low pressure).
Fuel issue might be solved by shutting fuel off, disconnecting tanks, reconnect tanks and SLOWLY turning tanks back on.
Could be a bad propane pressure valve.

jsb5717 · 07-22-2021, 09:37 AM

Could be the main circuit board or a sensor as suggested. On ours the problem was the OEM Fairview propane regulator. Your furnace has a high demand for fuel flow and if that rate is diminished then you will get the symptoms you're experiencing even though other propane appliances work just fine. Ours did the exact same thing. New Marshal Excelsior regulator solved it.

We have 2 propane tanks, one on each side of the trailer. I replaced both the main regulator as well as the small high-pressure regulator on the opposite side. If your issue is propane flow then it could be that either reg is causing the problem, although the main reg would likely be your culprit. You can replace the main one first and test the furnace, making sure that you are drawing propane from that tank. If that solves it then you can switch tanks to the other side to see if it still works. If so you're probably OK. I decided to switch both to make sure.

jcurtis934 · 07-22-2021, 09:51 AM

Since you have a means of sensing gas pressure, ie, a manometer, i would do this next...adjust the output of the changeover valve up to approx 13.75 inches, which is still below the 0.5 psi figure that the appliances can operate at. This will give you that boost to see if the furnace gas valve is the root of your issue. Yes, the furnace will burn hotter than before, but the idea is to see what effect it has on your issue. Just make sure you take a reading of the output of the changeover first, so you know what it was set to. I really doubt these are set to any standard, but mine started at 13.25 (0.478 psi) when i was diagnosing my furnace issues. If things start working, i would suspect that the furnace gas valve was sticking or had some kind of other internal issue. All i can think of to aid in your troubleshooting.

Mopar_Mudder · 07-22-2021, 10:04 AM

Quote:

Originally Posted by jcurtis934

Since you have a means of sensing gas pressure, ie, a manometer, i would do this next...adjust the output of the changeover valve up to approx 13.75 inches, which is still below the 0.5 psi figure that the appliances can operate at. This will give you that boost to see if the furnace gas valve is the root of your issue. Yes, the furnace will burn hotter than before, but the idea is to see what effect it has on your issue. Just make sure you take a reading of the output of the changeover first, so you know what it was set to. I really doubt these are set to any standard, but mine started at 13.25 (0.478 psi) when i was diagnosing my furnace issues. If things start working, i would suspect that the furnace gas valve was sticking or had some kind of other internal issue. All i can think of to aid in your troubleshooting.

Thanks I will give that a try tonight.

Does anyone know if the small test ports that come out of the side of the gas valve on the furnace should read the same 11" pressure or does the furnace valve lower the pressure, it has its own regulator in it. I did take the valve apart to look for any junk or hole in the diaphragm and it all looks good.

Also when I checked pressure I forgot to turn on all the other gas appliances and see if the pressure holds, I will try that tonight.

Mopar_Mudder · 07-22-2021, 07:35 PM

So I did some more testing tonight.

Gas pressure at fitting that hooks to furnace:
10-3/4" With nothing running
10" with Fridge, water heater and 3 burns on stove going.

Gas pressure at the test port on the furnace gas valve, would be the same as pressure at the orifice:
9" with the gas valve open calling for heat

I adjusted the regulator at the tank to give me 10" at the orifice which gave me exactly 11" at the line that hooks to the furnace. This made no difference in how it acts, still will not stay lit.

I can't seem to find good info on what the pressure at the orifice should be. I did find a posting on a forum that claims the pressure at the orifice on a new valve he tested was only 5" or so?

Bozwell · 07-25-2021, 01:17 PM

We had same issue. Cleaned the sail switch in the furnace and fixed it for a year or so. Happened again and we replaced it. Easy to fix yourself. Watch a youtube video👍

JLfromSTL · 07-25-2021, 01:22 PM

Quote:

Originally Posted by Mopar_Mudder

Well I have been trouble shooting with no success. I have an AFMD35131 that is giving me issues. RV is new to me and the furnace didn't seem to give off alot of heat and now it doesn't give any at all. Exhaust fan kicks on as expected and then furnace will light for a second or two and go out. This process goes on forever, light and go out. It almost seems like a lack of gas or air so that the fire can't keep going. It does have a new burner and igniter.

I know everyone's first thought will be the gas valve is being shut off because of no signal from the flame sensor, but that is not what is happening. The gas valve does shut off but it is a few seconds after the flame goes out. I also verified this by jumping constant power to the valve so that it stayed open, flame still goes out.

I have blown compressed air through the flue and found nothing blocking it. Air flow is good coming out when the fan is on. In have cleaned the orifice in the valve, it was not dirty. I checked gas pressure tonight at the fitting that hooks to the furnace and got 10-3/4", I think it calls for 11". Water heater, fridge, stove all work fine.

I can't see where it can be anything electrical when the valve is open and gas is coming out it doesn't burn. Only thing I see is where the metal shroud of the gas valve hits the plastic shroud of the blower the plastic had melted a little. So their is not a perfect seal for the fan blowing into the burner, but the way it is designed I don't think their ever was.

