Atwood Furnace will not light off

[Deepsea]

I have the following, Atwood AFMD30121 now (Dometic DFMD30121) Furnace Serial # 73299988, in an Eclipse Iconic 5th Wheel Model 2817CKG, two yeas old.

The furnace will NOT stay lit. 

The furnace goes through the proving/ignition cycle. 

The Fan motor runs, the furnace fires up and I feel hot air through the vent for about 5 seconds, then the propane turns off. 

This process happens two more times, then goes through the purge cycle and locks out.  

I can turn (off and on) the reset to begin the above again with the same results.  

The two blue wires were twisted together to bypass the thermostat. 

The sail switch was checked, removed, cleaned (did not need cleaning) and checked with Ohm meter.

The High Limit switched was Ohm, checked okay.

Removed the original circuit board and noticed one of the "capacitor or resistors" had rubbed against the housing. 

I then replaced the circuit board with a NEW Dinosaur circuit board. 

A NEW Electrode was installed insuring 1/8” gap(s).

The gas orifice was removed and cleaned; barley dirty.  

Gas valved was removed, energized and established an air flow. 

All exhaust, returns and air vents are clean and clear. 

The 5th Wheel is connected to shore power and batteries are at 13.5 volts.

The hot water heater and stove work fine. 

The 5th Wheel is level.

Both propane bottles are verified full.

No pinched propane lines. 

Removed, inspected and clean “T” fitting in the propane line leading to furnace. 

I have Opened the propane bottles one at a time slowly, bled the propane lines to no avail. 

There is no access to the furnace from the outside. I remove the unit from inside of the trailer to gain access to the fan and control board side of the furnace. 

To remove unit, disconnect two power wires (- +) two blue wires from thermostat, disconnect four ducting, disconnect gas line and exhaust pipe. 

I brought the unit to the RV shop, they could not fix the unit. I called Dometic and they said it is out of warranty and they said cannot help. 

Can someone please help! 

[Kirk W]

First I suggest that you download a copy of the service manual here.

When you checked the high limit thermostat, how did you check it? What it sounds like to me is that for some reason the high limit thermostat is opening after a short run time, closing again as the heat exchanger cools down. That is exactly how it should operate but it could be opening at too low a temperature. 

There is a service bulletin about the high limit problem also. It would appear that your serial number is not part of it, but I would check your furnace just to be sure. 

[Deepsea]

Wow! this is good information. I checked the high limit thermostat (HLT) with an Ohm meter, whited tested fine. However, like you stated, it may be shutting down at a lower temperature. I will replace the HLT and install a fiberglass sleeve as suggested in the service bulletin. Thank you for your help! 

[Kirk W]

15 minutes ago, Deepsea said:

Thank you for your help! 

I spent 40 years making my living fixing things and still enjoy troubleshooting so am happy to get the opportunity to assist. Let us know if that does take care of the problems. And examine those wires carefully before you install the sleeve. You probably know that but just wanted to make sure.

[Deepsea]

I purchased a direct replacement High Limit Thermostat and installed the 7" sleeve as per service bulletin. However, I am still having the same problems as described in my first post. Please advise. 

[Wrknrvr]

  This may seem to be really drastic, but I would suggest replacing the furnace.

  I have worked a few of the new Dometic Atwood furnaces. Never ever again. I am a Dometic dealer. If you get stuck on a few of those units you will finally quit servicing them.

 

  What I would suggest is to buy a used original Atwood furnace . From a older rv. It has to be the before Dometic took them over.Check to see if the burn chamber is in good condition. Install a new motor and dinosaur control board and enjoy it.

 

 

  PM me if you want to talk.

  Vern in a T-shirt 

 

  

[Wrknrvr]

  What I think happens to that furnace is the flame wants to lift off the flame diffuser witch is where the flame sense is located. They use the igniter to sense the flame. With the igniter probe in the flame the gas valve is told to stay open. If the flame lifts off the burn diffuser then the control boards shuts the furnace off.

n

Never could get Dometic to say so.  So I quit working on there new model furnaces.

 

  Yep they irritated me once too often?

 

  Vern in a T-shirt 

[oldjohnt]

With 49 years of RV ownership (and allllllll the necessary associated repairs) Ive had my share of furnace problems grrrrrrrrrrrr and understand what Vern is talking about. The flame sensor circuitry relies on an extremely small millivolt signal subject to the flames intensity and location. Often its the ignitor tips location and spark gap distance or its failure that wouldn't ignite the flame in the first place, but once underway the flame sensing technology is controlling and fairly critical.    When I experienced an OPEN high temp limit switch the furnace wouldn't light at all (no signal to the board) and sure a faulty temp sensor OR A BAD HIGH TEMP LIMIT SWITCH (opening prematurely) could be causing the flame to extinguish. With a new high temp limit switch if the problem persists either the board (but that new Dinosaur is prob fine, check the connection to the board, iffffffffffff its the flat ribbon type Ive had luck cleaning up the contacts) or what Vern spoke of (flame sensing) could be the problem???????? Inspect and clean up the ignitor tip, take a look at the flame and location of the ignitor etc. BUT LOOK AT THE MANUAL AND WHAT VERN OFFERS (and the bulletin Kirk posted) I HAVE NO SPECS OR DATA REGARDING THE FLAME OR IGNITOR OR LOCATION, sorry, just my best guess and experience. 

