Suburban sf-35f furnace not working

[Jessicawhetsell]

Last night our furnace decided to stop working, I went in the storage where it was at and spun the fan a few times and it kicked back on. It stayed of for a minute or 2 then shut back off. When it's on it will blow out heat but it just won't stay on. Everytime I go spin it the time it says on gets shorter and shorter. Fuses and breakers are good here in the house so I'm not to sure what to check.

[Toddupton]

Are you on shore power or running on battery only?

[cavie]

The furnace is not a DIY item. Unless you are a very good handyman. There are multiple things the need checking. First is the Sail switch. Unit must have good air flow for gas valve to stay open. igniter must work. Gas pressure must be correct. Control board must be checked. Can't get to the control board with the unit installed. It requires a special board tester. Must be done on a work bench. That requires a 12 volt electric supply and a gas supply.

[Jessicawhetsell]

Shore power

Cavie I definitely understand what you're saying. By no means am I a pro at this or think I know it all. Its Saturday (NO ONE IS OPEN) in Pennsylvania and it's cold so I was just asking to see if there's anything I could personally do to try to get it going.

[Mr. Dan]

Jessica, what method did you use to spin your fan? I would have to do some serious down-on-the-floor contortions to get to my fan to spin it.

[Jessicawhetsell]

I stuck my hand in there and spun it and yeah it wasn't easy lol

[Larry-NC]

Originally Posted by Jessicawhetsell

Last night our furnace decided to stop working, I went in the storage where it was at and spun the fan a few times and it kicked back on. It stayed of for a minute or 2 then shut back off. When it's on it will blow out heat but it just won't stay on. Everytime I go spin it the time it says on gets shorter and shorter. Fuses and breakers are good here in the house so I'm not to sure what to check.

If it shuts off, it's not the high limit switch. If it were, the fan would keep running but the flame would be off and you would feel cold air.

When it shuts off, does it restart by just adjusting the thermostat, or do you have to switch it from Heat to Off and back? It is possible that there's a lot of dust (or mud dauber nests) in the air passages and there's enough airflow to start but then things "flap" into the way and impede air passage. Airflow is measured by the Sail Switch. It's in the air path and turns on if there's sufficient airflow.

Here are three things you can try.

1. Get into the furnace area and vacuum the heck out of it. Get rid of dust, cobwebs, pet hair, etc.
2. Go outside of where the furnace is. You will see two vents, one above the other, each about 2-3 inches in diameter. Mud dauber bees love to build nests in these vents which restricts airflow. The best thing to do is to take this vent assembly out, clean it, and replace it using fresh butyl tape to keep water out. Instead, you may be able to clean it by poking in with a dowel or straightened metal coat hanger. Use something strong--the dried mud is almost like concrete. Maybe use flashlight. Vacuum when finished.
3. Make sure there is no furniture, area rugs, boxes, etc. across ANY of the floor vents.

Write back and tell us whether this works.

Note to Cavie: These things are all safe for a skilled amateur to do.

Larry

[Jessicawhetsell]

Hey Larry thanks for the info! No blockage in the vents I pulled it out and have it setting on my tailgate now cleaning everything real good the motor has some bumps when turning it and the sail switch has no resistance so I'm guessing that is messed up too.

[Larry-NC]

Originally Posted by Jessicawhetsell

Hey Larry thanks for the info! No blockage in the vents I pulled it out and have it setting on my tailgate now cleaning everything real good the motor has some bumps when turning it.

This is a brush-type motor. It is normal to feel some drag when spinning it.
A brush-type motor (also called AC-DC or universal motor) has two graphite brushes which conduct electricity to the spinning rotor. Ever look through the vent of your electric drill while it's running?

Suburban service manuals say "Do not lubricate the motor".

I have used an old toothbrush to clean the squirrel cage blowers.

Originally Posted by Jessicawhetsell

the sail switch has no resistance so I'm guessing that is messed up too.

Jess, the sail switch SHOULD conduct (zero resistance) when it's moved to the position where the fan would blow it. It should not conduct (infinite resistance) in the resting position.

