8500-IV Series Furnace Thermostat Dilemma

[RandyJ55]

I own a 1995 Sunline Solaris Model 2970. Last winter the furnace worked fine after I replaced the TDR and the DSI board. For some reason last week for the first time this Fall when the thermostat was turned up to call for heat, nothing happened. I had the DSI board tested, replaced the TDR and all wiring to the TDR, at the breaker, the grounds, and the on and off switch and cannot figure out why nothing happens. I followed the thermostat wires to the furnace and even touched the red and white wire together to see if I could start the furnace; nothing happens. I would greatly appreciate any input as to why this suddenly happened and if there is an issue at the power source, possibly a breaker. I would appreciate any input to guide me in the right direction. Thank you

[JohnB]

Hi,

Can you confirm if you have a ducted air conditioner (there are multiple ducts across the ceiling for the air to come out) And do you have only one thermostat for both the AC and the furnace? Or 2 separate ones?

John

[RandyJ55]

Yes I have a ducted air conditioner and have the wires separately for that unit not wired into the heat only thermostat from Dometic. I did check the breakers, the fuses, and the wires going to the furnace from the converter and from what I can tell, there is no obvious issue. I just got my thermostat installed and will attempt to get the furnace going with the standard startup process today. I'm still confused as what specifically supplies the voltage from the thermostat to the furnace.

[JohnB]

Originally Posted by RandyJ55

I just got my thermostat installed and will attempt to get the furnace going with the standard startup process today. I'm still confused as what specifically supplies the voltage from the thermostat to the furnace.

Hi Randy,

In these cases it helps if you can post pics of the wall thermostat for both the AC unit and the furnace. And the model number of the AC control box up in the ceiling next behind the mesh filter

Without the AC unit control box model number I am going to have to “guess” what vintage of the AC control board you have. Since you say you have a separate furnace T stat that points to you might have what Dometic called there bi-metal thermostat control setup.

I am away from my normal supply of service manuals to point which one fits your system, we also have them for you to download in our FILES area on the forum

Not knowing what you have unhooked between the AC unit and the furnace, I can tell you how the furnace is wired to get a run signal. And from there you may be able to back into the problem

Look inside the camper at the inside sheet metal side of the furnace. Look on the side where the control board is at all the way back by the wall. There should be a white plastic plug in the side of the furnace sheet metal with 4 wires coming out of it in a pigtail harness. Those 4 wires are wire nutted by Sunline into the camper system. The 2 thicker wires, are the 12 volt supply from the camper power converter fuse block system. Odds are high the black Sunline wire is +. Pos 12 volts and the white wire is - neg 12 volt . That’s the power supply to the furnace. Make sure you have 12 volts on those 2 wires. You can trace them into the furnace if you cannot reach them at the wall due to your layout

The other 2 wires are to control the furnace. They are very thin and Sunline may have used thermostat wire to connect to the 2 furnace pigtail wires. The furnace sends + 12 volts out one of those 2 wires to the T stat that runs the furnace. The other thin wire is the run control Signal To the furnace. If you jump those 2 wires together, the furnace should start running and will run non stop until they are separated. The T stat is the switch that makes and breaks that run circuit.

If you jump those 2 run control wires close to the white connector where the pigtail wires are, and furnace does not run, the problem is in the furnace. If the furnace does run when jumped at the furnace pigtail, the problem is in the T Sat or wiring in the wall to the T stat

Hope this helps

John

PS. Why this deals with the AC control, pending the vintage and type of AC control board, the AC system interlocks the furnace to not run the furnace the same time the AC is running. And sometimes that interlock has issues and will not allow the furnace to run. But, the furnace is a stand alone system, if jumping the run control at the pigtail area it should run.