PS I once had an ignitor tip with gray accumulation deposits on its end and when I removed it and cleaned/shined it up things started working again

Hope you get it going

John T

Edited December 31, 2020 by oldjohnt

[Wrknrvr]

  Just to leave you know my experience. I removed one of those units and replaced it with a new one that Dometic shipped to me. The new one worked. I did it under warranty. Sent it back for them to inspect. Never got a word from them about what was wrong. Never got paid for that job.

  So I quit working on the new Dometic Atwood  furnaces. The only person that knew at Dometic, there was a problem because so many people having problems with there furnace. Retired.  He did give me his thoughts a few weeks before he was done.

 

  I do hope you can get it fixed.

 

  Vern in a T-shirt 

[Deepsea]

John and Vern, I appreciate your combined years of knowledge. I am curious what Kirk thoughts are. I will remove the board and check for clean connections. I attempted to post a video, but I could not get it down to the allowed file size. 

[Deepsea]

I published the video on YouTube https://youtu.be/6dTm5VV1nt8 Hopefully this may provide a more insight. Note that the board was re-located to prevent any rubbing as stated in my first post and now I am able to see the ignitor light turn off/on. 

 

[Kirk W]

First, let me say that I have no experience with this model furnace, but have pretty extensive experience with earlier models from Atwood that should be pretty similar. As I watch and listen to the video, it sound like it is failing to detect the flame as it lights for a short time 3 times and then appears to go into lock-out. But the LED does not seem to be giving any flash codes to a failure, which I would expect it to do, after reading that sticker on the furnace on ignition failure. 

If it had been the high temperature limit, I'm sure that it would not attempt to light again. For that reason I would not consider it probable and you could easily make sure that isn't the case by putting a jumper wire across  the high limit thermostat to take it out of the circuit and test again. If you get the same symptoms that high limit is not a part of the problem. 

Assuming that it is not the high limit thermostat, it sure acts as though it is failing to detect the heat of propane burning. I would look to see if that spark probe is directly in the flame as they should get very hot. The heat sensing function does sometimes fail so I would probably replace it before replacing the circuit board. I have always used igniter circuit boards from Dinosaur Electronics which have a better track record and warranty than those from Atwood, but in looking at them, I am not sure that they have one that would replace yours on this new furnace so be careful. Examining the circuit board in your video, I do believe that it would work. I still do not understand why you aren't getting any of the flash codes listed on the sticker shown in that video.

[Deepsea]

The circuit board, as mentioned in my initial post is a new Dinosaur board that is visible in the video. I am not sure if the Dinosaur will flash the trouble codes like the OEM board. The probes are at factory specs, 1/8” apart and from the burner. I could bend the probes closer or father away from the burner to catch the flame....?

[oldjohnt]

5 hours ago, Deepsea said:

I could bend the probes closer or father away from the burner to catch the flame....?

Deepsea, while I have had success slightly bending/moving or re gapping the probe (in some models NOT yours) as well as a good clean up and shining in cases where a flame sense technology failure caused the flame to go out,,, I'M NOT RECOMMENDING IT, I don't want to trash YOUR unit or void any warranty...FWIW On some models Ive found the gap distance and location to be critical !!! You're ONLY talking about a small voltage here.   If temp limit is working  correctly ?????? thus allowing the flame to initially light yet it goes out shortly afterward, as I already posted above, I agree with Vern and I'm back to a flame sensing circuitry problem (depends on location of flame relative to probe) unless the new board is bad which I doubt????? A few of the Dinosaur boards I used had minor variations versus the OEM but I suggest you talk to their tech support regarding your problem, maybe they have an answer, this is getting above my pay scale lol. I mentioned cleaning and checking contacts and grounds etc., but your unit looks in much better shape then many older ones I had to repair over the years lol  

Give Dinosaur tech Support a call, they always helped me, I'm down to guessing here NO WARRANTY lol

John T   Yall have a Happy and Blessed New Year   Live from the RV in Avon Park Florida 

[Kirk W]

7 hours ago, Deepsea said:

The circuit board, as mentioned in my initial post is a new Dinosaur board that is visible in the video. I am not sure if the Dinosaur will flash the trouble codes like the OEM board. The probes are at factory specs, 1/8” apart and from the burner. I could bend the probes closer or father away from the burner to catch the flame....?

