I need help with Suburban NT-12SE Furnace!

[PatrickFisherPr]

I have a 1984 Toyota Sunrader. I've completely renovated my Sunrader. I'm approximately 98% done with renovations, but I'm having issues with my Suburban furnace.

I have a Suburban NT-12SE furnace. The blower operates but the gas valve does not open and the ignition does not fire.

Initially I thought it was the control board. I purchased and installed a new board. Even with the new board installed the valve does not open and the ignition does not fire.

I emailed Suburban and they said it could be my limit switch. The wire going from the limit switch to the control board is hot. Could the limit switch be the problem even if the wire is hot?

I based wiring off the diagram provided in the Suburban manual.

I would appreciate any insight.

[opnspaces]

enblethen wrote:

This may help. Manual from link from Bryant RV.
Service manual

In the manual that Enblethen linked the troubleshooting on page 14 looks for 12v between the red 12v+ and yellow 12v-. You have 12v from red to ground, but do you have 12v from red to yellow? I would think so, but it is the next step in the troubleshooting flowchart. From looking at the chart it seems you're getting close to a bad micro switch or a spark gap to fix this.

[enblethen]

Check for 12 volts DC on gas valve immediately after turning on furnace. If you have power there, use a long lighter to try to light the furnace with it.
Have you checked the clearance on the ignitor?

[PatrickFisherPr]

Yes, there is power at the red wire to the circuit board.

[enblethen]

But is there power at the circuit board red wire?

[PatrickFisherPr]

There are 12 volts on both sides of the switch and the grounds are good. All screws were sanded down and covered in graphite.

[enblethen]

Are you getting 12 volts on the red wire connected to the circuit board?
Verify that you have a good ground via the mounting screws for the circuit board.

[westend]

From the service manual linked to above on Page 18--"8. Check for 12 V DC on both sides of the limit switch. If there is voltage on one side of the switch but not on the other, replace the switch. If voltage is present on both sides of the switch, proceed to step 9."

I'd suggest you read through the troubling shooting guides as Suburban is very thorough with their service manuals.

[PatrickFisherPr]

I appreciate your responses.

Here is the furnace manual. The wiring diagram is on the last page. http://manuals.adventurerv.net/Suburban-Install_NT-12SE_NT-16SE_NT-20SE.pdf

The power going in to the furnace is a bit over 13 volts. The blower kicks on. It runs continuously without the gas valve opening or the ignition sparking.

After the blower runs the sail switch is closing. It is remaining closed.

I didn't mean the wire from the fail switch is hot in temperature. I meant it is live.

I think the issue is the contact arrangement on the board. From left to right they are as follows:

Number one is the red wire, +12V DC
Two and three have no wires
Four is the brown wire, which turns gas on
Five is the black wire, which goes down to the electrode
Six is the yellow wire, -12V DC

I just bought a wiring kit, model number 520840, hoping that will resolve my issue.

Any additional thoughts would be greatly appreciate. If someone can confirm the contacts that would be a massive help.

[enblethen]

The wire being hot as in temperature could indicate a miss wired circuit and the wire is being heated by direct short.
The limit switch should be in series with the sail switch.
Disconnect the wires from the limit switch. Use an Ohm meter to verify that it has continuity, and there is no continuity to ground.
Locate the wires going to the sail switch.
Wire from fan relay should go to the sail switch then to the limit switch then to the circuit board.
This may help. Manual from link from Bryant RV.
Service manual

[hohenwald48]

Can't say about the limit switch without looking at a diagram. If you have a link you could post it.

However, the normal course of events is for the fan to come on for some time (usually about 30 seconds) to ventilate the furnace of any residual propane fumes. Normally the 30 second timer does not start until the "sail" switch tells the controller that the fan has started. After the 30 seconds, the solenoid gas valve opens and the igniter tries to light the burner (usually max of 3 tries). If burner lights it heats the thermocouple telling the controller that the flame is burning.

Based on that and the fact that you have changed the controller, I'd be suspect of your sail switch operation.