Suburban SF 30 Furnace Troubleshooting and Repair

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Suburban SF 30 Furnace Troubleshooting and Repair

August 21, 2018, 04:26:05 pm

My furnace quit on the last trip. I thought it might be the sail switch, a common failure, and replaced it not knowing that the blower had to be functioning (mine wasn't) for the sail switch to work. I have an extra sail switch now. The digital thermostat would click when turned on and I could hear a click at the furnace. This told me the thermostat was working and was the end of my diagnostic expertise. Dan, the mobile RV tech came out and quickly determined it was the circuit board, which he replaced. He determined this using his multimeter, following the 12v current to where it stopped - at the board. Now that I have seen the board replaced, I could do it myself. It's quite ease to unscrew the silver holding bracket and pull it out from the side of the furnace.

I attached a photo of Dan at work and of the failed circuit board. Dan said it was probably a solder failure at the black arrow. This could likely be easily repaired, so I have a repairable spare circuit board now.

As a side note, Dan, who has been repairing RVs for nearly 40 years, said he always uses original manufacturer's parts where possible. He does this so that if the equipment fails, the manufacturer has liability if the part fails and causes damage to the rest of the equipment, e.g., the new circuit board starts a fire in the LD.

Chris

Re: Suburban Furnace Troubleshooting and Repair

Reply #1 – August 21, 2018, 04:39:16 pm

Does the furnace have to be turned on to check the board?

Jim

Re: Suburban Furnace Troubleshooting and Repair

Reply #2 – August 21, 2018, 04:52:16 pm

Chris,

By the looks of the solder failure, it makes me wonder what caused it to fail. Old age, vibration while traveling, overheating...?

It would be interesting to repair the original board and reinstall it to test its road worthiness.

You have to wonder sometimes what causes these errors. https://youtu.be/CD9YqdWwwdw

Kent

Re: Suburban Furnace Troubleshooting and Repair

Reply #3 – August 21, 2018, 05:55:37 pm

Probably not soldered correctly in the first place, would be my guess. At least it looks easy to repair. RonB

Re: Suburban Furnace Troubleshooting and Repair

Reply #4 – August 21, 2018, 06:30:43 pm

"Dan said it was probably a solder failure at the black arrow."

An Ohmmeter would confirm that diagnosis and indicate the use of a low wattage iron, a solder sucker and some solder wick. Careful soldering should then restore the board (or not).

Re: Suburban Furnace Troubleshooting and Repair

Reply #5 – August 21, 2018, 07:37:35 pm

Quote from: Jim & Gayle - August 21, 2018, 04:39:16 pm

Does the furnace have to be turned on to check the board?

Jim

Yes. As soon as he connected the new board it fired up.

One note: In the photo of the furnace, note that the exterior cover is hanging from either the exhaust tube (top) or the intake tube (lower) with one of the tubes on the cover inserted. I don't remember which. When the furnace came on, I started to pull it off (out) and Dan said not to because it would cause a recirculation problem and cause damage. I don't remember exactly what he meant.

Chris

Re: Suburban Furnace Troubleshooting and Repair

Reply #6 – August 21, 2018, 07:51:03 pm

In the photo showing how the wiring is connected, there is a rocker switch attached to the board.
What is the switch for?
There are two versions of the furnace, those that access the interior by removing the exterior panel that has the intake and exhaust ports and models that are accessed from inside the rig.

Larry

Re: Suburban Furnace Troubleshooting and Repair

Reply #7 – August 21, 2018, 09:23:43 pm

Quote from: Larry W - August 21, 2018, 07:51:03 pm

In the photo showing how the wiring is connected, there is a rocker switch attached to the board.
What is the switch for?
There are two versions of the furnace, those that access the interior by removing the exterior panel that has the intake and exhaust ports and models that are accessed from inside the rig.

Larry

The rocker switch (on/off) is attached to the plate (not the board) to which the module board is attached. The plate is part of the curved silver piece that screws into the 2 empty holes on the fan shroud. The current from the thermostat flows through the on/off switch to the control module board. Access is is from exterior per the photo.