Am I correct that you had the same symptom with the old circuit board and with the new one from Dinosaur? Did the old board give you any flash code? It might be worth putting it back just to run some tests, if nothing changed when you replaced it. On the probe location, I'd watch to see if it is in the flame when the propane is burning. If it is, leave it alone as that is where it should be. If too close that can cause problems. I am very slow to start bending things.

EDIT: One thing that is of critical importance is the continuity of the wire from the probe to the circuit board and the quality of it's connection. If you have not replaced that probe, try it as I have found that they have a much higher failure rate than do the circuit boards or other components. 

 

Edited January 1, 2021 by Kirk W
Add another thought.

[oldjohnt]

46 minutes ago, Kirk W said:

I am very slow to start bending things.

I AGREE 100%  While more then once (to correct gap or relocate better to flame etc) Ive cured problems by doing so and NEVER had a problem NO WAY IM RECOMMENDING SOMEONE ELSE DO IT LOL I don't want to be responsible for damaging their unit or voiding a warranty YIKES

Happy and a Blessed New Year to you and yours

John T

[Wrknrvr]

  This for Kirk. You will notice the new style burn chamber.

  This is the new type of burn chamber in afmd series’ furnaces.

Totally different than the ones before the design changes. I have seen many of the new furnace boards have moisture on them. I mean water on them. I inspected one that a dealership had chased a water leak as they thought it was dripping on the board. They replaced the board. I looked at it a few weeks later and it was wet. I was nice about the situation,but told her take it back to where she bought it.

 

Absolutely irritating to work on those things.

 

  Vern in a T-shirt 

[Kirk W]

47 minutes ago, Wrknrvr said:

You will notice the new style burn chamber.

Wow! That is a major change. 

Old style...    

[oldjohnt]

17 hours ago, Wrknrvr said:

I have seen many of the new furnace boards have moisture on them. I mean water on them. I inspected one that a dealership had chased a water leak as they thought it was dripping on the board. They replaced the board. I looked at it a few weeks later and it was wet. I was nice about the situation,but told her take it back to where she bought it.

Hey Vern, indeed those boards don't tolerate water to well lol. While Ive never had a failure using Dinosaur replacement boards if water gets on them THATS NOT GOOD especially regarding the extreme low and sensitive flame sense circuitry. 

Take care Vern

John T

[Wrknrvr]

  What you show Kirk is the diffuser for the flame. They have one in the new style furnace.

  To me it seams as though when the engineers were developing the new unit. It was in a controlled environment. Not out in the real world.   Now I had one extremely knowledgeable person that told how to fix it. I think he was so mad at Dometic he retired from them.

 

 

   Vern in a T-shirt 

[RandyA]

I recently had exactly the same problem and went through the same troubleshooting issuses you described initially.  After going through the possible outside items that could cause this to happen I pulled the furnace out, removed the blower so I could access the burner tube and igniter and found that the burner assembly like the one KirkW showed above was badly burnt, missing some of the area where the dot matrix holes are and had a bad physical distortion.  This area is directly over the igniter so it is important to adjust the igniter gap to 1/8" or less.  You have "about" 6,000 volts going to the igniter and anything over 1/8" may not allow a spark. I ordered a new burner assembly on Amazon ($32), replaced it and the furnace works beautifully - with the exception of eating propane faster than my diesel truck does its fuel.  I betcha a cookie to a doughnut you have a bad burner.  It is NOT hard to replace - the only tool you must have is a long reach 1/4" nut driver or a 1/4" socket set with a long extension and of course the square drive Robertson driver for the outside screws - a #2 Phillips may also be needed.  Removing the propane supply tube requires care as it would be easy to break something if you do not support it well.  Honestly, not a difficult job. But, you have had the furnace out so you know this.  BTW, the exhaust and fresh air intake tubes on the outside MUST be installed to test the furnace in the camper.  Without them the furnace senses a problem and shuts down.  I learned this part the hard way 🤕.