[RandyJ55]

Hi John,
Tuesday evening I found where the white and red thermostat wires each connect with a white plastic wirenut to a blue wire. I was going to disconnect the red and white wire from the blue wires and check for continuity from the thermostat to make sure there was no broken wires in the path to the furnace with my tester. If I understand you correctly, under normal conditions if I jump the two blue wires together, the furnace should run. In regards to the a/c unit I haven't had a separate thermostat for a few years. I have the wires capped off separately. If the furnace still doesn't run after touching the blue wires together then I will follow the blue wires into the furnace area to make sure all wires and connections are intact. Thanks again for your insight. I will let you know what I find and if resolved, the issue.

[RandyJ55]

Hi John,
I have an update regarding the thermostat wires to the furnace. When I disconnected the pigtails which connected the red and white wires to the blue thermostat wires going to the furnace and connected the blue wires together, the blower motor kicked on. When I tested the red and white wires together with my tester after turning the thermostat system switch on and closing the contacts on the thermostat, I got nothing. Normally, with my tester it would beep. I have my old functional thermostat which I will wire as a test unit close to the furnace with two different thermostat wires to see if all goes well from there. I will let you know if that problem is resolved. Thanks again for all your insight .

[Torskdoc423]

There may be a Thermal limit switch in the ducting on the furnace before it splits. If this gets too hot it will open, then it's supposed to close when it cools down. Sometimes it doesn't close again, leaving you with an open circuit in the power circuit.



With the furnace at room temp, Lift one terminal on limit switch, then ohm it with a meter. O/L (out of Limits) indicates an open (NO GOOD). IIRC somewhere in range of 100- 250Ω is ok. (it's been a long time since I had to meter one.




2 examples Both are limit switches.

[JohnB]

Originally Posted by RandyJ55

Hi John,
I have an update regarding the thermostat wires to the furnace. When I disconnected the pigtails which connected the red and white wires to the blue thermostat wires going to the furnace and connected the blue wires together, the blower motor kicked on.

When I tested the red and white wires together with my tester after turning the thermostat system switch on and closing the contacts on the thermostat, I got nothing.

Normally, with my tester it would beep. I have my old functional thermostat which I will wire as a test unit close to the furnace with two different thermostat wires to see if all goes well from there. I will let you know if that problem is resolved. Thanks again for all your insight .

Hi Randy,

The blower is the first thing to start on the furnace. If the blower does not run, the rest will not work. By unhooking the camper T stat wiring at the white plug pigtail plug, and then jumping the 2 blue furnace wires on the pigtail and the blower starts, that is a major find. At this point, the furnace has made it past the first test, the blower has to run.

Now back to those T stat wires. There seems to be a problem in those wires. There may be a break in the wire somewhere between the pigtail at the furnace and the T stat. With no thermostat in place up at the wall where the T stat goes, if you jump those 2 T stat wires hooked while being hooked up to the pigtail at the furnace, the furnace blower should start after a very sort delay, just like when you jumped the 2 wires at the pigtail. If I understood you right, you tried this before and get nothing. If so, you just proved there is a wire issue somewhere.

Worst case, get a long piece of test wire and your ohm meter and see if the red and white wires from the plug to the wall location at the T stat have the proper continuity with nothing hooked to them. There may be a wire break buried in the wall.

If you have a spare 2 wire set, you can connect the T stat with test jumper wires from the T stat to the pigtail out in the open and check if your old T stat or the new is working. Again, the T stat is just a temperature controlled switch.

Also, confirm your pigtail and plug at your furnace looks like these pics. Only 4 wires in the white plug that plugs into the furnace.






As a troubleshooting thing and just to understand, those 2 blue wires in that plug. One is + 12VDC. On some vintages of this hydroflame furnace, they print in tiny letters + + + or + 12 V on the blue pigtail wire. But it is hard to see. Yours being as old as it is, may not have that printing, but one of those wires should read battery or converter voltage when using a volt meter, the blue wire at the positive meter lead and the other end of the meter on - 12 DC ground. You can use the white wire Sunline ground wire brought to the pigtail or pick it up elsewhere. The other blue wire, is the run signal to the furnace.

