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Old 05-20-2019, 04:17 PM   #1
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Atwood furnace

I have a 2015 Thor ACE with an Atwood furnace model AFMD35141. The blower will come on but flame will not light. An online service tech thought the problem might be the limit switch. I got one today but wonder if it more than I can do to change. Are there any videos on this procedure or does anyone with the same problem have an other remedies?
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Old 05-21-2019, 12:21 AM   #2
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The manual for your furnace can be downloaded here. May do better to troubleshoot from sequence of operations in the manual. (voltage, blower, sail switch, ignition etc.)
http://manuals.dometic.com/download.php?file=2642
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Old 05-21-2019, 12:36 AM   #3
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Atwood furnace

Somehow I got the furnace working without changing the limit switch. However, I did replace the circuit board and the furnace is working. Another question, can propane perform differently at higher altitudes? The furnace began to fail when we were at higher altitudes than we are now.
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Old 05-21-2019, 12:38 AM   #4
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With your RV being a 2015, the first thing I'd suspect would be the sail switch. If there's debris on the switch, the furnace will act just as you described. Check this video; it's a quick and easy repair.
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Old 05-21-2019, 03:19 AM   #5
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Is it clicking like its "trying" to light?

If YES.... perhaps something is blocking the air intake or exhaust that prevents it from lighting? Just an idea.

If NO clicks..... then its something electronic?

Just some thoughts .... Trying to help you eleminate things.
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Old 05-21-2019, 04:07 AM   #6
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Originally Posted by longknocker View Post
Somehow I got the furnace working without changing the limit switch. However, I did replace the circuit board and the furnace is working. Another question, can propane perform differently at higher altitudes? The furnace began to fail when we were at higher altitudes than we are now.
Yes, all combustion appliances/engines function differently when at altitude with less air pressure.
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Old 05-22-2019, 11:47 PM   #7
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Well, it ran for 1 day then won't light again. Next I will check the sail switch, then have the limit switch replaced, then replace entire unit. Also thinking of replacing the coach.
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Old 05-23-2019, 10:44 PM   #8
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Problems with RV systems can be very frustrating at times, but hopefully you will find the cause soon and will be able to focus on travelling and camping instead.

You said that the furnace control board replacement enabled the furnace to work for about a day.......which suggests to me the basic furnace control systems were working (at least for that long). Possibly while installing the new board you temporarily moved a loose or intermittent connection/wire.

I was also wondering if you are having any operational issues with other propane appliances? Weak flame on stove; fridge or hot water heater not firing up properly? It is not uncommon for the propane regulator near the tank, to leak or fail to regulate gas pressure properly.
Long & Winding road had some valid suggestions and questions that are important to respond to for the forum to try to help you. Providing as much information about what is happening (or not) during the failure time, makes diagnosing easier for the forum. (i.e., the blower ran Ok but did it shut down after 30 seconds or so?; did you smell gas at all during the start sequence?; any sound of spark attempts? You may find a mobile tech to be a good solution if getting into a Dealer is not in your time frame requirement.
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Old 05-24-2019, 03:36 AM   #9
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Atwood furnace

The furnace ran for about 24 hours and then, this afternoon, won't relight. The blower comes on when I engage the thermostat and the burner tries to ignite 3 times and the everything times out. I don't have any propane issues with any other appliance, the range, refrigerator, or water heater. They all work fine. I could understand it if it shut down and never worked again but it's intermittent, works then doesn't then works again for a while. I'm thinking of maybe replacing the entire furnace. I've got about $1000 wrapped in repairs with different mobile techs so far.
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Old 05-24-2019, 01:20 PM   #10
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Hi, unfortunately not all techs in any industry are created equal. In fairness, intermittent failures are the hardest troubles to find since when the device is working......there is nothing to fix.


