Suburban furnace problem - Problem Solved

[danemayer]

Check the connection of the furnace wires to the trailer wiring, probably above the furnace. When the thermostat calls for power, you should get 12V there. If not, go back to the relay output in the A/C return.

 

[6.7LMegaCab]

Thanks, Dan.

Working on this now.

According to the schematic on the bottom of the case cover, I'm guessing the left connector is Furnace Y4 and the right connector is Furnace Y3.

With the thermostat off, Y4 has +13.22V. Y3 shows zero.

With the fuse installed and heat called, both Y3 and Y4 show 13.22V.

With the fuse pulled and heat called, 0V on both Y3 and Y4. With the thermostat off, Y4 shows 12V.

I'm going down to pull the basement wall by the furnace to access those wires. I'll post those findings next.

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[6.7LMegaCab]

Alright, so unless I pull the furnace, I can't access the crimp connections that ties into the trailer wiring. Instead, as painful as it was, I just tested the two white wires through the insulation that I'm guessing are part of Y3 and Y4 and both show 13V as well. There is also a combined red and black wire (14 AWG perhaps) going to the furnace. The red wire has +13V and the black wire has good continuity to ground. I'll seal those pin holes back up after I figure this out. Where I tested is about within 2 feet of those crimp connections, so power is getting close to it.

That brings me back to the control board. The red wire that is tied in with the trailer wiring above the furnace is hard to tell what it is tied to, but should have 12V. This wire is on the same connector on the board as a blue wire that goes to a N.C. Valve Switch (On/Off switch)...which is part of the thermostat wiring according to the schematic.

I'm literally stumped on this. I'll pull the furnace, probably tomorrow, to get better access to those wire crimps above the furnace. I find it hard to believe those crimps are involved...but won't discount them yet.

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[danemayer]

It's either a bad connection - could be at the board btw - or a board problem. Board failures are not uncommon.

 

[6.7LMegaCab]

I'm willing to buy a new board (or repair this one), but having trouble understanding why there isn't power going to the board itself on wires that I think should have power. I'll sleep on it tonight to ponder these schematics a bit more.

Where is the limit switch and would it cause any problems with power not getting to the board if it fails?

 

[6.7LMegaCab]

Is there some sort of over-temp lockout somewhere that would otherwise prevent +12V getting to the board? I've been running through my noggin going back to when the issue started. While we were out hunting, bags were placed in front of the return air grill to the furnace. I didn't notice the bags there when I adjusted the thermostat before I went to bed but saw them there when we got up bright and early to a very cold trailer. So now I'm wondering if this could have had something to do with it?

 

[danemayer]

I'm willing to buy a new board (or repair this one), but having trouble understanding why there isn't power going to the board itself on wires that I think should have power. I'll sleep on it tonight to ponder these schematics a bit more.

Where is the limit switch and would it cause any problems with power not getting to the board if it fails?

I believe the limit switch is on the inside as it detects an overtemp condition. It's in series with the sail switch as shown in the wiring diagram. Note there is an alternate path for the board to get power to the blower, independent of the limit and sail switches.

I've attached a troubleshooting flowchart.

 

[2019_V22]

Possible broken wire? I don't know your system, but have extensive HVAC background. If your furnace uses a separate thermostat, (I have a 1 thermostat for heat/AC, and furnace) than verify voltage to the thermostat, and voltage out of the thermostat when calling for heat. if voltage is present at both in and outs of the thermostat, then you need to follow the out voltage wire that goes to the input of the furnace board.
If you are certain of the voltage input wires on the furnace board, you can remove those input wires, and connect a 12v source to them, IE: battery like you did with the fan. That should make the board react as if the thermostat is sending voltage to it.

 

[wdk450]

What you posted in this sentence seems to indicate that the wiring from the 12 volt fusepanel to the furnace power input is good: "Is there some other hidden fuse or CB somewhere that is between the combo 120 main breaker/ 12V fuse panel that controls power to the furnace? There is a fuse for the furnace on that main panel and when heat is called for and pull the fuse, the LED lights up, so I know that is good."

Change that fuse just for the heck of it.

Did I understand correctly that you measured with a voltmeter where the power is supposed to come in to the furnace control board, and got 0 volts instead of 12 volts DC? On perplexing problems like this you may need to run an external test wire from a good 12 volt dc source directly to that circuit board 12 volt dc power input point, then see if it works.

On edit (after viewing the schematic) : Incoming power is +12 vdc on a red wire and -12vdc (ground) on the yellow wire. Are BOTH connected correctly?

