Troubleshooting Atwood 8531-IV DCLP furnace (fixed!)

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Back2PA

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Helping someone in my campground troubleshoot his furnace - thermostat brings fan on but no audible spark and no heat. I have Gary's troubleshooting guide from the library which I will be using, but my first questions are:


1) since the sail switch is inside the housing for this model, is it accessible without removing the furnace?
2) if the sail switch is not accessible while the furnace is installed, are the sail switch wires accessible for the purpose of shorting them to confirm if the sail switch is suspect?
 

Henry J Fate

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Here is the service manual for the unit.

https://www.google.com/url?sa=t&source=web&rct=j&url=http://bryantrv.com/docs2/docs/hflamefurn04.pdf&ved=2ahUKEwj_ybnxr6XkAhUCmlkKHcIDDbQQFjABegQIBBAB&usg=AOvVaw0f90jb1oMm1WoxBiPkulLi&cshid=1566988929024

Removing the heater is the best option. You may be able to access some areas of the installed heater but generally removal should be expected.
 

Back2PA

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John From Detroit

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Thought accessing the sail switch may be an issue you can access the leads that connrct to it at the control board connector and measure voltages relative to ground.  Thus you can test it without even seeing it.

Alas you will need the very fine manual to figure out which two pins feed the switch
 

Gary RV_Wizard

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I agree with John that you should be able to jumper the terminals to simulate closing the switch. Basically all the sail switch does is close the circuit that powers the controller board, so it should be practical once you identify the terminals.

The other good possibility is to reach in with a home-made bent wire hook and push or pull the sail switch closed. You don't have to actually get your hands on it to test. Replacing it may be more challenging.
 

Back2PA

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Thanks. Is it your experience that in order of likeliness the sail switch being the culprit is near the top of the list?
 

Gary RV_Wizard

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Despite internet wisdom to the contrary, sail switches don't often fail. However, with these symptoms (fan but no spark), the switch and its wire to the controller are probably #1.  First, though, verify that there is in fact no attempt to ignite (gas valve open and DSI spark).  Inexperienced observers are often wrong about that, so double-check.  If the controller has initiated the ignition cycle, then the sail switch has done its thing and you have to look elsewhere.
 

NY_Dutch

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One simple, but often overlooked, test is to simply unplug the ignitor board connector and plug it back in 2 or 3 times to clean the contacts. If that gets the furnace going, then remove the board and clean the contacts completely with a pencil eraser. I don't know how many RV furnaces I've "repaired" for folks over the years with this simple fix, but it's been more than I've actually had to replace parts to fix.


I should add that jumping the white and blue wires at the plug bypasses the sail switch and high limit switch.
 

Back2PA

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NY_Dutch said:
One simple, but often overlooked, test is to simply unplug the ignitor board connector and plug it back in 2 or 3 times to clean the contacts. If that gets the furnace going, then remove the board and clean the contacts completely with a pencil eraser. I don't know how many RV furnaces I've "repaired" for folks over the years with this simple fix, but it's been more than I've actually had to replace parts to fix.
I should add that jumping the white and blue wires at the plug bypasses the sail switch and high limit switch.


Good suggestions Dutch thanks.  :)Looks like I'll be working on it next week
 

Back2PA

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Update: I had some time today to play with his furnace. Unfortunately the ignitor board connector miracle fix didn't work.  :( Unplugged/plugged several times, cleaned with eraser and also used contact cleaner.

Fan comes on, about 10 seconds later I can hear gas valve solenoid click (and confirmed 12V to solenoid). I can hear ignitor ticking, and 99% sure I'm getting a slight whiff of LP out the exhaust. Unit goes through its three tries, does not light.

As I understand the troubleshooting guide, this eliminates the sail switch and high limit switch. I can hear the ignitor, but could be weak (I just had a bad ignitor on my water heater and although I did have a spark, it was weak and was only lighting the water heater about 50% of the time). I guess we don't know if the gas valve is opening all the way - I just went back to the service manual and they talk about checking continuity on both coils on the solenoid so that will be next. Also wondering about the orifice.

Good flame at stove and water heater.
 

Alfa38User

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Check for spider webs etc. in the gas path. They have a nasty habit of dispersing the gas so that it will not light even with a good spark.
 

Back2PA

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Alfa38User said:
Check for spider webs etc. in the gas path. They have a nasty habit of dispersing the gas so that it will not light even with a good spark.

I was editing when you replied. Yes, I'm thinking that as well. If the gas valve checks out tomorrow, will probably be down to pulling the unit. Owner says at that point he's planning on replacing the ignitor and orifice.
 

Henry J Fate

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Probably a bad ignitor. Very common for those to fail either in ignition or flame verification. It does both functions. However, the most common failure is flame verification so it still leaves the door open for a gas problem. You can light the flame manually if you can get at it but be careful and don't try to manually light it if the ignition cycle has more than one try as gas will build up and you risk the possibility of a pretty good flash.


Also.....If it is an ignition problem, the board is responsible for sending the voltage needed to create the spark. It could be a bad board.

If you are able and have the ability to pull the unit and get it on a bench with power and gas, troubleshooting becomes much easier.

 

John From Detroit

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Gary RV_Wizard said:
I agree with John that you should be able to jumper the terminals to simulate closing the switch.

Though I do agree with that Gary.. I said "Measure the voltage on the two switch wires"  One should be 12 volt the other should be too once the blower is up to speed.

But yes jumpering may work as a test as well.
 

Old_Crow

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If you end up needing a gas valve for the 8531-IV PM me. 
I've got a brand new in the box one that I ended up not needing.  Just taking up space and I'd send it to you.
 

Back2PA

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Old_Crow said:
If you end up needing a gas valve for the 8531-IV PM me. 
I've got a brand new in the box one that I ended up not needing.  Just taking up space and I'd send it to you.


Thanks Wally. I'll be checking the valve on Monday and we'll see what happens.


I'm thinking igniter and/or orifice. When I spoke to Dinosaur regarding the board on my water heater, they said that for a board to put out low ignition voltage in most cases you'd need low voltage to the board. I'll be checking that as well
 

Back2PA

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Update: tested impedance on both gas valve solenoids, both came in around 40 ohms, spec is 30-50. Parts arrived so I pulled the burner/ignition assembly - Houston, we found the problem(s). Burned out burner assembly, corroded igniter/electrode and bug in the orifice (see attached). Replaced the electrode and orifice, hadn't considered that I'd need a burner assembly so had to order, should be back together for a test run Thursday. LP line capped with 3/8" flare plug so he can still use the stove
 

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Back2PA

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And the final chapter. Burner assembly arrived today. Reinstalled components including new ignitor (electrode), burner assembly, and orifice. Fired up the furnace, flame lit on first click of ignitor. Success!
 
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