Strange Atwood (Dometic?) Furnace problem. Stuck Relay?

[Jevad]

BLUF: Furnace not providing 12V to Thermostat, "Fixes" itself with a thump to the side of the RV. Thoughts on what could be wrong?

Atwood/Dometic Model Number AFMD30141/ Part Number 32830

Very new to me/ just purchased a used 2018 Thor Four Winds 30D. We are on our second trip and went to upstate NY in mid-November = COLD and Snow!! Everything seemed to be going well, although woke up to cold coach this morning. I'm having a problem with our Furnace where after working fine for the last 3 days/night, the furnace fails to start/kick on.

Propane is good, Plugged into shore power, also tried on battery and with the chassis running - to make sure it wasn’t a powerline issue - still no luck. I noticed the thermostat wasn’t "clicking", happened to have some tools and pulled out the voltmeter and found no power to the Thermostat. Sliding setting left/right had no effect (no click for thermostat to kick on heat). After checking this forum I saw there was a reset breaker likely on the furnace. (it had been windy… so maybe that triggered something).

I went out started to remove panel over furnace and suddenly it kicked on!! The blower motor spun up, then 30 seconds or so later furnace fired and started as normal. I finished removing the panel and saw the reset switch, everything looked normal, dry, no water (no louvers on this coach that others had water issue), nothing to indicate why it hadnt started. It’s a fairly new coach and all the wiring is in good shape (looks new), and I verified each connection was solid. I Buttoned everything back up and furnace now running worked as expected. It kicked back off when the coach warmed up to temperature as normal.

When a while later we noticed the coach was getting cold again… and once again, nothing - no power on the thermostat. (it had not kicked back on automatically since I touched it last), I went back out and removed the furnace cover panel again, this time it didn’t start on its own as I removed it. Poked around, tried to test Sail switch and burner switch, (Not knowing the normal state of the switchs - I cant find a wiring diagram.) I figured maybe one of the switches might have been stuck, so I tapped the housing, and suddenly the blower motor started (followed by the furnace again as normal).

Again, furnace ran to proper temperature, the thermostat shut down at temp, but again it would not restart the next cycle. After the furnace ran/shut off as normal, I failed to get 12Vdc on the thermostat wires again (between Red 12V and Blue Ground). I went outside this time and decided just to give the outside wall a bang or 2 with my open hand. Went back inside and I now had 12 vdc on the thermostat and was able to run again.

I suspect either a switch or more likely relay is sticking? Any thoughts? (Not sure how/what provides power to thermostat? Relay?). Help!?!

Thank you!!

[DenverTransplant]

Sounds a bit like a loose wire connection or a bad wire. Attached is an AFM wiring diagram from a technician training manual I found while I was fighting what turned out to be a bad gas valve solenoid. I tried attaching the manual itself but the file is too large.

[Jevad]

Thanks for the image, may have to try and trace out the wiring... It looks like solid connections/wires, I gave each one a little tug, unplugged the ones to the limit and sail switches, all look clean/solid connections. Nothing happened when I jostled the wires, but the thump on the RV did. I should also note, that the diagnostic LED on the controller board did not blink any codes... no lights at all.

THANKS AGAIN!!

Dave.

[javelin]

Typically the +12VDC voltage is applied directly to the thermostat (RH and RC pins – thermostat switched on to heat position) and a call for heat closes a relay in the thermostat which then supplies the +12 VDC to the Atwood furnace control board/sail switch etc. Since you said all the wiring in the furnace (outside access) looks solid, clean, dry and new..... possibly the time you opened the outside panel and the furnace came on was only coincidental with the thermostat calling for heat? You said when not working, there is no power on the thermostat. I assume you are measuring DC volts on the RH terminal of the thermostat? Do you hear the thermostat relay clicking when it calls for heat? Does the thermostat use its own batteries for internal control of its relays and if so are the batteries good? Perhaps a bad 12VDC connection at the thermostat RH and RC connections; or where +12VDC wire in the wall connects to the thermostat RH/RC wiring?


