Flame sensor on an older Atwood Furnace

[Rugb]

I’m putting a new board 31501 in my 8525 lll Atwood Furnace. My flame sensor wire plugged into the board on the old board with a side connector. The wiring diagram I find for the new board shows it grounded. Does anyone understand how this works? Do I need a newer flame sensor style for the new board to work? I get it to ignite and burn for a short time, and does this 3 times, then locks out and flashes code 3. Any help appreciated
Thanks
Picture is of the new style board where there is the ignition wire and a ground wire to the electrode

 

[Gary RV_Wizard]

I think you are misreading the diagram. The burner is grounded by its connection to RV frame. The igniter, which is also the flamer sensor, is wired only to the board. It receives a high voltage spike to provide the ignition spark, and the burner must be grounded in order for that spark to leap to the burner. The igniter wire then carries a very low voltage signal back to the board IF there is sufficient flame to heat the sensor. The ground is not relevant to that.

Your symptom is that there is no flame sense signal received at the board. The most likely causes of that is either that the igniter tip is not close enough to the burner flame or a poor quality wire connection at the board. The high voltage igniter spark can get through a poor connection but the very low voltage flame signal cannot.

 

[John From Detroit]

One possible cause (This is theory but wellread the story) is the sensor chip on the control board.
Rounding shamelessy the spark voltage is about 1,000 or 1,500 peak

The flame sensor is looking for abut half a volt My theory is some device is supposed to insure spark voltage does not reach the sensor and pop goes the chip. NOTE sensor refers to the chip on the control board.

Thus I ordered a new board from Dinosaur boards. One of the things I did in my youth... Certified electronics technician, Never worked as such but got the cert. When the Dino board arrived my first impression was quality product. Then on closer inspection I found a part NOT on the Attwood board. A gas discharge tube. These are used as spike suppressors and by its placement on the board is there to protect that very chip.

Oh. Furnace worked with the new board.

 

[Kirk]

The igniter, which is also the flamer sensor, is wired only to the board.

Gary gave a great answer. The ground for the igniter probe is that it is mounted metal to metal on the burner. You might need to replace that pobe and if you do be careful to get the right one as they are not the same on all Atwood models.

The flame sensor is looking for abut half a volt

The signal returned to the circuit board is actually measured in microvolts and is too small and short to be detected with a commonly available meter, while the spark is made by a very high voltage that is of less than 1 second duration.

 

[Rugb]

I think you are misreading the diagram. The burner is grounded by its connection to RV frame. The igniter, which is also the flamer sensor, is wired only to the board. It receives a high voltage spike to provide the ignition spark, and the burner must be grounded in order for that spark to leap to the burner. The igniter wire then carries a very low voltage signal back to the board IF there is sufficient flame to heat the sensor. The ground is not relevant to that.

Your symptom is that there is no flame sense signal received at the board. The most likely causes of that is either that the igniter tip is not close enough to the burner flame or a poor quality wire connection at the board. The high voltage igniter spark can get through a poor connection but the very low voltage flame signal cannot.

Thanks for your reply. I’ll send a photo of my original wiring. Even though I can’t see it, there seem to be a sensor that is on the end of the black wire. Then the black wire goes to the connector to the board. With the new board, are the high voltage wire and the sensor wire the same? That this low voltage produced by sensor feed back into the high voltage coil on the board? Just wondering if I have to update my sensor to make it work?
Thanks

 

[Gary RV_Wizard]

In a modern furnace there is only one wire to the igniter/sensor and it serves both purposes. It delivers the high voltage pulse that triggers the ignition spark and also allows the tiny micro-volt flame sense signal to travel back to the circuit board. If your old board & igniter/sensor used two separate wires, you will need a new igniter/sensor to be compatible with the new board.

 

[Rugb]

In a modern furnace there is only one wire to the igniter/sensor and it serves both purposes. It delivers the high voltage pulse that triggers the ignition spark and also allows the tiny micro-volt flame sense signal to travel back to the circuit board. If your old board & igniter/sensor used two separate wires, you will need a new igniter/sensor to be compatible with the new board.

Thanks
I found the one wire sensor/igniter and ordered that