Another furnace won't light problem

[DiegoWest]

I think this is normal. If I am reading the manual and your comment correctly, the wire you checked "at the ignitor" is the brown wire that goes to the gas valve. Gas valves operate on very low voltage - they don't want anything high enough to create a spark inside the valve. On a heater with a pilot light (which this one does not have), there is a thermistor that sits in the pilot flame, and generates about 0.25 VDC when the pilot flame is good. This low voltage is what opens the gas valve. DO NOT apply 12 VDC here, or you will burn out the valve and quite possibly the module board.

Bill

Hi Bill,
Here's a picture of the spot where I measured .25 volts, Where the brown/red wire connects to the terminal. Are you saying that this is the spot which should only have .25 volts? I thought this spot is what supplies the igniter and it needs over 12 volts.

 

[DiegoWest]

Hi Bill,
Here's a picture of the spot where I measured .25 volts, Where the brown/red wire connects to the terminal. Are you saying that this is the spot which should only have .25 volts? I thought this spot is what supplies the igniter and it needs over 12 volts.

Upon further research, I'm pretty sure this is the wire that connects to the igniter. I'm reading now that this spot can go to 1,000's of volts in order to generate a spark, so it probably wasn't even prudent to try and measure it with my multimeter.

 

[Bill]

You are right - you were looking at the thick high voltage wire to the spark electrode. Say away from that.

The wiring diagram shows three normal-size wires coming from the control module board - Red (12 VDC from the limit switch), Yellow (ground), and Brown (to the gas valve). It is this latter that would run 12 VDC when it is trying to open the gas valve. It is worth checking this voltage, since it is one of the basic steps in lighting the burner.

Bill

 

[Bill]

Rob -

At this point, I am concluding that you have two issues going, and we can't proceed until you have remedied them.

Issue 1. The sail switch does not move when the fan is running. This should be a simple fix, and you might as well do it right up front, since the furnace will never do you any good until you have done so.

Issue 2. It appears that the limit switch (item 17) may be faulty, stuck open. If so, it must be replaced. Again, the furnace will never do you any good until this is corrected.

Let us know when you have taken care of these issues. At that point, we can stop shot-gunning a bunch of random measurements, and begin a more organized approach.

Be sure to read the experience of LoveToCamp in post #12 in the earlier thread, now located here.
https://www.trailmanorowners.com/forum/showthread.php?t=21678
I don't know if it is directly applicable, since it is a different furnace. But the reported missing knockout, and the resulting airflow change, might help explain the sail switch failure.

Bill

 

[Bill]

When I was testing, I did wait a minute or two holding the sail fan forward before giving up on the clicking sound. I think the air flow is probably good just because it comes out the front side real strong.

Again, if the air flow is good but you still have to move the sail manually, that is a problem you must address. By the way, are you keeping track of the fact that the furnace contains two fans? One moves room air in and out of the unit's air-heating chamber. The other moves outdoor air in and out of the unit's combustion chamber, via the outdoor vents. You can see these two fans as items 32 and 41 in that same exploded parts diagram in Partshop.com (post #9 above). The two air streams do not mix.

I'm thinking about pulling the furnace out within a few days although I'll have to have a way of reconnecting the 4 wires that I cut to reconnect them with longer lengths to do more troubleshooting.

This should not be hard. For DIY-ers like you and me, a good way to disconnect and reconnect wires is to get a bunch of insulated crimp connectors, male and female, of the size appropriate to the wires involved (probably #18), a GET A DECENT CRIMP TOOL INTENDED TO BE USED WITH THOSE CONNECTORS. As I have discovered the hard way, you cannot reliably attach a crimp connector to a wire with a pair of pliers, a pair of dikes, a screwdriver and hammer, a cold chisel, a vise, or any other method that I have tried.

Once the connectors are properly attached, most of them have a clicking retainer that keeps them from easily slipping apart. Never separate a pair of connectors by pulling on the wires. Pull only on the connector bodies, using fingers or a carefully-applied set of needle-nose pliers.