I think your manual can be found here: (Schematic on page 8).


https://manuals.heartlandowners.org/...wood%20AFM.PDF

[Jevad]

Thanks Javelin, I searched all over for that manual... and could not find it (and it was the one that I didn't get from dealer either.) Looking at the wiring diagram, the 12V TO the Thermostat seems to come from the Off/Reset switch located on the mounting bracket for the Ignition Control Board on the Furnace. (hard to see, but its on the left side of picture below).

To answer some of your questions for clarity.... I'm quite sure its not coincidental for furnace to come on as I started opening it. In fact, for the last 8 hours or so, I have now gone out and simply gave the wall a knock (not even too hard), and immediately the blower fan starts up (followed by the ignition of the furnace in a about 30 seconds - as normal). When the system is "stuck" and not working, I can slide the temp setting from one extreme to the next (low to high), I do NOT hear the thermostat click/engage relay to start the furnace. ALSO, the fan nor AC kick on (switching from Heat to any other setting). Since this is an analog/slide type Coleman Mack thermostat, there are no batteries in the unit. I was measuring the voltage from W1 (Red) wire to W2 (Blue) farthest wire (see picture). When working I can read 12VDC across those 2 points, when stuck I read only a couple of volts. (something pulling the voltage down?). again, all the wiring looks good, and I've moved them around to make sure nothing is shorting.

I REALLY appreciate the thoughts. I'm in upstate NY with my wife and son, who aren't too keen on freezing out in the "RV"... first experience for them is not going well :-(.

[javelin]

Seems like a loose or intermittant connection in the furnace .....especially if banging on the side of the furnace access door outside starts normal operation. Check ground and 12 volt connections are good and tight at the furnace end. Also check if closing the outside furnace door causes any intermittant short (pinched wire) somewhere in the wiring. Does the air conditioner come on normally when the coleman mac is set for cooling and you temp set call for cooling? If both heat and cool are intermittant then the 12vdc connection, source or thermostat are in question.


I converted my coleman mac to digital many years ago and did a drawing related to that conversion which I will try to attach for reference (left side is the analog coleman mac I used to have). If our coleman macs are the same, I think the red wire is the +12VDC coach battery, the Blue is Ground and the White should be the switched +12v feed to the furnace when heat is called for (coleman mac temp set above current room temp; heat switch on if applicable).


When thermo calls for heat it should short red and white leads to provide +12v to the furnace on white lead (but not sure if that is how your coach operates or is wired). Strange that you say when “not working” the red to blue is only a couple volts........you mean thermo is calling for heat but nothing is happening at the furnace and you only see a couple volts measuring red to blue? If yes, sounds like a heavily loaded circuit dragging voltage down but you would think the thermostat fuse would blow if true (depending on current flow of course). The load is the furnace control board, fan motor etc. (assuming a faulty thermostat is not causing the voltage dip somehow). You could try bypassing the thermo completely with appropriate in line fusing between R and W, but banging on the outside furnace cover seems to imply outside furnace wiring issues.
Sorry I can’t help more.

[javelin]

Try that upload file attach again.....past my beauty sleep time I guess.

[javelin]

I re-read your postings again and you said there were no codes on the control board so probably not in lock out mode. A reset or recycle of power switch in the furnace should fix the lockout assuming control board is good and no codes exist. From the schematic it looks like the coach 12VDC source does connect directly to the furnace power switch and is then sent to the thermostat via blue wires from the furnace. So when not functioning I would try carefully shorting (jumping) the blue thermostat wires at the furnace to see if the fan motor starts running as is normal startup for the furnace. Intermittant wiring issues are always a PIA to track down because when working there is no probem to trace; so you have to wait until it fails again in order to troubleshoot. A multimeter and the manual schematic will help you trace where your 12vdc is failing to connect to. You also said tapping in a certain place or device area caused things to start working again....can you re create that scenario? Good luck.

[javelin]

Hi I assumed BL on the furnace schematic was blue...could be black also....just make sure from the schematic that you are carefully jumping the thermostat wires only of course.....don't want to create more costly repair issues.

[Jevad]

Thanks AGAIN Javelin!!! your input is helping, and I will definitely move to a digital thermostat once this problem is resolved (I prefer setting to exact temp instead of doing the analog slide guess).


