Help with Suburban SF-35 furnace.

[House Husband]

On occasion my furnace will lock out and not refire until I remove the furnace fuse for 5-10 seconds and reinstall. The sail switch is functional. The thermostat is sending the proper signal on demand. It does have a Dinosaur control board. All else is OEM.

Thanks for your thoughts.
Richard

 

[Isaac-1]

I suspect it may be the flame sensor, I think the Dinosaur board allows 3 attempts at ignition before locking out retries.

 

[Kirk]

My first thought is the same as Isaac. The circuit boards from Dinosaur work exactly the same as the original from Suburban. I suggest that before you do too much you download this Suburban Service Manual and use it to continue. It is quite common for the ignition probe to fail intermittently as it reaches late life. That probe not only supplies the spark to ignite the propane but it also detects the heat of the burning propane to tell the circuit board that it is burning. Either the lack of a spark or the failure to detect the heat can cause the same symptoms that you are experiencing. It is a common problem.

 

[Gary RV_Wizard]

I think the question being asked is why is it necessary to remove the fuse to reset it. "Normal" operation would be to simply turn the thermostat off for several seconds and them back on again. It should not be required to remove power to the board by interrupting the DC power source.

A failing flame sensor would trigger the shutdown but should have no effect on the reset action.

 

[John From Detroit]

On occasion my furnace will lock out and not refire until I remove the furnace fuse for 5-10 seconds and reinstall. The sail switch is functional. The thermostat is sending the proper signal on demand. It does have a Dinosaur control board. All else is OEM.

Thanks for your thoughts.
Richard

Somewhere there is a text document (forum files section) that describes in time order how the furnace works from the T-Stat calling for heat to shut down.

It is important to know what is and is not happening
If the T-stat calls for heat the blower starts blowing
Then the sail switch closes and the gas valve opens and the sparker starts sparking
Then ignition happens and the thermocouple (Flame sensor) Warms up and sends current down the wire.. The Control board detects the current and says" AH Working" and you get warm.

There are many things that can cause lock out
From your description Ill skip to the Ignition.. Does it happen all the time or not?
If not check the spark gap. Might be too big (Seen it in two out of Two furnaces I've worked on on RV one Domestic)

IF it ignites. Good burn or pathetic? (Clogged nozzle)

Ok got a good burn but it still shuts off re-tries (total 3 tries) and locks out.


Possible the flame sensor (Thermocouple) is bad but if it looks good rather good chance it is good... I do know how to test but not comfortable telling you how.

Flame sensor chip on the board.. Some systems use a "Single wire" ignition system the same wire that carries oh about 1,000 volts AC to spark cares about (yes folks I know I said ABOUT) 1/2 volt DC the other way. A switch on the main board swiches from Spark to Sense.. What happens if the swich over hits the poor chip with 1414 volts (The peak of 1000 AC) .. POP goes the chip.

Doinosaur boards inluded one part not on my DOmetic board.. A Gas Discharge Tube..
From it's location I suspect it is there to protect that sensor chip.. Works too.

 

[House Husband]

I took the furnace out and inspected the ignitor. It was in good shape and located properly. I did find a small mud dobber nest in the combustion fan housing. I wouldn't think the small nest would cause a lockout issue. We shall see.
Also, I recently hook a manometer to the propane system and adjusted the regulator from 9.5" to 11" of water column. I suspect the spring in my 29 year old pressure regulator has lost some tension.

Isaac-1, thanks for your input.

Garry, the Dinosaur board has added features that will cause lock out, that will only be reset buy removing 12 volt power. That's why I included the Dinosaur board in my original post.

Thanks KIRK for the link, I'll get a head full of it.

John, thanks for you thoughts.

Richard

 

[CharlesinGA]

I am in an area where mud daubers are very common, my shop is a prime place for them to invade. In the summer its terrible when I leave the doors open they build in toolboxes and elsewhere. I have bug screens on everything on my trailer.

I uploaded a PDF on bug screens some time ago. Follow the link and find the red Download button in the upper right of the screen. You may find it of some help.

Charles

 

[Kirk]

I took the furnace out and inspected the ignitor. It was in good shape and located properly.

In my experience, looking at the ignitor probe does not mean that it isn't the problem. A new ignitor from Amazon is less than $20 and they are by far the most common failing part in an RV furnace or water heater.

 

[House Husband]

Yeah! I finally found the problem.
The connector that plugs into the control board was the issue.
The connector has curved connectors that slide on to the circuit board.. Over 29 years the connectors had lost some of their curve, causing an intermittent connection. I carry a denial pick that I used to recurve the connectors. The same thing happens in older water heaters.

Richard