750mv vs 12v Thermostat Question UPDATE 2

If it's a suburban furnace, one blue wire is connected to +12VDC, the other blue wire is connected to the sail switch, limit switch time delay and module control. so when the thermostat closes you are connecting +12VDC to the sail switch and limit switch time delay circuit. once it sees the 12VDC the fan motor comes on and the after a time delay 12VDC is applied to the control module to signal opening the gas valve and firing the ignitor. the gas valve won't open if the limit switch is open or if the sail switch is open. (e.g. if it is either to hot in the heat exchanger or if the fan motor doesn't start running to give enough airflow to close the sail switch)

I'm sure other furnaces are similar since virtually all of them can use a common aftermarket ignitor board.


see link for schematic.
http://gooddy.org/suburban-rv-furnace-wiring-diagram.html