Bill

 

[DiegoWest]

Rob -

At this point, I am concluding that you have two issues going, and we can't proceed until you have remedied them.

1. The sail switch does not move when the fan is running. This should be a simple fix, and you might as well do it right up front, since the furnace will never do you any good until you have done so.

2. It appears that the limit switch (item 17) may be faulty, stuck open. If so, it must be replaced. Again, the furnace will never do you any good until this is corrected.

Let us know when you have taken care of these issues. At that point, we can stop shot-gunning a bunch of random measurements, and begin a more organized approach.

Be sure to read the experience of LoveToCamp in post #12 in the earlier thread here.
https://www.trailmanorowners.com/forum/showthread.php?t=21678
I don't know if it is directly applicable, since it is a different furnace. But the reported missing knockout, and the resulting airflow change, might help explain the sail switch failure.

Bill

Thanks Bill,

I removed the furnace from the RV and then hooked it up to an external DC power source. I was able to duplicate the issue that I was having of the fan coming on but not the heater with the furnace removed.

At this point, I could test the sail switch independently and see that it's working. It's a mechanical only switch and when you close it via manually moving the sail an ohm meter shows the resistance going from infinite to zero.

I could then see that my limit switch is also good because I get 12 volts on both sides of it when the fan turns on. I also did your suggested test of pulling the leads off and measuring the resistance and that was very close to zero. Not sure why on my previous tests it only showed voltage on one side but must be for a different reason or I just didn't test correctly.

Based on further research I learned that the clicking sound comes not from the igniter but from the control board. But I never hear any clicking. I took the igniter electrode out and it looked to be in fairly good shape but I brushed it off and then set the gap to 1/8th inch but it was already pretty close to that. Still no luck.

At this point, I was able to get to the control board, unscrew it and see that it was getting 12 volts to the board. At least at the point that the connector connects to the board.

Oh. That open red connector with the blue wire coming off of it I could see plugged in to the control board. It plugs in to a port named test so that explains that. There's very few other connectors coming in to the control board. Just a ground wire and then a brown wire going to the gas valve. That's not even needed for this testing. At this point it seems as if I have a bad control board. But I'm still pondering it. Any thoughts?

 

[DiegoWest]

Again, if the air flow is good but you still have to move the sail manually, that is a problem you must address. By the way, are you keeping track of the fact that the furnace contains two fans? One moves room air in and out of the unit's air-heating chamber. The other moves outdoor air in and out of the unit's combustion chamber, via the outdoor vents. You can see these two fans as items 32 and 41 in that same exploded parts diagram in Partshop.com (post #9 above). The two air streams do not mix.



This should not be hard. For DIY-ers like you and me, a good way to disconnect and reconnect wires is to get a bunch of insulated crimp connectors, male and female, of the size appropriate to the wires involved (probably #18), a GET A DECENT CRIMP TOOL INTENDED TO BE USED WITH THOSE CONNECTORS. As I have discovered the hard way, you cannot reliably attach a crimp connector to a wire with a pair of pliers, a pair of dikes, a screwdriver and hammer, a cold chisel, a vise, or any other method that I have tried.

Once the connectors are properly attached, most of them have a clicking retainer that keeps them from easily slipping apart. Never separate a pair of connectors by pulling on the wires. Pull only on the connector bodies, using fingers or a carefully-applied set of needle-nose pliers.

Bill

In response to the above questions. The sail switch seems to be working fine. Now that I have good visibility to it I can see that it definitely engages without me having to do anything manually.

Both fans are working fine too. The top port that goes to the exterior vent is definitely blowing out. Theres another port below that one that is sucking in also.

One other interesting fact. One end of the limit switch. The red wire is what provides 12 volts to the control board. And voltage is definitely getting to the board.

So my previous message still stands. It looks like a control board issue based on the fact that no clicking sound is coming out of it and as far as I can tell it only needs the 12 volts from the red limit switch wire to make it do it's thing and generate high voltage to the brown output wire which goes to the igniter.