Edited January 2, 2021 by RandyA

[Deepsea]

Let me start off with saying THANK YOU for everyone's input on this frustrating issue. As everyone has thoroughly read my first post indicated my findings in detail and what I have tried or replace. However, Kirk did ask if my old board provided flash codes. I could not see the flash codes because the there is no access to the back of unit unless, I removed the whole unit. It was not until I figured out how to remove the unit to bench test it so I would have access to the back of the unit. At that time all indication pointed at the circuit board and replacing the OEM board with a Dinosaur. So getting back to Krik's question, I decided to install the original circuit board to see the flash codes and provide this information to Kirk. Upon installing the original circuit board and by passing the thermostat, the furnace fired up as normal and continued to function as designed. I removed the by-pass and try the thermostat controls. The unit continued to function. I fired up the furnace three more times prior to a full installation and totally button up,  getting the same results.  Again, this morning the furnace continues to function as designed. This leads me to believe the Dinosaur board has a malfunction but, I do not know what the cause of all this furnace issues was. I did make some improvements, such as the service bulletin that Kirk provided, installing a 7 sleeve also as I indicated in my original post, the circuit board was rubbing on the furnace housing, I secured the board with a couple of zip-ties to prevent any more chaffing. If all you guys saw this unit, and the parts removed and examined, this unit looks brand new, nothing worn out. I will report any other issues, good or bad and continue to hypothesize the possible issues of this simple furnace. Happy New Year's, Frank 

[Kirk W]

1 hour ago, Deepsea said:

Again, this morning the furnace continues to function as designed. This leads me to believe the Dinosaur board has a malfunction but, I do not know what the cause of all this furnace issues was. I did make some improvements, such as the service bulletin that Kirk provided, installing a 7 sleeve also as I indicated in my original post, the circuit board was rubbing on the furnace housing, I secured the board with a couple of zip-ties to prevent any more chaffing. If all you guys saw this unit, and the parts removed and examined, this unit looks brand new, nothing worn out. I will report any other issues, good or bad and continue to hypothesize the possible issues of this simple furnace.

When I go back through the thread, it is my understanding that the symptoms were exactly the same both before and after replacement of the circuit board with a Dinosaur and the same for replacing the ignition probe. It is possible that the Dinosaur board is not a direct replacement for the one that came with your current model furnace and that something like the update may have resolved the problem, but that does seem to be a reach. I would try to contact the tech people at Dinosaur Electronics and talk with them about what you have experienced. It has been several years since I last had reason to call them, but they used to be very good about helping to deal with unique or new problems.

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[oldjohnt]

On 1/1/2021 at 7:56 AM, oldjohnt said:

Give Dinosaur tech Support a call, they always helped me

 

On 1/2/2021 at 4:15 PM, Kirk W said:

I would try to contact the tech people at Dinosaur Electronics and talk with them about what you have experienced.

Looks like two of us agree !!!!!!!!!!!! Maybe factory tech support (don't hold your breath lol) or Dinosaur (but maybe not if its a flame or probe issue) can help ???? 

Some time back I called Dinosaur Tech Support and got instant expert advice, no holds or wait times or talking to a computer first. As I noted above and experienced "some" of their replacement boards may not be EXACTLY the same even though they still function.

PS if the problem was the same with the old or a new board its possible there was an issue with the probe or flame or the relationship and/or proximity of the two ?? Perhaps the factory or its service bulletin or Dinosaur (unless its a flame or probe issue) can provide the answer to this riddle... Once you followed what the service bulletin described plus replaced the high limit it would seem that's ruled out as the cause, so what's left ???? Board or still some sort of flame or probe problem  ?????  One last potential problem is a loose, corroded or oxide coated resistive connection, check and clean all connections and grounds and grounding sheet metal screws. Also check the HV ignitor wire FROM the boards HV Coil/Transformer TO the Ignitor, as the flame sensing circuitry involves low voltage. I've seen those HV cables (maybe a sharp bend or a nick or a kink etc) cause problems ????  PLEASE LET US KNOW what you find.

Watch it turn out to be NONE of these lol

Thanks for the feedback keep us posted

John T

Edited January 4, 2021 by oldjohnt

[Deepsea]

Sorry for the delay, family issues and attention. 

It was mentioned in the string, "what was the number of times the red light flashed on the circuit board before I removed it." I was not able to see the light flash because I could not see the back of the furnace. During the troubleshooting phase all indications pointed to the circuit board which was replaced with a Dinosaur, and since I built a test bench for the furnace to easily trouble shoot and test the furnace. Now I wanted to answer the flashing red light question, I put the old circuit board in to see what the red light indicated. After installation of the old circuit board, the furnace fired right up and functioned as designed. Moving a head, we went camping last weekend, the first night I set the thermostat to heat and the A/C fan turned on then the furnace fired up. The A/C fan stated on and the furnace maintained the temperature through out the night. Second night same as above, but the furnace would not light. The trailer was hooked up to shore power with the electric water heater on. (Glad I had my back up oil heater.) Brought the trailer home, the following day on battery power, the furnace lit off with no A/C fan. A friend of mine strongly suggested I replace my dual propane regulator. This is so frustrating, I am ready to replace the furnace with a Espar Airtonic diesel heater. Of course I appreciate anyones input on this matter. 

Frank