 

[6.7LMegaCab]

I believe the limit switch is on the inside as it detects an overtemp condition. It's in series with the sail switch as shown in the wiring diagram. Note there is an alternate path for the board to get power to the blower, independent of the limit and sail switches.

I've attached a troubleshooting flowchart.


View attachment 66604

Dan, thank you! This is the chart I've been trying to follow from the installation manual that came with the rig. It seems like something is missing from it as the motor has 3 or 4 wires coming out of it...I'm baffled. I'll look at it again in the morning.

Possible broken wire? I don't know your system, but have extensive HVAC background. If your furnace uses a separate thermostat, (I have a 1 thermostat for heat/AC, and furnace) than verify voltage to the thermostat, and voltage out of the thermostat when calling for heat. if voltage is present at both in and outs of the thermostat, then you need to follow the out voltage wire that goes to the input of the furnace board.
If you are certain of the voltage input wires on the furnace board, you can remove those input wires, and connect a 12v source to them, IE: battery like you did with the fan. That should make the board react as if the thermostat is sending voltage to it.

The furnace is connected via the Dometic proprietary AC thermostat. So when you set it to heat, you get an audible click from a relay on the AC control board, and then an audible click from the furnace - but I don't think the click is coming from the control board...hard to tell though.

From what I can find, there are only 4 wires coming from the electrical cabinet area that go to the furnace. Two white wires, which come from the ceiling AC unit and are the Furnace Y3 and Y4 and drop behind the cabinet and go across the other side via the basement to the furnace. The other two wires are a red and black wire that looks like it is coming from the bottom of the cabinet, possibly from the fuse panel...hard to tell. Both white wires have +12V when heat is called for. Y4 has a constant +12V regardless of whether heat is called for or t-stat is off. The red wire has +12V and the black wire is confirmed good continuity to ground.

I was trying to get the red "Power" wire off of the board to isolate that and connect a battery directly to it, to see what it does, but that spade connector wouldn't release. I'll try again tomorrow and attempt your recommendations.

What you posted in this sentence seems to indicate that the wiring from the 12 volt fusepanel to the furnace power input is good: "Is there some other hidden fuse or CB somewhere that is between the combo 120 main breaker/ 12V fuse panel that controls power to the furnace? There is a fuse for the furnace on that main panel and when heat is called for and pull the fuse, the LED lights up, so I know that is good."

Change that fuse just for the heck of it.

Did I understand correctly that you measured with a voltmeter where the power is supposed to come in to the furnace control board, and got 0 volts instead of 12 volts DC? On perplexing problems like this you may need to run an external test wire from a good 12 volt dc source directly to that circuit board 12 volt dc power input point, then see if it works.

On edit (after viewing the schematic) : Incoming power is +12 vdc on a red wire and -12vdc (ground) on the yellow wire. Are BOTH connected correctly?

Luckily, I got a lot of spare fuses...I'll change it tomorrow.

Yes you did read correctly. If I can get the red power wire off (see below), I'll hook up a separate battery to see if this gets the circuit to work.

If I recall, the connector with the 4 wires on it that connects to the PCB has the red and yellow wires on it (along with brown and blue wires)...there is zero voltage on either of them (I'll double-check again tomorrow though). The Red wire with a female spade connector is connected to a double male spade soldered to the board, the other male has a blue thermostat wire that goes to an on/off switch just to the left of the board - YES the switch is on...in fact, never knew it existed until this unit stopped working. This red wire is what the schematic shows as the POWER wire for the board, which, according to the schematic, it appears that is the first point for the blower motor to kick on to get the whole circuit working. If the power wire would work, it should send the +12V and -12V to the red and yellow wires you mentioned...again, if I'm reading the schematic correctly. I read the schematic again - I don't recall seeing another yellow wire anywhere (the yellow wire that has the -12VDC annotated next to it). It might be part of the wires I can't get to unless I pull the furnace.

There is another red wire on a separate male spade soldered to the board right next to the other two wires. This goes directly to the blower motor. I disconnected that wire and connected a separate battery to it which immediately powered up the blower.

I have never touched this system and for the 4 years we've owned it, the furnace has always worked...until this past week...and only worked for one night. So I can assume they are all hooked up correctly. I'll dig some more to find that other yellow wire.

Thanks again for all the replies folks! This is helping....I'm all ears for other ideas. I'll be back at it tomorrow.