When that run circuit wire receives +12 VDC it powers up the TDR (time delay relay) coil, which pulls in the relay contact. and powers up the fan motor. And it passes power to one side of the sail switch. Again, with the 2 pigtail blue wires unhooked, one should be + 12VDC with a voltmeter to ground. The other would not have positive voltage on it.

Since you said you have the 8500-IV and you changed the TDR, that is the older vintage of the 85 series, rev IV control. Your model number should have 2 other numbers in place of the 00 like 16, 20, 25 , 31 or 35 like 8525-IV which the 2 digits stand for BTU size of the furnace. But they all operate the same, just different heat output.

If you have not found it, see this service manual for you furnace from our files section. https://www.sunlineclub.com/forums/d...o=file&id=5631

When logged in, download is and flip to page 10. At the bottom left, see the wiring diagram, 85-IV Local Sense. That should be the one for your furnace. They use wire colors in place of numbers but this may help in your troubleshooting efforts.

It appears you have to sort out your T stat wire issue, then you can get to what else might be an issue with the furnace.

Hope this helps

John

[JohnB]

Originally Posted by Torskdoc423

There may be a Thermal limit switch in the ducting on the furnace before it splits. If this gets too hot it will open, then it's supposed to close when it cools down. Sometimes it doesn't close again, leaving you with an open circuit in the power circuit.

With the furnace at room temp, Lift one terminal on limit switch, then ohm it with a meter. O/L (out of Limits) indicates an open (NO GOOD). IIRC somewhere in range of 100- 250Ω is ok. (it's been a long time since I had to meter one.

Hi Larry,

Randy's furnace does have an overtemp thermal disk switch like what you showed. And if it is stuck open, the ignition and gas valve will not fire off like you said, but the blower motors should still run. There is also the blower sail switch in the mix that has to make, but again it needs the fan running at speed to engage the sail switch then go through the over temp limit switch. I did not hear Randy say yet, the blower would run but no gas valve or ignition would work. Not sure he made it that far just yet.

At this point, if I read it right, Randy cannot get his T stat to turn on the blower motor and with no blower, nothing is going to work on the furnace. I think he found a good issue tonight as he jumped the 2 blue wires right at the furnace pigtail plug and the blower started. That is a major step, he had nothing before. Is seems to point to his T stat wire in the wall is an issue.

More as this unfolds.

John

[RandyJ55]

Hi John, I haven't tested the blue wires to see which one as positive current. I wonder then if it makes a difference which thermostat wire is hooked to the positive blue wire? I cannot reach that plug from which the pigtails emerge, but I can reach the wires which are longer than the ones showed in your picture. I will definitely have to check the limit switch, once located, because it does seem as if there is a power loss somewhere. After running a section of new wire wired to a thermostat, the thermostat does not work as it should. I will have to do some more testing in regard to both issues I described. Fortunately, all of this work can be done inside. I'm going to stay engaged with this issue. One last thing I wanted to mention is the fuse board in the parallax converter. Typically, when I pull out the fuses to check them, a light comes on for each fuse, indicating that individual fuse is out or blown. Something has changed and now only a few of them light up and not all of them across this green circuit board on the right side of the panel box. I will check out those items you referenced and let you know what I found. Thanks for your assistance. I believe I'm closer to finding the cause.

[JohnB]

Originally Posted by RandyJ55

Hi John, I haven't tested the blue wires to see which one as positive current. I wonder then if it makes a difference which thermostat wire is hooked to the positive blue wire? I cannot reach that plug from which the pigtails emerge, but I can reach the wires which are longer than the ones showed in your picture.

Not sure which T stat model you have. Can you post a pic and model number? This much I can say, if the T stat only has 2 wires, room for the 2 blue wires, then odds are there is no polarity difference.

It would look like this, this most likely is not yours


But, if there is a DC common, ground (-) wire on the T stat, then the T stat may use some of the 12 volt power to run the T stat. If so, then there could be a polarity issue. I have seen many of the Atwood 2 wire only T stats where polarity does not matter, but that does not mean yours is like that as there was some interlock that was unhooked from the AC unit.