 

[Wavery]

The clicking sound (quite loud) is actually coming from the ark that is created by the ignitor. However, the ignitor will not click/ark until the gas valve is open. If you don't have propane pressure going to the gas valve, the ignitor will not fire.

The process goes as follows:

*The thermostat sends power to the MB which immediately turns on the fan for several seconds to be sure that the burner chamber is free of gas.

*As the fan blows, that should open the sail switch and the sail switch sends current to the gas valve.

*If the gas valve has the correct amount of gas pressure, it will open and release gas into the burner chamber and simultaneously charge the ignitor to fire (I think 3 times X3).

If any of these parts fail or the furnace just doesn't light), the entire system will shut down for a period of time. At that point the thermostat must be turned off, (wait a few seconds) then back on to reset the MB

 

[Bill]

Rob -

That is a much more organized approach, and I'm glad you followed through on it. You've eliminated the sail switch and the limit switch, which were previously suspect.

Wavery, you pretty well outlined the process, though I will take minor exception to the second step.

"*As the fan blows, that should open the sail switch and the sail switch sends current to the gas valve."​

Actually, the schematic shows that the sail switch sends 12 VDC power (via the limit switch) to the control board via the red wire, and the control board sends 12 VDC power to the gas valve via the brown wire. EDIT: I was wrong when I previously said that the gas valve probably operates on small voltage.

So Rob, can you monitor the voltage on the brown wire when everything else seems to be fine? If proper voltage appears on the brown wire, the valve is most likely bad. If no voltage appears on the brown wire, I tend to agree with your conclusion that the control board has a fault. A replacement for either is available, but I bet it ain't cheap.

Let us know.

Bill

 

[DiegoWest]

Hi Bill and Wayne,
Good inputs. I'm not sure now what causes the clicking sound. Wayne could be correct that it's only the sparking sound but I've read it could also be the sound of the relay on the control board. I do believe that the spark should occur though even in the absence of opening the gas valve.

Per Bills suggestion, As a matter of fact, I just got done doing another test to see if the brown prong on the control board connector which goes to the gas valve got to 12 volts. It never did. But the voltage across the Red and yellow one did reach 12 volts. Just for good measure I could do the test again tomorrow and see if I can pick up 12 volts on the gas valve itself but I doubt it if I'm not picking it up on the control board and that's where it should be getting generated.

I've attached the Suburban furnace manual and if you look at section C on page 18 which is headed "BLOWER RUNS BUT FURNACE DOES NOT LIGHT" it actually tells you at one point to check the voltage at the gas valve and then it even recommends to replace the control board if it's not present. So this and the lack of clicking sound gives me more confidence that the problem is the control board.

I looked around for replacement control boards and I found one that looked perfect. It was the right dimensions and with the connector in the right place but it did not list this model of furnace as a correct replacement.
https://www.amazon.com/520814-Contr...a?sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY

On the other hand there were other listings that said they were a correct replacement and yet they were not the right dimensions and or the connector came out the bottom instead of the side. Frustrating. The model number shown on the board is CCA-1202 with a part number of 233527 but searching on those numbers did not come up with much.

Thanks again guys for your input. Feels like I'm making progress.

 

[Kidkraz]

Rob, using the model number from your picture, Suburbans site lists this relay for the NT series of furnaces.



https://suburbanrvparts.com/suburban-furnace-time-delay-relay-board-521313-nt-qt-models/

 

[Bill]

Rob -

Be careful of the item in that Amazon ad. I can't tell for sure, but the text of the ad says the thing is powered by 120 VAC, and there is no AC power anywhere in your furnace. Your furnace contains only 12 VDC power. Beyond that, on the left end of the board in the second picture, I see two AC rectifier diodes designated D1 and D5, along with a power supply filter capacitor C2. If I am right, this board is intended for use in a Suburban water heater (such as the SW6-DE in the TM) where AC is available, but not in a furnace where no AC is available.

