Duotherm Pilot Models
Pilot okay, blower will not run
Find the fan relay on your furnace. It will have 6 wires leading to it. With a test light probe the red, white and two orange wires. With the thermostat off, there should be power at the red, the white and one of the orange wires.
Turn up the thermostat. The relay should engage and now supply power to the blue wire. The blower motor should start. If you get no power to the blue wire, replace the fan relay. If you do get power to the blue wire, check or replace the blower motor.
Pilot on, Blower runs, No burner
As the blower motor comes up to speed, the force of the air flow acts on the sail switch and closes it's contacts. There should be power on both sides of the sail switch at this time. This switch is a safety feature that is provided to ensure there is enough battery power to turn the blower fan at the proper speed. Adequate flow is required to supply combution air and to distrubute the heated air throughout the coach. If you have power on one side of the sail switch and no power at the other terminal (when the motor is running at full speed), replace the sail switch.
The current now flows through the sail switch to the limit switch. The limit switch is normally in the closed position- meaning that it normally allows current to flow through it. It's job is to open the circuit, disconnecting and closing the gas valve (thus extinguishing the main burner flame) if the furnace heats up beyond it's design limit. There should be power on both sides of the limit switch when the blower motor is running at full speed. If not, replace the limit switch. You can test the limit switch with a continuity tester or an ohm-meter. It should be closed at room temperature and show zero ohms.
Next in line is the gas valve. This is a simple electrically operated solenoid valve. It's job is to open or close the main gas valve. If it gets power, it opens the valve. If it gets no power, it closes the valve.