Interestingly, if you look at the module board, most of the board and nearly all of the soldered circuits are covered with a shiny transparent coating/glaze. The circuit that appears to have failed is not. Wonder if that means anything?

Chris

Re: Suburban Furnace Troubleshooting and Repair

Reply #8 – August 22, 2018, 01:13:13 am

Quote from: Chris Horst - August 21, 2018, 09:23:43 pm

Interestingly, if you look at the module board, most of the board and nearly all of the soldered circuits are covered with a shiny transparent coating/glaze. The circuit that appears to have failed is not. Wonder if that means anything?

I'm guessing the shiny stuff is flux. As Ron said, the failed joint was probably not soldered correctly in the first place.

Re: Suburban Furnace Troubleshooting and Repair

Reply #9 – August 22, 2018, 01:39:06 am

Quote from: Chris Horst - August 21, 2018, 04:26:05 pm

I attached a photo of Dan at work and of the failed circuit board. Dan said it was probably a solder failure at the black arrow. This could likely be easily repaired, so I have a repairable spare circuit board now

Chris, this looks from the square cross-section of the joint insert that it is a post where a wire connector is pushed on. This is a common failure mode of this sort of connector post, a situation I have run into innumerable times over my career. Physical stresses on the connection cause the solder joint to fail, and arcing then occurs, causing the black ring on the failed solder joint. The problem is the solid post. A less stressful joint would be if a wire were soldered directly there. The wire could then be cut and male and female connectors crimped on to allow easy removal.

Steve

Re: Suburban Furnace Troubleshooting and Repair

Reply #10 – August 22, 2018, 02:56:43 am

Hi Steve, a keen eye indeed!. That film is called solder mask applied with a silk screen technique. It protects the circuitry from being coated during 'wave' soldering. The lighter colored areas where the solder connections are, are holes in the mask. At first I thought this board was sloppy enough to be hand soldered, but the blobs of solder on the date code show that it was wave soldered. Holes in that mask allowed the heavy duty conductors to have solder applied, leading to that pin. This enhanced the ability of the printed wires to carry high current. Probably to the fan circuit. A better quality board would have that square post be staked or swaged into the board having the look of a rivet from this side. A better mechanical connection to the board. The post that is there has a high current density right at its base. This , over time, causes 'solder joint thermal fatigue'. A difference in thermal mass between the post and copper circuit trace, causes the solder to crystalize (I'll call it that) and it gets brittle, causes it to not conduct as well as it should, and fail faster. Source material I read up on, thinks that a better alloy of solder, would help this situation. (I did have to read up on this problem). The good news, you should be able to resolder that one joint, and have a good board in reserve. RonB

Re: Suburban Furnace Troubleshooting and Repair

Reply #11 – August 22, 2018, 10:08:14 am

"This enhanced the ability of the printed wires to carry high current."

Notice also that the B+ and ground traces are wider = higher capacity. Engineering a printed circuit board is an art form.

Re: Suburban Furnace Troubleshooting and Repair

Reply #12 – August 22, 2018, 10:55:38 am

Since I do not have an engineering background, I have to read Ron's, Steve's (2), Larry's, Art's, and the many other engineers' posts twice, squinting, and running my finger under the words in the sentences, and looking up unfamiliar terms while trying to visualize the "parts", equipment, tools, and whatnot involved. Following this MO, I can now usually manage to attain about a 60%-75% comprehension rate!

So, thanks! We're very lucky to have such high levels of expertise and experience on the board; I have learned a great deal over the years!

Re: Suburban Furnace Troubleshooting and Repair

Reply #13 – August 22, 2018, 11:47:20 am

Quote from: Chris Horst - August 21, 2018, 07:37:35 pm

Yes. As soon as he connected the new board it fired up.