Bill

 

[DiegoWest]

Rob, using the model number from your picture, Suburbans site lists this relay for the NT series of furnaces.



https://suburbanrvparts.com/suburban-furnace-time-delay-relay-board-521313-nt-qt-models/

Hi Kidkraz,
The problem with getting one like this is that you also need another wire kit to go along with it in order to allow it to mount to the board because the size and position of the connector is different. Those wire kits seem to be hard to come by also.

I found the exact correct part on ebay and even though it's used, I may go that way anyway if I can't find the exact correct part new.

 

[DiegoWest]

Rob -

Be careful of the item in that Amazon ad. I can't tell for sure, but the text of the ad says the thing is powered by 120 VAC, and there is no AC connection in your furnace. Beyond that, on the left end of the board in the second picture, I see two AC rectifier diodes designated D1 and D5, along with a power supply filter capacitor C2. If I am right, this board is intended for use in a Suburban water heater (such as the SW-6DE in the TM) where AC is available, but not in a furnace where no AC is available.

Bill

Thanks for pointing that out Bill.

 

[Bill]

Rob -

Since the fan comes on when it is supposed to, I don't think the time-delay relay is faulty.

The diagram shows an item called a Valve Switch. I'm not sure what it is, but since everything downstream of it seems proper, I don't think it is faulty.

On page 19 of the manual you linked, step 12, there is mention of a neon light that should be flashing as the unit tries to light. Have you checked that?

Bill

 

[DiegoWest]

Good question. I looked for that light and couldn't find it. I'm not sure if it wasn't there at all or just wasn't flashing.

 

[Bill]

I think you are approaching the end. Everything around the control board seems to be good, but the control board is not trying to open the gas valve when it should, and it is not generating a spark when it should. Incidentally, it should produce an audible spark regardless of whether the gas valve actually opens. (If sparks are produced but the flame doesn't start, then the flame sensor will signal the board to shut down, and quit sparking. It will reset and retry 3 times, then lock out.)

Unless you have some more ideas, I think you are in for a new board.

Bill

 

[DiegoWest]

I think you are approaching the end. Everything around the control board seems to be good, but the control board is not trying to open the gas valve when it should, and it is not generating a spark when it should. Incidentally, it should produce an audible spark regardless of whether the gas valve actually opens. (If sparks are produced but the flame doesn't start, then the flame sensor will signal the board to shut down, and quit sparking. It will reset and retry 3 times, then lock out.)

Unless you have some more ideas, I think you are in for a new board.

Bill

I agree Bill,
You should hear a spark even if the gas valve doesn't open. That's fortunate for testing. Otherwise I'd have to hook it up to a gas supply just for testing.

Now the challenge will be to find the correct board.

 

[DiegoWest]

Update:

I got a used control board on ebay. I wanted to get a new one but suburban no longer sells the furnace and doesn't sell parts for it either. They do have a substitute part but that part was not the same size and also required an extra cabling adapter to make it work for another $80. To make matters worse, no one seemed to have that adapter in stock.

But good news! I got the used part for just $50. Plugged it in and it worked! I noticed that it made the clicking sound about 15 seconds after the fan came on when I had it on the work bench. The clicking sound it made sounded like a relay coming from the board. Should have unplugged the wire going to the igniter to verify that the sound was not from the igniter but I forgot to do that.

Then I put the furnace back in the TM and hooked the wires back up. Everything works fine now and the heater comes on. Kind of odd to have a control board problem like that on such a new Trailmanor, (2023), as everyone said the sail switch and limit switch failures are much more common.

Thanks Bill and Wayne and others for weighing in on this project! It helps to get second opinions before diving in on some something like this.

 

[Bill]

That was quite a process, and I'm glad you stuck with it to a successful end.

Thanks Bill and Wayne and others for weighing in on this project! It helps to get second opinions before diving in on some something like this.

That's what this Forum is all about - no one-and-done answers. Congratulations.

Bill