As far as this issue, although the furnace sporadically worked properly, we continued to have the same thing happen again throughout the weekend, but we were able to make due. Luckily we were staying in an RV Campground with power so I ran a space heater which helped keep us from freezing in the sub 30s snowy weather we were getting. I also went out and did the "Heat Tap" to kick on the furnace when it didnt work by itself and it was getting cold in the coach (when the space heater couldnt keep up). We got home late last night, and I had to do the thump, but its been running overnight since without ... I hate intermittent problems!!!.



I was able to record some video of the Thermostat, with voltage readings, when it was not working, when I did the Heat Thump to get it working and voltages afterwards.

[javelin]

Well at least you or family have not frozen....yet. The main thing is to make sure your pipes don’t freeze in the coach.....people can huddle around a space heater or put on more clothes to stay warm. You may want to call a mobile tech as that may be less costly than a major freeze and water leak in your coach..... or a divorce. LOL.


In the mean time, if the weather permits (no rain/low winds), I would be tempted to leave the furnace door off in the day time while you standby for it to fail again.......and then carefully move select “start circuit” wires (like +12v and ground; on off reset switch or thermostat or control board; motor) in the furnace area to determine if/when the fan starts up (during a constant thermostat call for heat of course). If no intermittant wiring trouble is found then try your tapping technique around furnace devices to home in on the fault location.


You said previously that you measured a few volts only (between red to blue on the inside thermostat during the failure time – which is essentially +12V and ground per your schematic). For voltage to drop to a couple volts (implies high current loading or high resistance intermittant connections) is not normal especially with no apparent fuse blow or smoking components. Maybe just a bad meter reading ......or flakey high resistance +12v or ground wire connection or wire conductor itself causing intermittant high resistance conditions.


One other option which requires the +12 feed turned off to the furnace (fuse pull best at source) is to use the ohmmeter setting on your multimeter and place it across individual wires that should provide short ccts (low resistance on the meter) and then wiggle each wire being measured to see if the meter reading changes). This could indicate a bad wire or broken conductor inside the insulation. It is best to have small alligator clips on the meter leads so you can clip on each wire end and wiggle wires. Ohmmeter readings must be taken on dead circuits – reading a live circuit will potentially damage your meter. Remember to reset meter to read volts again when you are done with any ohmmeter tests.
That’s about all I can suggest.

[camperguy99]

Isn't there a thing called "sail switch" that determines if the fan that purges the exhaust (I think?) that often sticks? That's why a good bang on the side frees it up.

[javelin]

I thought about that too but normally the furnace motor starts first when the thermostat calls for heat and then the air flow from the blower is proven by the sail switch connecting 12v through the limit switch to the control board to continue the gas light process etc.

One other option is what if the sail switch remains stuck in the blower prove position sometimes.......maybe the 12v being applied to the control board through the prove circuit before it should be is causing the issue.....but there are no codes shown. Then banging the side of the coach frees the sail switch to return to normal open position. For such a new coach you would not think the sail switch would even be a problem....famous last words.

[mk34techrep]

I would also check any splice connectors you have in the circuit, had a problem with mine doing about the same thing, turned out the splice connection came loose, once found replaced connector with solder joints and tape, no further problems, good luck, hope you locate the problem...

[Jevad]

Originally Posted by mk34techrep

I would also check any splice connectors you have in the circuit, had a problem with mine doing about the same thing, turned out the splice connection came loose, once found replaced connector with solder joints and tape, no further problems, good luck, hope you locate the problem...

Thanks Mk34TechRep.... I will try that if all else fails. Really odd the behavior, I suspect its a stuck switch (Sail switch, Limit Switch or relay to start the motor) since its been so consistent and did not correct when I was fooling around with the wires - tugging around.

[JamieGeek]

You've had to tap on it enough: Did you narrow down the area you have to hit to get it to work? Perhaps there is something directly behind that? (Stuck switch, loose wire, etc.)

[Jevad]

Originally Posted by javelin

I re-read your postings again and you said there were no codes on the control board so probably not in lock out mode. A reset or recycle of power switch in the furnace should fix the lockout assuming control board is good and no codes exist. From the schematic it looks like the coach 12VDC source does connect directly to the furnace power switch and is then sent to the thermostat via blue wires from the furnace. So when not functioning I would try carefully shorting (jumping) the blue thermostat wires at the furnace to see if the fan motor starts running as is normal startup for the furnace. Intermittant wiring issues are always a PIA to track down because when working there is no probem to trace; so you have to wait until it fails again in order to troubleshoot. A multimeter and the manual schematic will help you trace where your 12vdc is failing to connect to. You also said tapping in a certain place or device area caused things to start working again....can you re create that scenario? Good luck.