One note: In the photo of the furnace, note that the exterior cover is hanging from either the exhaust tube (top) or the intake tube (lower) with one of the tubes on the cover inserted. I don't remember which. When the furnace came on, I started to pull it off (out) and Dan said not to because it would cause a recirculation problem and cause damage. I don't remember exactly what he meant.

Chris

Thanks to all who commented. This has been a learning experience for me and a valuable troubleshooting procedure for folks.

Jim, one more reason to have the furnace on when troubleshooting is that the first thing Dan did after removing the exterior cover and having access to the wiring bundle was to go to the blue wire and check for current. This is the wire to the thermostat and he wanted to make sure it worked before going any farther.

This is link to the manual and has contains detailed troubleshooting steps. Suburban RV FURNACES SERVICE MANUAL | Thermostat | Ignition System

Chris

Re: Suburban SF 30 Furnace Troubleshooting and Repair

Reply #14 – August 22, 2018, 01:07:16 pm

Quote from: Chris Horst - August 21, 2018, 04:26:05 pm

My furnace quit on the last trip. I thought it might be the sail switch, a common failure, and replaced it not knowing that the blower had to be functioning (mine wasn't) for the sail switch to work. I have an extra sail switch now. The digital thermostat would click when turned on and I could hear a click at the furnace. This told me the thermostat was working and was the end of my diagnostic expertise. Dan, the mobile RV tech came out and quickly determined it was the circuit board, which he replaced. He determined this using his multimeter, following the 12v current to where it stopped - at the board. Now that I have seen the board replaced, I could do it myself. It's quite ease to unscrew the silver holding bracket and pull it out from the side of the furnace.

I attached a photo of Dan at work and of the failed circuit board. Dan said it was probably a solder failure at the black arrow. This could likely be easily repaired, so I have a repairable spare circuit board now.

As a side note, Dan, who has been repairing RVs for nearly 40 years, said he always uses original manufacturer's parts where possible. He does this so that if the equipment fails, the manufacturer has liability if the part fails and causes damage to the rest of the equipment, e.g., the new circuit board starts a fire in the LD.

Chris

Since some of us are enjoying this thread, I added 3 more photos (scroll up), 2 of which are closeups of the failed (we think) connector post and one of the business side of the module. The white spot on the black relay identifies where the failed connector post enters the relay underneath from the other side. The bad post also connected to the power spade circuitry on the other side.

Chris

Re: Suburban SF 30 Furnace Troubleshooting and Repair

Reply #15 – August 22, 2018, 02:46:51 pm

Quote from: Chris Horst - August 22, 2018, 01:07:16 pm

Since some of us are enjoying this thread, I added 3 more photos (scroll up), 2 of which are closeups of the failed (we think) connector post and one of the business side of the module. The white spot on the black relay identifies where the failed connector post enters the relay underneath from the other side. The bad post also connected to the power spade circuitry on the other side.

Looks like Steve nailed this one, based on his years of experience in electronic repairs.

Larry

Re: Suburban SF 30 Furnace Troubleshooting and Repair

Reply #16 – August 22, 2018, 06:52:28 pm

Quote from: Lazy Bones - August 22, 2018, 10:08:14 am

"This enhanced the ability of the printed wires to carry high current."

Notice also that the B+ and ground traces are wider = higher capacity. Engineering a printed circuit board is an art form.

Designing printed circuit boards was my profession (over 30 years of experience) What Ron B stated is true. Whoever designed that board did a good job. When I started designing I used black tape and stick on donut holes. Later in my career we started using computers. These were really needed as technology made electronics smaller and smaller and more compact. Imagine trying to design a printed circuit board from todays cell phones without a computer. Well I am glad I am retired and don't have to deal with technology anymore. Glad I still have some memories that I can bring to the front of my brain. Thanks to all for the refresher! ☺

Re: Suburban SF 30 Furnace Troubleshooting and Repair

Reply #17 – August 22, 2018, 11:22:34 pm

"Entertaining" is following detailed troubleshooting thread on a problem on somebody else's rig,
Thanks to all for the info.
joel

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