My original problem was the usual story. The fan would come on and the furnace would fail to light and the fan would then shut down. I, standing outside could hear the switch and smell gas but the spark, I couldn't hear due to fan noise and it was failing to light the gas. I brought the furnace home and checked the over-temperature thermostat and sail switch. They were good. I hooked up a 12 volt battery and opened up the combustion chamber to check to see if I was getting a good spark, which I was.Took it back and reinstalled it and the fan no longer came on when I called for gas heat. I could, however, use the external 12v battery to supply power through the green and yellow wires while jumping the two blue wires together. With the gas hooked up, the furnace came on and burned, just like it should. I had the internal thermostat out of the circuit and the only hook up to the rv was the gas source. I hooked internal wires back, just like they were originally but nothing would happen.I'm guessing that I have a thermostat problem. I may have caused it when I back fed power by accident, blowing the 2 amp glass fuse in the thermostat.Lacking any expert suggestions I guess I will go spend $80 for a replacement thermostat. This one is a 3 way, AC, heat pump or gas furnace. The ac and heat pump functions still work.

Locate the two wires feeding the furnace either at the thermostat or in the furnace. Connect them together. The furnace should start and then produce heat. If this happens, separate the two wires. The furnace should go into shut down mode and turn off.If this happens, the problem is in the thermostat.Make and model of thermostat? Number of wires in thermostat? Does it control the A/C?

enblethen wrote:Locate the two wires feeding the furnace either at the thermostat or in the furnace. Connect them together. The furnace should start and then produce heat. If this happens, separate the two wires. The furnace should go into shut down mode and turn off.If this happens, the problem is in the thermostat.Make and model of thermostat? Number of wires in thermostat? Does it control the A/C?Here on p.35 is the schematic of my unit. The wires coming down from the thermostat are bk, gn, y, wh and they connect to r, bl, bl, and y, in order, on the furnace. I have no idea which pair of the bk, gn, y, wh to tie together. I see 6 possible pairs and don't know what further damage could be done by picking the wrong pair. I do know that when powering the furnace with an external battery that the red and yellow wires power it up with the two blues tied together. Gas connected and the furnace lights and runs.

It is the two blue wires that come from the thermostat.Red and yellow is the 12 volt DC supply.

enblethen wrote:It is the two blue wires that come from the thermostat.Red and yellow is the 12 volt DC supply.The wires from the thermostat are black, green, yellow and white.The blue wires are both on the heater side. They are blue, blue, red and yellow. Thermostat__________Heater black------------------------red green------------------------blue yellow-----------------------blue white------------------------yellow

enblethen wrote:It is the two blue wires that come from the thermostat.Red and yellow is the 12 volt DC supply.Red is 12VDC + (positive)Yellow is 12VDC - (negative)Red supplies 12VDC+ to the thermostat via the On/Off CB Switch (local/on furnace frame) (Blue) and DC Power to 'power' circuit on boardThermostat closes and sends DC+ back to circuit board (Blue) and sail switch/limit switch circuit Connecting the 2 Blues wires at thermostat makes up 'run' circuit...bypassing thermostatIf Furnace runs (Fan/Flame/Heat) then thermostat is problemIf furnace does NOT...check if the on/off CB switch is tripped///push button reset

Need help with a SF 30 Suburban Furnace | Good Sam Community

enblethen
Nomad
Nov 12, 2020
Locate the two wires feeding the furnace either at the thermostat or in the furnace. Connect them together. The furnace should start and then produce heat. If this happens, separate the two wires. The furnace should go into shut down mode and turn off.
If this happens, the problem is in the thermostat.
Make and model of thermostat? Number of wires in thermostat? Does it control the A/C?
mikestock
Explorer
Nov 12, 2020
enblethen wrote:
Locate the two wires feeding the furnace either at the thermostat or in the furnace. Connect them together. The furnace should start and then produce heat. If this happens, separate the two wires. The furnace should go into shut down mode and turn off.
If this happens, the problem is in the thermostat.
Make and model of thermostat? Number of wires in thermostat? Does it control the A/C?

Here on p.35 is the schematic of my unit. The wires coming down from the thermostat are bk, gn, y, wh and they connect to r, bl, bl, and y, in order, on the furnace.

I have no idea which pair of the bk, gn, y, wh to tie together. I see 6 possible pairs and don't know what further damage could be done by picking the wrong pair.

I do know that when powering the furnace with an external battery that the red and yellow wires power it up with the two blues tied together. Gas connected and the furnace lights and runs.
enblethen
Nomad
Nov 12, 2020
It is the two blue wires that come from the thermostat.
Red and yellow is the 12 volt DC supply.
mikestock
Explorer
Nov 12, 2020
enblethen wrote:
It is the two blue wires that come from the thermostat.
Red and yellow is the 12 volt DC supply.

The wires from the thermostat are black, green, yellow and white.

The blue wires are both on the heater side. They are blue, blue, red and yellow.

Thermostat__________Heater
black------------------------red
green------------------------blue
yellow-----------------------blue
white------------------------yellow
Old-Biscuit
Explorer III
Nov 12, 2020
enblethen wrote:
It is the two blue wires that come from the thermostat.
Red and yellow is the 12 volt DC supply.

Red is 12VDC + (positive)
Yellow is 12VDC - (negative)

Red supplies 12VDC+ to the thermostat via the On/Off CB Switch (local/on furnace frame) (Blue) and DC Power to 'power' circuit on board

Thermostat closes and sends DC+ back to circuit board (Blue) and sail switch/limit switch circuit

Connecting the 2 Blues wires at thermostat makes up 'run' circuit...bypassing thermostat
If Furnace runs (Fan/Flame/Heat) then thermostat is problem

If furnace does NOT...check if the on/off CB switch is tripped///push button reset
mikestock
Explorer
Nov 12, 2020
Old-Biscuit wrote:
If furnace does NOT...check if the on/off CB switch is tripped///push button reset

Are you saying this switch is a breaker? Mine is on.

I don't have two blue wires at the thermostat. I do have the two blue wires on the heater that make up when the over-heat thermostat is closed and sail switch closes. As I said originally, when the two blue wires from the heater are tied together and 12vdc are supplied by an external battery to the red (+12v) and yellow (-12v), hooked up the gas line and the heater started and ran. When I matched up the wires as they were, originally (see last post) matched, nothing happens.

The other 2 functions of the thermostat, ac and heat pump, work correctly.

I don't have a clue where the relay is located where the tiny wires from the thermostat transfer to the heavier wires to the furnace.
enblethen
Nomad
Nov 12, 2020
If the fan starts, the circuit breaker on the Fan housing is good.
On the terminal strip in the furnace is the best place to jumper the blue wires to bypass the thermostat. You can see what color wires from this terminal strip goes to the thermostat for the heat function.
Again, what is make and model of thermostat?
mikestock
Explorer
Nov 13, 2020
There's no telling who made it. I have seen it with Coleman in the label. Here is a picture. My Phaeton has two just alike.
enblethen
Nomad
Nov 13, 2020
This should be the Manual for your thermostat.
mikestock
Explorer
Nov 13, 2020
I have a stack 3" thick but so far, I haven't been able to find the manual for the thermostat.

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