Relay; Sail Switch; Limit Switch - Atwood 79 Series Service Manual

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This component is commonly referred to
as a time delay relay. The same relay is
used on the 7900, 8500 and 8900 series
- The relay has one primary
to purge the plenum of heat and the
chamber of any unburnt gases after each
heating cycle.
- The motor voltage path of relay
is normally open. There should always be
voltage from the circuit breaker to the relay
terminal of the circuit breaker. There
should always be continuity between the
thermostat terminal connection and ground
terminal connection or the relay.
Only when the thermostat contacts are
closed is voltage supplied to the
The sail switch is an air proving device. It
is a safety component that will not let
ignition occur until it sees 75% of the
motor's rpm's. It insures that the
combustion wheel is rotating fast enough
so that there is a proper air and gas
mixture for smooth ignition.
There are different size sail switches. The
noticeable difference is the size of the
paddle on the switch. Each switch is
matched to the size of motor it must
respond to.
: All 85-IV use the same sail switch.
The limit switch is a very important safety device on a
furnace. The function of a limit switch is to protect the
furnace from overheating. These switches come in a variety
of temperature ratings and are located at critical locations
above the heat chamber on the various models of furnaces.
Therefore, it is very important that when one of these switches
is replaced, you do so with the properly rated switch.
79-II, 89-II, 89-III, 2540
Part # 37021 (was 34781)
3/16˝ / 3/16˝ terminals
L77 or L170
on Switch
thermostat terminal of the relay. This
voltage heats a coil in the relay body. In
approximately 20 seconds this heated coil
causes a bimetal disc to close. Voltage
now passes through the relay and on to the
motor, which in turn should allow the
furnace to ignite and start a heating cycle.
When a heating cycle is complete, the
contacts of the thermostat open and
voltage ceases to the heater coil of the
relay. In approximately 45 - 90 seconds,
the heater coil cools down, the bi-metal
disc opens and voltage ceases to the motor
as well.
AMP Draw - The relay should draw no
more than 1 amp. If the relay should draw
more than 1 amp, it will burn out the
When the paddle of the switch is
depressed, there should be continuity
through the switch. If a sail switch needs to
be replaced, it should be replaced with the
exact same size. If a larger switch than the
original is used, it will probably not close
when the motor reaches 75% of its rpm's
and therefore keep ignition from occurring.
The most common problems with these
switches are bent paddles, loose wire
connections or an obstruction between the
paddle and switch contact.
Failure to do so could cause an unsafe condition with the
heating system. When in doubt as to whether you are using
the proper limit switch for a furnace, look at the temperature
at the base of the switch and match it to the temperature and
related switch noted below.
Part Number 36205 is a thermal cut-off and was a specific
safety component used on some 89-II furnaces. It was
located above the chamber and tripped in the event of a burn
through chamber.
85-II, 89-I
Part # 35132
1/4˝ terminals


Ground (-)
breaker (+)

Sail Switch

Limit Switch

85-III, 85-IV, 1522, 2334
Part # 37022 (was 36176)
1/4˝ terminals

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