Had similar problem. It was the "SAIL SWITCH" which allows the flame to ignite because the switch detects air flow from the blower. Open outside furnace panel, there is a wire connected and single screw that holds the switch. Remove, Clean and/or bend it so the flag part detects air flow and closes the switch. Wish I had pics but rig in storage

jimc100 · 07-25-2021, 02:09 PM

As has been suggested check the sail switch, if bad switch or not enough air it will shut down the furnace. Just on a whim check the burner also for blockage, without going into details I got water into the combustion chamber and it rusted the burner solid. It would try to start, I would hear the valve open, smell some gas and then it would shut off. The burner is just a pot metal and will rust easily if there is water. I had a break in the shower, above it, and didn't realize water had gotten into the furnace. Good luck, it's a pretty simple appliance.

bcrvman · 07-25-2021, 02:27 PM

Quote:

Originally Posted by Mopar_Mudder

Well I have been trouble shooting with no success. I have an AFMD35131 that is giving me issues. RV is new to me and the furnace didn't seem to give off alot of heat and now it doesn't give any at all. Exhaust fan kicks on as expected and then furnace will light for a second or two and go out. This process goes on forever, light and go out. It almost seems like a lack of gas or air so that the fire can't keep going. It does have a new burner and igniter.

I know everyone's first thought will be the gas valve is being shut off because of no signal from the flame sensor, but that is not what is happening. The gas valve does shut off but it is a few seconds after the flame goes out. I also verified this by jumping constant power to the valve so that it stayed open, flame still goes out.

I have blown compressed air through the flue and found nothing blocking it. Air flow is good coming out when the fan is on. In have cleaned the orifice in the valve, it was not dirty. I checked gas pressure tonight at the fitting that hooks to the furnace and got 10-3/4", I think it calls for 11". Water heater, fridge, stove all work fine.

I can't see where it can be anything electrical when the valve is open and gas is coming out it doesn't burn. Only thing I see is where the metal shroud of the gas valve hits the plastic shroud of the blower the plastic had melted a little. So their is not a perfect seal for the fan blowing into the burner, but the way it is designed I don't think their ever was.

In the Files section is a troubleshooting manual. Pretend you are 3 yrs old and follow it word for word. My money is on a bad board. Make sure replacement is a Dinosaur as in this one (same board for HWm, which is why I carry 2)
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Mopar_Mudder · 07-25-2021, 02:36 PM

I appreciate the suggestion but I have posted multiple times it is NOT the sail switch. If the sail switch was bad it would not light at all, this can be verified simply by unplugging the sail switch and it will not spark.

It really come down to it has to be an air to fuel ratio problem, and I don't know how you can adjust that.

jsb5717 · 07-25-2021, 02:44 PM

Quote:

Originally Posted by Mopar_Mudder

I appreciate the suggestion but I have posted multiple times it is NOT the sail switch. If the sail switch was bad it would not light at all, this can be verified simply by unplugging the sail switch and it will not spark.

It really come down to it has to be an air to fuel ratio problem, and I don't know how you can adjust that.

I said earlier that I changed our our propane regulator and that solved the problem. For me that was partly a guess after eliminating other things. Like you I figured it was likely either the main board or the reg. The reg was something I could get and try right away so went that direction first. Fortunately that was it. Cost me $46 to find out but it's worked perfectly since.

bcrvman · 07-25-2021, 02:47 PM

Quote:

Originally Posted by Mopar_Mudder

I appreciate the suggestion but I have posted multiple times it is NOT the sail switch. If the sail switch was bad it would not light at all, this can be verified simply by unplugging the sail switch and it will not spark.

It really come down to it has to be an air to fuel ratio problem, and I don't know how you can adjust that.

What code is the board showing?

bcrvman · 07-25-2021, 02:56 PM

Quote:

Originally Posted by Mopar_Mudder

I appreciate the suggestion but I have posted multiple times it is NOT the sail switch. If the sail switch was bad it would not light at all, this can be verified simply by unplugging the sail switch and it will not spark.

It really come down to it has to be an air to fuel ratio problem, and I don't know how you can adjust that.

You might have accumulated a lot of oil in the gas lines. I assume you fix that by blowing out the lines. I have heard it is more prevalent west of the rockies.

Also the high limit switch can cause this according to the troubleshooting manual.
https://www.dinosaurelectronics.com/Limit_switch.htm

bcrvman · 07-25-2021, 03:00 PM

Quote:

Originally Posted by Mopar_Mudder

I appreciate the suggestion but I have posted multiple times it is NOT the sail switch. If the sail switch was bad it would not light at all, this can be verified simply by unplugging the sail switch and it will not spark.

It really come down to it has to be an air to fuel ratio problem, and I don't know how you can adjust that.

Do you see a blinking red light as in this video

https://www.youtube.com/watch?v=0t8vRuGdZLw

Mopar_Mudder · 07-25-2021, 03:22 PM

Their never is a blinking light because it never stops trying to light and their is no code to show. Their is no electronic part failing to create a code.

At this point I thing I am going to order a new gas valve, orifice and burner. Even though to burner look now. May get a new pressure regulator also even though my manometer shows no pressure problems.

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