Originally Posted by RandyJ55

I will definitely have to check the limit switch, once located, because it does seem as if there is a power loss somewhere. After running a section of new wire wired to a thermostat, the thermostat does not work as it should. I will have to do some more testing in regard to both issues I described.

The hi temp limit switch ( a normally closed switch) is buried deep inside the furnace, you cannot even see it short of taking the motor housing apart. BUT, there is a sail switch, (a normally open switch buried inside the blower housing) also in series with the hi temp limit switch that has to have the fan running at proper speed to trip the sail switch that the motor is running at speed. When the motor runs at the proper voltage, the power flows through the hi limit closed switch, then through the sail switch closed by wind, then onto the gas & ignition control board to fire up the gas valve and the igniter. This all comes back to, the motor has to run at speed, so testing wires down stream of the sail switch/hi limit will have no power until the fan runs. While you cannot get to the hi temp switch & sail switch, you can get to the power wiring going to the hi temp switch to see if power is going to it, and you can test the wire coming out of the sail switch heading to the PC board to see if power is coming out when the fan runs if needed. If you get the fan running but no gas valve/ignition, then you have to sort out the sail switch and hi temp limit and the ignitor PC board.

If you want to see the hi temp limit switch, this link is to a furnace rebuild. This 8500 rec IV series is newer then yours, there is no timing relay and the PC board controls the fan motor and the ignition where yours is separate, but the hi temp limit and the sail switch is the same concept. https://www.sunlineclub.com/forums/f...tml#post154909

Originally Posted by RandyJ55

Fortunately, all of this work can be done inside. I'm going to stay engaged with this issue. One last thing I wanted to mention is the fuse board in the parallax converter. Typically, when I pull out the fuses to check them, a light comes on for each fuse, indicating that individual fuse is out or blown. Something has changed and now only a few of them light up and not all of them across this green circuit board on the right side of the panel box. I will check out those items you referenced and let you know what I found. Thanks for your assistance. I believe I'm closer to finding the cause.

When you get into the Parallax converter issue, create a new post with pics and the model number. While I have helped change out the power converter piece with a replacement on the parallax, I have not had to deal with the PC board on the fuse lights. We have others here on the forum who have and may see your converter post and chime in. As long as power is still coming out of the fuses, even if the lights do not work for blown fuse, you still have something that works to get you going.

Hope this helps

John

[RandyJ55]

Hi John,
Yesterday, I made a couple of discoveries. First, after touching the two blue wires to start the furnace I tried wiring the thermostat with the new wire I purchased. The thermostat would not work. I began to examine the thermostat and discovered that at sometime my brother must have bent the metal strip for the system on and own switch on the thermostat which kept it from making contact with the metal piece below when turned on. Once I re-bent the metal piece the thermostat worked normally. Next , my brother went out and used a compressor and wand to blow out any dust and debris in the motor and wiring harness section of the furnace. Afterwards, I ran a few cycles with the furnace and the furnace works at start up and after 20 seconds the heat comes on and then the cool cycle. I still have some issues to work out with the circuit board but it got down pretty cool last night and it was good that my brother had heat. Thank you for your assistance of making me aware of some components and tests that I wasn't aware of and especially locating and testing the furnace with touching the blue wires together. I will recheck all connections and be sure to make sure the outside door is sealed against moisture. Thanks again for your insight and help.

[JohnB]

Originally Posted by RandyJ55

snip...

Afterwards, I ran a few cycles with the furnace and the furnace works at start up and after 20 seconds the heat comes on and then the cool cycle. I still have some issues to work out with the circuit board but it got down pretty cool last night and it was good that my brother had heat.

Hi Randy,

You made great progress! Good for you, and thanks for reporting back.

Not sure what PC board issue you are having, but we are here if needed.

Stay warm!

John