Restated:
Switch resting: infinite resistance (off, not conducting)
Switch deflected: zero resistance (on, conducting)

Of course, measure across the two terminals that have wires connected to them. Some switches have three terminals:
--Normally Open (NO)
--Normally Closed (NC)
--Common
You want to measure between Normally Open and Common.

Reassemble the unit. (Be sure the gas flare fitting is tight.) See if the problem recurs.

The big enclosure you pulled the furnace from is the "plenum." Right at the front of the enclosure, just above the cover you pulled off is a sensor with a wire coming into each end. It is called the High-Limit Switch. It cuts off the furnace if the temperature is too high, just like the high-limit switch on a clothes dryer.

The high limit switch should conduct now. If there is not enough airflow, it will open (stop conducting). So do this: When it's reassembled, leave the grille off. Run the furnace. You should have 0 volts DC across the high limit switch. If the furnace stops, measure the voltage. If it's 0 volts, the switch has not tripped. If it's 12 volts, it's tripped.

Let us know what you find.
When you're all done, be sure to put a little detergent and water solution on the flare fitting to make sure it doesn't leak.

Larry

[Jessicawhetsell]

I ordered a new motor today and sail switch just waiting for it to come in to see if that's the problem. Thank you for your help Larry!

[Larry-NC]

Originally Posted by Jessicawhetsell

I ordered a new motor today and sail switch just waiting for it to come in to see if that's the problem. Thank you for your help Larry!

Jessica, I hope that's it. Try just the sail switch first; it's easier to change.

While you are waiting, step through the troubleshooting steps on page 31 of this manual.

Also see page 28 of this manual.

These two manuals also contain a complete Sequence of Operation. Sometimes you can simply watch and listen and see how far through the process the unit gets and figure out what's wrong.

Since you obviously don't have a full test-bed on the tailgate of your truck, you might have to fasten clip leads to some of the test points and then slide the assembly into the housing. Run your test leads out the side, towards the back, same place the thermostat and power wires go in.

Larry

[rockfordroo]

Originally Posted by Larry-NC

Of course, measure across the two terminals that have wires connected to them. Some switches have three terminals:
--Normally Open (NO)
--Normally Closed (NC)
--Common
You want to measure between Normally Open and Common.

Larry

Jess - Make sure you remove at least one of the wires from the switch before you try to read it's resistance. If you don't, you will also be reading back through the two wires and into the rest of the circuitry, not just the switch.

[Jessicawhetsell]

Thank ya'll for all the help! As soon as I get my parts in I'll let ya'll know.

[Larry-NC]

Originally Posted by Jessicawhetsell

Thank ya'll for all the help! As soon as I get my parts in I'll let ya'll know.

Maybe do the troubleshooting chart before you start "easter-egging" new parts in. If one of the new parts happens to be defective, you will have two problems to solve.

I was once "advised" by someone to change the sail switch, even though the furnace went way past the part of the sequence where the sail switch is checked. The "advisor" had suggested changing it because that was the only part for which he understood the function. Needless to say, I didn't do it. Still using the original one.

Larry

[Jessicawhetsell]

We kept all the old parts and we will be doing 1 by one until it's fixed. I'm no electrician my trade is pipefitter so I'm not confident on checking that stuff

[rockfordroo]

Here's a service manual that may help you.

[Jessicawhetsell]

Thank you!

[TomnGerri]

We had the same issue the fix was corrosion on high limit switch and loose wiring. Removed and cleaned high limit switch, removed crappy wire nut and replaced with a real electrical wire nut (yellow) have not bad any other issues

[Larry-NC]

Originally Posted by Jessicawhetsell

We kept all the old parts and we will be doing 1 by one until it's fixed. I'm no electrician my trade is pipefitter so I'm not confident on checking that stuff

Jessica, are you there? Any progress?

Usually folks come back to the forum and tell us what they found--even if they are red-faced. We all learn from those experiences.

Larry

[Jessicawhetsell]

Yes Larry I'm here lol. My apologies for not getting back sooner. The brushes were bad in the motor so we replaced it. While we had it out we deep cleaned it and made sure nothing else was lose.