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Old 11-19-2024, 04:29 PM   #1
CraigB in FL
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Join Date: Jul 2021
Location: Niceville
Posts: 51
M.O.C. #29201
Atwood 8535-IV DCLP Problems

On our last trip, the furnace quit working. The fan didn't run, so it wouldn't ignite. Being late at night we opted to just use our electric space heater. The next day, when I turned the furnace on to check things out, it worked as normal and continued working for several more days. Then, the night before we got home, it quit again.

Initial troubleshooting: When thermostat cycles it on the fan is not starting. There's a buzz/clicking sound at the furnace and then everything quits with a Fault Code indicating "limit switch or airflow problems". Checked the sail switch with the VoM & it's good.

From the parts diagrams, the limit switch appears to require a significant amount of teardown to access it. Is there an easier way to test it?

On the other hand, if the fan isn't blowing, has it progressed to the point where the limit switch will come into play?

New development: I reassembled everything after checking the sail switch. When I cycled the furnace back on to reconfirm the fault codes, it now produces as slow continuous blinking. I don't find any explanation for that in any of the literature.

Any thoughts and recommendations are appreciated.
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Old 11-19-2024, 05:53 PM   #2
ChuckS
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M.O.C. #20949
When 12 VDC is applied to the furnace control board via the signal wires from thermostat the First thing that Must occur is blower motor spin up..

The high limit switch and sail switch are not in play UNTIL AFTER blower motor spins up, the air from the blower motor then trips the sail switch “Closed” amd then signal is passed to the high limit switch

When the signal passes through the high limit switch it is routed back to furance control board to signal the board to do the next two steps..

Which are LP solenoids open to allow LP to flow and ignitor fires to ignite the LP mix.

Your issue is with either an intermittent 12 VDC to the furnace control board, a bad delay timer on the board or a bad blower motor
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Old 11-19-2024, 05:58 PM   #3
ChuckS
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M.O.C. #20949
This short PDF might be useful regarding blower motor checks
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File Type: pdf Atwood and Suburban Blower Motor Power Test.pdf (630.2 KB, 39 views)
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Old 02-04-2025, 09:13 AM   #4
Cline88
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Chuck , I am having a similar issue with my 8535 IV furnace,
furnace worked, furnace then failed to turn on blower, then blower worked became noisy, failed to work at speed needed, replaced circuit board before noticing blower was failing, blower motor and furnace burner were then replaced ( burner was rusted through, electrode is functioning and not replaced) everything was working heat coming from all registers, issue now is -- set temp to 70 ,,,furnace gets to 70 then stops ,,,temp drops ,,,furnace does not cycle back on alone must be manually turned off and on ( I can hear the click of the thermostat in my AC area which is followed by ignition) circuit board will sometimes code " internal control failure" do I need to replace the thermostat over head or on the wall now....any thoughts are appreciated!!! Just need to make it through this winter. ( I know nothing about furnaces , youtube learner here)
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Old 02-04-2025, 10:00 AM   #5
ChuckS
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Quote:
Originally Posted by Cline88 View Post
Chuck , I am having a similar issue with my 8535 IV furnace,
furnace worked, furnace then failed to turn on blower, then blower worked became noisy, failed to work at speed needed, replaced circuit board before noticing blower was failing, blower motor and furnace burner were then replaced ( burner was rusted through, electrode is functioning and not replaced) everything was working heat coming from all registers, issue now is -- set temp to 70 ,,,furnace gets to 70 then stops ,,,temp drops ,,,furnace does not cycle back on alone must be manually turned off and on ( I can hear the click of the thermostat in my AC area which is followed by ignition) circuit board will sometimes code " internal control failure" do I need to replace the thermostat over head or on the wall now....any thoughts are appreciated!!! Just need to make it through this winter. ( I know nothing about furnaces , youtube learner here)
What is the Age of the RV? The "Click" in the AC area is a relay that is energizing and sends the "SIGNAL" from thermostat to furnace control board to Turn On.

You've got a new control board and blower motor that has resolved previous issues... Now you have a "Signal to Turn On" that is intermittent..

It could be a faulty thermostat or if the thermostat routes its signal up to the roof AC there is a "Gateway" board that routes the signals sent to it from thermostat to AC unit as well as furnace...

I would "RESEAT" all connections on that gateway as well as the TWO SIGNAL wire connections located on the furnace control board and then evaluate operation..

It appears the thermostat "is sending" the signal to the gateway to turn the furnace ON ( hence the click sound of the relay you hear overhead )
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Old 02-04-2025, 10:15 AM   #6
ChuckS
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Checked your profile and it appears you have a 2015 Montana..

That year used one of two style thermostats...

Either the Dometic CCC2 multizone thermostat or the Dometic SZ single zone thermostat..

The CCC2 thermostat sends and receives its signals from the Zone 1 AC unit ( most likely Dometic Blizzard II..

When you remove the outside cover off this unit there is a control board mounted inside and that is what controls the signals from the CCC2 to either the AC unit or the furnace..

The click heard is a relay on that board that appears to be energizing but the signal is not reaching the furnace to tell it to run UNITL you turn the thermostat OFF and back ON.. This tells me the control board is failing..

*** Info provided based on my assumption you have the CCC2 thermostat unit

** THe circuit board will be mounted in a larger metal colored box inside the AC area..

** Caution - before doing any work in this area remove shore power and turn of the 12 VDC coach batteries

You will need to identify the PN of the CCC2 control board used in your unit and order a replacement IF AFTER CHECKING FOR LOOSE CONNECTIONS FAILS TO RESTORE NORMAL OPERATION
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Old 02-05-2025, 08:52 AM   #7
Cline88
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Thank you ,

ChuckS / Montana forum . Thank you kindly for your response. I did a little search and found I could reset the thermostat on the wall . I really only did this because it has blinked E4 since it was installed 5 years ago and it just bugged me. This actually solved my problem like maybe all the new parts were not familiar with each other now it is working fantastic cycling on and off as it should. Thanks again for all time invested in your response !
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Old 02-05-2025, 10:51 AM   #8
ChuckS
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When you did the CCC2 thermostat reset what that does is "clears out" all previous Zones that were detected on the CANBUS ..

Then the CCC2 goes out and establishes a new CANBUS connection to the devices it finds in the loop..

In your case Zone 1 ( AC and also control for furnace ) as well as Zone 2 ( usually just an AC )....

Glad all is working well...
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