So you have a new control board; everything worked OK for 24 hours; you have propane pressure to other appliances and they are all working OK and now it sounds like your furnace is getting all the way to the ignition cycle but not lighting and it is then going into lockout after three attempts to light. To restart you have to turn off the thermostat and wait 15 seconds then turn on again; OR cycle the on/off switch in the furnace. Did you smell any propane being released during the ignition attempt sequence?


So some possible failure issues that may be preventing ignition at this point in the furnace cycle (I wonder if any techs checked these items?):


1. While unlikely (infant mortality), the new Control Circuit Board “may” be a problem and either does not open the gas valve or does not trigger the igniter (but you said you heard the igniter sparking for the three attempts). Check for 12V at the gas valve (if you can) during the ignition cycle which confirms the board is sending the signal and the connection is good to the gas valve.
2. The Gas Valve fails to open, so no propane in the burner/oriface (no gas smell either after several start attempts). “Assuming” no issues with gas line feeder to the gas valve; replace the valve.
3. The Gas Valve is open OK, but no propane flows. You verified other gas appliances work OK so propane regulator is good, but is gas getting to the furnace gas valve? Unless the furnace feeder is restricted or kinked somehow the gas feed line to the gas valve should be OK (famous last words). Check possible dirt or bugs in the gas line or burner oriface after the gas valve. Clean out debris from the lines/oriface.
4. The Igniter fails to spark. Could be a defective igniter or no power is reaching it. It is also possible the gap at the ends of the Igniter electrodes is too large (or that they are shorted together). Inspect and adjust/proximity to gas oriface also.
5. Igniter does not confirm that ignition was successful (no flame is sensed by control board), so the Control/Ignition board has shut down (lockout stops restart attempts).
6. Propane and spark are present but the burner doesn’t light. This can occur if the Combustion Air Intake is clogged (mud duaber nest/spider webs etc.), and there is insufficient air flow through the combustion chamber.
7. Loose or corroded connections in any of the above components.




Not sure how comfortable you are with getting into this, but the tech(s) you hired should have gone through a process to repair your furnace. Intermittent troubles are the worst thing to work on though.
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Old 05-24-2019, 01:40 PM   #11
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Very thorough possibilities. The gas valve is new but doesn't mean it might be bad. I do remember not seeing the red light come on the circuit board. I have another board and will replace. Most of the suggestions are beyond my comfort or knowledge level but will save for RV tech, thanks.
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Old 05-24-2019, 04:02 PM   #12
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Very thorough possibilities. The gas valve is new but doesn't mean it might be bad. I do remember not seeing the red light come on the circuit board. I have another board and will replace. Most of the suggestions are beyond my comfort or knowledge level but will save for RV tech, thanks.
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Old 05-26-2019, 12:45 PM   #13
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Atwood furnace

The furnace will work for about 24 hours each time I replace the circuit board even though I don't see the red light come on the board. After that the fan will come on but burner won't relight. It sounds like it's relighting but then stops each time after about 3-5 seconds and 3 attempts. Also, now the outside switch on the furnace won't shut off the unit, I have to control everything from the thermostat.
I have checked everything mentioned in the previous suggestions at one time or another (no mud daubers, cleaned ends of gas tubes, had a new gas valve installed, sail switch cleaned). During the unsuccessful relight attempts, I have smelled propane from the exhaust area.
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Old 05-26-2019, 12:57 PM   #14
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Quote:
Originally Posted by longknocker View Post
The furnace will work for about 24 hours each time I replace the circuit board even though I don't see the red light come on the board. After that the fan will come on but burner won't relight. It sounds like it's relighting but then stops each time after about 3-5 seconds and 3 attempts. Also, now the outside switch on the furnace won't shut off the unit, I have to control everything from the thermostat.
I have checked everything mentioned in the previous suggestions at one time or another (no mud daubers, cleaned ends of gas tubes, had a new gas valve installed, sail switch cleaned). During the unsuccessful relight attempts, I have smelled propane from the exhaust area.
If the outside switch won't shut off the unit, it is either wired wrong or 12 VDC is being backfed from the thermostat; which should only be getting its 12 VDC from the furnace.