Josh

 

[travelin2]

Good suggestion about changing the fuse! I forgot that the first cool weather we experienced in our ‘17 BH we had some furnace issues. During trouble shooting the fuse looked ok but while brainstorming with others we were rallying with someone suggested changing the fuse just for giggles.
Bingo!! That was the ticket

 

[6.7LMegaCab]

Alright, so I changed the fuse, still no dice. I found the yellow wire which looks to be connected to a that black wire (part of the red/black wire pair I mentioned earlier) above the furnace and it checks out okay as well. I was able to break the cable tie loose and pull the wires to the outside and test them. All of those wires appear to be testing just fine. I then discovered that the flat connector with the red, yellow, brown and blue wires has to be connected to the board. Absent of those connected, nothing will happen. The more I look at the schematic and comparing the wires, the more it seems that those must be connected to complete the circuit. It's the yellow wire providing the ground (as well). Since I had the board mounting plate disconnected, there was no additional ground to the board, thus why I wasn't getting voltage at the power wire. When I completed the grounding path, the power wire at the board checked out at +13VDC.

However, it still appears that there is something in the circuit that is not clear on the schematic that is preventing the blower from turning on. I hear the click at the furnace, which is confirmed to be at the board. I'm guessing the missing piece in the schematic is the relay that might be providing the final ground path to start the blower.

I'm now leaning towards the board being the culprit and due to me not clearly wrapping my head around everything on the schematic yesterday, explains why I wasn't getting the voltage at the board (yellow wire providing the ground path).

I'm going to pulling the board and test the relay. It looks like I might even have the capacitors on hand from other projects (they all appear to be the same physical size). If I recall, you can safely upside a cap by about 10%, so I may replace them and the relay if I can narrow the relay down to being bad.

Thanks again everyone for the replies! It definitely helped to get me to look at this differently and wrap my head around it all.

I'll report back with my findings on the board.

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[6.7LMegaCab]

Looks like the relay or the board is the problem. The inside pin for the power wire connects to a pin on the bottom of the relay and the relay side is burned. That then connects to a relay pin that traces back to the inside pin for the blower connector (its the long trace just below the burned pin). I should have just pulled this to begin with instead of over analyzing this system.

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[6.7LMegaCab]

Welp...further diagnosing is officially over! I was looking at that solder joint, researching the relay, asking some questions about an upgraded relay (with 12VDC nominal coil voltage instead of the current relay which is a 9VDC nominal coil voltage - but within close min-max range). I looked at the solder joint again and it dawned on me that the burned spot was due to a poor soldering job and the slight vibrations would have caused arcing and excessive heat, resulting in the solder joint breaking. I cleaned it up, flowed new solder, temporarily hooked it up, and the blower powered up immediately.

Now, my criticism is direct to Suburban as they could have added a relay into their schematic that shows the blower only powers up via the relay on the control board. The way the schematic looks is that it is directly powered once the thermostat is set to heat. That explains why I was so confused. This simple addition would have saved me countless hours of troubleshooting.

I do wonder how many of these boards have failed because of this?

Can't thank y'all enough for taking the time to reply and point me in the right direction.

 

[danemayer]

Suburban has published a number of wiring diagrams with slightly varying content. But I think the board designs change more than the diagrams.

 

[6.7LMegaCab]

Ah! I'll go to their website and try to find the latest install sheet for the model line I have. Not certain where I grabbed the pdf that I downloaded yesterday, but it is the same as the printed one that came with the trailer.

Thanks again!

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[thewanderingeight]

Nice work tracking this issue down! Thanks for posting your findings.

 

[6.7LMegaCab]

I hope this helps others that has a similar problem and can perhaps save some money.

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[wdk450]

Congratulations on your persistence in troubleshooting this problem. Thanks for sharing your problem resolution with all the forum readers out there, and everyone who does a websearch on solving this problem.

The melted solder joint on a switch or relay connecting pin can be the CAUSE or the RESULT. High currents through a bad relay (or switch) contact can cause enough heat transmitted from the contact through the device terminal metal to melt the solder joint, too.

 

[tomhank]

There could be an electrical problem if the fan isn't running and the furnace isn't producing heat. Check to see if your RV batteries are still charged and capable of producing a consistent 12 volt output. You may have a blown fuse or a tripped circuit breaker if the batteries are still OK.
Check your utility box to see whether it needs to be replaced. If the batteries are in good shape and all of the fuses are in good working order, the thermostat should be checked next. Remove the thermostat's cover and look for a "anticipator"-labeled adjustment control. Turn the thermostat to the highest setting if the furnace's fan won't start.