Suburban furnace problem

[Doctarie]

My suburban sf a 30 will fire up and run for about 1 1/2 to 2 minutes and shut down , no blower or refire attempt . I can turn thermostat to off and back still no restart , I can wait 45 minutes try again I get the same short run and shut down . I have 14 volts to ground on both sides of limit so it’s not opening , there is plenty of propane , l,m guessing control board any other ideas Or thoughts ?

[Larry-NC]

The Service Manual states:

If ignition is successful the module board will monitor the flame sense, sail switch and limit switch circuits, and the thermostat inputs during the heating period.

The flame is sensed through the spark wire and electrode. Therefore, it is essential that the electrode is properly positioned in the burner flame.

My money is on the flame sensor.

The manual is attached. Relevant pages are 24and 28.

[Doctarie]

Thanks for you reply , What puzzles me is it waits so long before it will retry , a lock out would likely need to be reset if flame sensor could be weak or malfunctioning does it send it to lock out ?

[Capacitor]

To start with, remove and clean igniter with emery paper or sandpaper and set gap to 3/16” and try again or replace it. Take a 1/4” nut driver with you, don’t bother with the flathead screwdriver.

[Doctarie]

Larry NC I don’t find the attached manual!

[Larry-NC]

Originally Posted by Doctarie

Larry NC I don’t find the attached manual!

Should be attached to this post.

[Doctarie]

Now im really frustrated, pulled furnace out replaced the igniter , flame sensor , cleaned the burner , did find wasps, wasp nest material and stink bug body’s checked all connections on control board test ran on the floor great spark , blower ran well all good . Put back together installed back in camper . Fired up ran 4 1/2 mins stoped. Dead , not satisfied thermostat, reset at thermostat nothing ,wait an hour ,fires up , runs 45 min satisfied thermostat cycles 2 other times . Something it failing but not everytime . I guess only thing left is the board . Thoughts ?

[Larry-NC]

Only other option is for electronics geeks like me. I would replace all the capacitors on the existing board first, just out of stubbornness.

[jim1]

Try a new sail switch. It worked for mine.

[Doctarie]

Also replaced the sail switch !

[GolfingDave]

Originally Posted by Larry-NC

Only other option is for electronics geeks like me. I would replace all the capacitors on the existing board first, just out of stubbornness.

After looking for any cracked solder joints on the backside.

Just out of curiosity, do you find any capacitors bad that are not bulged? That was always my sign one was bad.

[Capacitor]

Have you ruled out the thermostat?

[GolfingDave]

Originally Posted by Capacitor

Have you ruled out the thermostat?

Good idea. Just because it clicks does not mean the contacts are good.
Jumping a couple wires is too easy not to try.

[Chuck_S]

A couple of alligator clips and short length ow wire can hot-wire the thermostat to an Always ON condition.

-- Chuck

[Larry-NC]

Originally Posted by GolfingDave

After looking for any cracked solder joints on the backside.

Just out of curiosity, do you find any capacitors bad that are not bulged? That was always my sign one was bad.

Sometimes an electrolytic capacitor will get dry without bulging. Like when I was restoring a 40-year-old Fender guitar amp for my son.

On my 1992 Mitsubishi, they mounted the radial-lead (upright, both leads at one end) electrolytic capacitors onto the PC board of the Powertrain Control Module with some kind of goo (maybe RTV) to prevent vibration. The goo attacked the copper traces. To avoid buying a $700 PCM, I removed all the capacitors, cleaned off the goo, added copper wires bridging the open traces, and mounted new capacitors. Cost of parts: under $5.

[Doctarie]

I believe she’s going to get a new board , thermostat is a hunter I put in a couple years ago , still calling for heat when it quits. Furnace ran all night last night and cycled properly but can’t count on it . Thank guys for all the reply’s !

[GolfingDave]

My furnace acted up like this two falls ago, I believe mine would relight though if I shut the thermostat off and on but would not relight on its own. Blew cold air then shut down. Might have gone into lockout mode then if memory serves me correctly. That is when I would shut the thermostat off and on.

Never did figure it out but I did run my water heater on gas instead of electric for a few days and the heater ran fine after that and has been. I winter camp.

They are next to each other. Only reason I did that was another member here said that is what he did and it worked. Why???? Who knows.

I do not believe I had disconnected any propane tank prior to this either.

[Larry-NC]

Originally Posted by GolfingDave

My furnace acted up like this two falls ago, I believe mine would relight though if I shut the thermostat off and on but would not relight on its own. Blew cold air then shut down. Might have gone into lockout mode then if memory serves me correctly. That is when I would shut the thermostat off and on.

Never did figure it out but I did run my water heater on gas instead of electric for a few days and the heater ran fine after that and has been. I winter camp.

They are next to each other. Only reason I did that was another member here said that is what he did and it worked. Why???? Who knows.

I do not believe I had disconnected any propane tank prior to this either.

Easy. Mixed air and gas in the lines. You probably had gas off at the tanks before you went camping and some gas diffused out and was replaced by air. Happens to us all the time.

I frequently post here to new campers to Always run the range burners until they burn steadily before attempting to light the furnace. It takes over a minute with all burners open for them to burn clear on the Cherokee 38P. I won't even guess how many furnace lighting cycles that corresponds to.

[MR.M]

Originally Posted by GolfingDave

Good idea. Just because it clicks does not mean the contacts are good.
Jumping a couple wires is too easy not to try.

There is also a high limit cut out switch at the back to the heat exchanger it could be tripping and then resets after awhile . look at the high limit . if the sail switch was the problem it would not light , since you did everything else high limit

[GolfingDave]

Originally Posted by Larry-NC

Easy. Mixed air and gas in the lines. You probably had gas off at the tanks before you went camping and some gas diffused out and was replaced by air. Happens to us all the time.

I tried to rule that out at the time with the stove. Stove is right above the furnace and water heater. Pretty sure I was even using a propane grill too.

This was on going for a while, at least a week or two as I had time to order a thermostat and install. Troubleshooting and not finding anything. Still persisted and then cleared up after the water heater trick. Furnace would run fine for a day or two and then kick on and no heat. Erratic.

Air was my guess too but as I said I am almost positive I had not disconnected any gas lines at the time prior to this.

I did change my outdoor LP quick disconnect at one time but I am sure I would have put the two together if it was shortly before the problem. Grill is connected to it.

Not a problem since and have filled tanks numerous times and left disconnected while getting propane.