Double check your wiring and if you feel you need to replace the controller get one from Dinosaur Electronics with a 3 year warranty.
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Old 05-26-2019, 01:05 PM   #15
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When you say "replace the controller" do you mean the circuit board or the outside control switch?
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Old 05-26-2019, 02:49 PM   #16
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Here is a forums link for someone who has the same furnace as you and they cleared a blockage with an air compressor I think.


Furnace won't light - simple fix - updated


Yes it sounds like you have a wiring problem, as the on/off switch in the furnace should turn off the furnace and 12v to the thermostat also. I am attaching the schematics from the manual linked in the above forum response. I think the schematic diagram at the left is correct, but the diagram on the right side might have an error where the thermostat wire from point 5 connects in the diagram. It shows the thermostat wired connecting between the sail switch and limit switch which is wrong I think. The thermostat wire should be connecting to the control board and sail switch. So, that’s a surprise if I am correct. Jpeg of it attached.
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Old 05-26-2019, 03:24 PM   #17
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One more comment. While your on/off switch is wired wrong I don't think that is causing your no start issue. Just follow the schematic on the left and make sure your wires are connected to the devices as noted correctly. Maybe a connector got reversed on the on off switch. Back to the no start issue though. Your furnace runs normally for about 24 hours each time to swap the control boards.....so to me both boards are probably OK (famous last words). The fact the furnace runs 24 hours after each swap seems to also indicate clear propane feeds; combustion chamber etc. So what else is there.....the spark unit/flame sensor perhaps is going flaky......works for a while (at night perhaps with regular furnace restarts?) and then does not sense the flame after it sits cold for a period. Flame sensor is much cheaper than a control board if that's any consolation. When you do get this working again you will have lots of spares.
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Old 06-01-2019, 08:12 PM   #18
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I have the same year and model Atwood furnace, was having same issues, dometic bought Atwood and after numerous discussions with dometic my RV tech got a gentleman there that knew his stuff, he said those particular furnaces had problems so he sent us a new circuit board and relocater kit as the has moisture problems hence a relocation kit then a baffle system that had to be installed inside the burn chamber. The burn chamber was a bad design and wouldn’t circulate air properly and in one corner of the burn chamber would get real hot and it was the corner that the limit switch was in also. Just for fun he installed a new igniter. Now it works flawlessly and actually has more airflow from the vents. So replace circuit board and get the relocation kit and the baffle for the burn chamber. Dometic sent everything for free.
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Old 06-04-2019, 01:51 AM   #19
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Originally Posted by longknocker View Post
The furnace ran for about 24 hours and then, this afternoon, won't relight. The blower comes on when I engage the thermostat and the burner tries to ignite 3 times and the everything times out. I don't have any propane issues with any other appliance, the range, refrigerator, or water heater. They all work fine. I could understand it if it shut down and never worked again but it's intermittent, works then doesn't then works again for a while. I'm thinking of maybe replacing the entire furnace. I've got about $1000 wrapped in repairs with different mobile techs so far.
We had a similar problem with our 2017 ACE. What I finally figured out was that the furnace ran just fine when the slider was in, but would not ignite when the slider was out. Had no problems with the other appliances, stove, water heater, refrigerator. Problem turned out to be the gas line. In the hole where it comes up through the floor, the gas line for the furnace, and only that line started getting squished flat when the slider was extended. Our fix was to loosen the bundle a bit, by replacing the zip ties with slightly looser ties, so that the gas line could move as the slider extended. It has worked great since. And yes, we paid a service company several hundred dollars to take apart the furnace and try to figure it out, however what none of us realized at the time was the issue with the slider. I figured that out later at home when I couldn't get the furnace to fire up after we took it home, and put the slider out as I was cleaning and stocking for an upcoming trip! Our gas lines come up under the kitchen sink, but that may not be the case with your unit.
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Old 06-04-2019, 02:04 AM   #20
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Never thought of that, thanks. I will check it out.
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