surburban furnace will not light

[booty51]

sf -35 # 963105899 in 97 allegro bus. Furnace goes thru steps but does not light. I have 12 volts to relay, temp sensor, sail switch , to circuit board. Fan runs ,But I do not smell propane out exhuast or hear igniter strike. Propane tank is full and valves from tank are open. I disconnected tubing on heater and opened isolation valve to heater and I get propane! I was about to pull heater out but discovered a plinum box on the bottom of heater and do not have clearance to pull it out. I wanted to check the ignitor as next step but I suspect it may be the board that has failed. I can get to it with out pulling the heater. Any thoughts by you heater Guru's. I left out that I have 12 volts from t stat and heater responds to settings . I also have screens on the heater intake /exhaust tubes. I did look inside tubes with a flashlight and didn't see any obstructions.

[Old-Biscuit]

Yes........if there is an outside panel it will come out that way.
(Some SF & SH don't have an outside panel just vent/exhaust thru side of rig...they have to be removed from inside)

2 screws hold outside panel in place
1 screw (bottom center) holds furnace in place

Then the whole assembly slides out..provided gas line/wiring disconnected

Here is a write up on SF Series......LINK

Check the ignitor board part number.......replace with one from Dinosaur Boards......LINK

[booty51]

finally had a chance to check heater again. I have 12 volts to/from sail switch and 12 volts to circuit board . I do not have any voltage coming out of board or any type of voltage on high tension wire. So Board is bad? I disconected high tension wire and the plug on the end of the board . Removed two mounting nuts, but cannot make board clear mounting screws without the big black module on board hitting side of blower cage. Removed one mounting srew on front of blower cannot see others to remove. Any I deas how to disassymble this . Heater is mounted under stove no overhead acess unless stove is pulled . Should the heater be able to be removed thru outside cover.

[Old-Biscuit]

Ex-Tech wrote:
Well I was going by what the OP stated and that was that he had power TO the sail switch not "through".
Did I misread?

YEP..3rd sentence
"I have 12 volts to relay, temp sensor, sail switch , to circuit board."

[Ex-Tech]

Well I was going by what the OP stated and that was that he had power TO the sail switch not "through".
Did I misread?

[Old-Biscuit]

Ex-Tech wrote:

Old-Biscuit wrote:
You have 12V DC power from t-stat to fan thru sail switch, high temp limit switch and to circuit board but no spark/gas valve.

Check for 12V to the gas valve during the opening cycle.....typically 15 seconds after power reaches board. Power is applied to gas valve for 7 seconds while ignitor sparks.

No power to gas valve.....clean circuit board connection
Still no power than circuit board is problem

If you have power to gas valve than ignitor circuit is problem
Bad connection on board for high tension wire....bad high tension wire
Ignitor spark gap/position incorrect (3/16" from back of lance, centered over burner with a 1/8" spark gap)

Suspect you will find no power to gas valve....circuit board.
Suburban part number 520820

Power to the gas valve could also mean bad gas valve.

OP mentions power TO sail switch but is there power going out when the switch is activated?

OP stated he has power at board thru sail switch and limit switch.fan runs.....no spark no gas valve function.

[Ex-Tech]

Old-Biscuit wrote:
You have 12V DC power from t-stat to fan thru sail switch, high temp limit switch and to circuit board but no spark/gas valve.

Check for 12V to the gas valve during the opening cycle.....typically 15 seconds after power reaches board. Power is applied to gas valve for 7 seconds while ignitor sparks.

No power to gas valve.....clean circuit board connection
Still no power than circuit board is problem

If you have power to gas valve than ignitor circuit is problem
Bad connection on board for high tension wire....bad high tension wire
Ignitor spark gap/position incorrect (3/16" from back of lance, centered over burner with a 1/8" spark gap)

Suspect you will find no power to gas valve....circuit board.
Suburban part number 520820

Power to the gas valve could also mean bad gas valve.

OP mentions power TO sail switch but is there power going out when the switch is activated?

[Old-Biscuit]

YES......
Brown wire from board goes to gas valve....12V DC(+)
Yellow is common ground 12V DC(-)
Red wire is 12V DC(+) from battery for fan motor and from limit switch (t-stat to sail switch to limit switch)

The heavy gauge wire (Orange...may look red if faded) is the high tension wire going to spark electrode. Sends high voltage from board to spark electrode and then is used for milivolt signal back to board to prove main flame.

[booty51]

IT will be a while before I can get back on this,I put rv back in storage shed and start back to work, do not know when i can follow up on this shed is 15 miles away and it is dark when i get home from work. But I do have a question about the circuit board. The connection on the end has a red yellow and brown wire. I had 12 volts to the red and yellow does the brown wire go to the gas valve? Also there is a heavy gauge red wire that comes off the board off of a black round piece , what is that and where would that go. I am sorry I don't know what to call electronic pieces on a circuit board. I should have studied the schematic on the motor a little more.

[Old-Biscuit]

You have 12V DC power from t-stat to fan thru sail switch, high temp limit switch and to circuit board but no spark/gas valve.

Check for 12V to the gas valve during the opening cycle.....typically 15 seconds after power reaches board. Power is applied to gas valve for 7 seconds while ignitor sparks.

No power to gas valve.....clean circuit board connection
Still no power than circuit board is problem

If you have power to gas valve than ignitor circuit is problem
Bad connection on board for high tension wire....bad high tension wire
Ignitor spark gap/position incorrect (3/16" from back of lance, centered over burner with a 1/8" spark gap)

Suspect you will find no power to gas valve....circuit board.
Suburban part number 520820

[kampinguru]

I just went through the same thing. Had the furnace pulled and there was a small birds nest sitting on top of the furnace right over the gas jets. Removed it and all is well. Have to get some screens for the furnace vents.

[Flyfisherman128]

Bad Gas valve?