The problem could be one of several things or a combination. The first
thing to understand is that the furnace has an airflow sensor that has to
close before the gas flow solenoid to the main burner can light. This
sensor is located at the back of the furnace in the air stream from the
blower. It is a long metal vane with a small microswitch attached. These
are prone to failure but are replaceable. It can be checked with an
ohmmeter as well as a visual and audibly. Once removed, you should be able
to hear the microswitch snap from open to close as you move the metal vane.
You should also see the small plastic button on the microswitch pop back to
the extended position once the metal vane is moved away from the switch.
And as I stated, check it with an ohmmeter. If the switch seems to be
functioning, then reinstall and with the furnace still removed, connect a
12v battery to the blower and ensure that the switch shows continuity with
the blower running. A slow blower will result in insufficient airflow to
move the switch upward closing the microswitch. If all that checks out,
then check the gas solenoid. This can be done by applying 12v to BOTH sets
of coils on the valve. The valve has 2 solenoids as a safety feature and
they both have to work. When you apply the 12V you can feel the solenoid
activate. With both open, you should be able to blow air thru the valve.
If the gas valve seems to check out as well as the air flow sensor, then
again power up the furnace while still removed and see if once the airflow
sensor activates that you feel the gas valve activate. There is also the
possibility that the orifice at the end of the gas tube leading to the
burner may be plugged. This does happen from time to time. A needle can be
used to clear the opening of the orifice.
I want to warn you that given the age of your furnace, you need to be aware
that the heat exchanger rusts from the inside out. This is due to the fact
that burning gas produces water vapor. We have found some furnaces where we
can extract 1-2 cups of rust flakes. As the furnace is a forced combustion,
if there are any pinholes in the heat exchanger, carbon monoxide gas will be
forced into the trailer when the heater is running. Don't operate the
furnace without a working CO2 detector as the gas is odorless and deadly.
If there is a lot of rust that comes out of the heat exchanger when you
removed the burner tube, don't assume it safe.
David Tidmore
GM Roger Williams Airstream
Weatherford, TX
> Happy New Year everyone ! My '73 Safara has a heating problem. The furnace
> pilot lights and stays lit, the blower comes on and off with the thermostat
> but no heat. Appears no gas flow to the chamber. Any thoughts before I pull
> the furnace out to get to the sensor?
>
> Rick B