Going through to reply to each post.... Yes, no codes on the control board, so yes I agree, probably not in lockout mode. (note I powered the coach on/off seeing if that would reset anything. - unplugged shore power, turned batteries/coach power off - did not reset the trouble. I wasnt able to troubleshoot/test voltages while it was out again due to where we were (nor wanted to do it in the dark/middle of the night, nor in a snow storm :-/ ). I am trying to get it to recreate the problem... and will try jumping as suggested.

THANKS AGAIN!!

[Jevad]

Originally Posted by JamieGeek

You've had to tap on it enough: Did you narrow down the area you have to hit to get it to work? Perhaps there is something directly behind that? (Stuck switch, loose wire, etc.)

I actually had taken video of the "Heater Thump" that I've gotten used to (this site is not letting me update it... may try YouTube and post link). No its not just a single spot, tried the Furnace Door and the spot on the wall above/between furnace and hot water heater.

[Jevad]

Originally Posted by javelin

I thought about that too but normally the furnace motor starts first when the thermostat calls for heat and then the air flow from the blower is proven by the sail switch connecting 12v through the limit switch to the control board to continue the gas light process etc.

One other option is what if the sail switch remains stuck in the blower prove position sometimes.......maybe the 12v being applied to the control board through the prove circuit before it should be is causing the issue.....but there are no codes shown. Then banging the side of the coach frees the sail switch to return to normal open position. For such a new coach you would not think the sail switch would even be a problem....famous last words.

I think we may be on to something... I wish I could recall exactly the state of the Sail Switch when I was poking around in the furnace the other day when first trying to troubleshoot before I had the drawings. I did remove one wire, and thought it was closed.... (which could have been the issue) but I cant be positive.

I think I'll be a Atwood furnace expert by the time this is debugged, but thinking this through....

(Note the left detailed schematic and right side logical schematic in the manual are slightly conflicted, the left detailed side shows the power feed going from thermostat -> Sail switch, which when closed feeds the Limit switch. The RIGHT SIDE logical schematic, shows the thermostat feeding BOTH the Sail Switch AND the Limit switch. I will verify when I open up the panel the next time.)

The schematic shows the 12v feed from the power source ->
-> Off/Reset switch on the furnace then feeds the ->
-> Thermostat, which when temperature demand triggers then feeds back to ->
-> (depending on which schematic) The furnace Sail Switch and the Limit Switch OR Input on the Control Panel (I think is incorrect).

I see there is a relay on the Control Board that feeds the blower motor, so I suspect under the start condition input from the Thermostat, the controller board closes the blower relay, and watches the input for the Sail Switch to close. Once the control panel gets a signal that the fan is running (by the sail switch closure), it then ignites the furnace.

Now I wonder, if the Sail Switch is sticking CLOSED, what does the Ignition Control Board do? If the Control Board believes the fan should not be spinning and the Sail Fan should be open, does it prevent the Furnace from starting? Why does the voltage on the thermostat read only 2-3 volts? what is pulling that voltage down?

When it occurs again, I will definitely check voltages on each of the points of input... and see what I get. I will also check the state of the Sail Switch, open/close and see if I can trick it. If that doesnt work, I can try tapping the relay on the Control Board... to see if that resets it.

THANKS AGAIN!!!

[Jevad]

Troubleshooting my 2018 Thor Four Winds 30D Furnace, that is not working starting until I thump the side of the RV. I have 4 videos in this series.
1. "Dead" thermostat showing no reaction to thermostat control - Showed no voltages on thermostat lines
https://youtu.be/1TTYQ90Rn1Q

2. Shows my "Heat thump" that triggers the furnace to start.
https://youtu.be/6qdnLHWRVzo

3. shows the "Live" thermostat and good voltages and control.
https://youtu.be/ZF_-U1x9xT4

4. shows the Live thermostat AFTER I shut the furnace off.
https://youtu.be/AFrWwZ2SCqw

(Sorry I wasnt able to load videos to this site for some reason).