Service manual

Section 3 Electronic Control Testing
3-18
The Heating Circuit:
The electronic control board applies power to the heating
circuit through the contacts of relay RL 2. When the
electronic control senses that the temperature in the
drum is below the programmed temperature, it closes
the contact of heater relay RL2 applying line 1 to the
high limit thermostat. The high limit thermostat is a
safety device that prevents the dryer from overheating if
the contacts of the relay RL2 fail closed. The contacts
of the high limit thermostat, normally closed, are set to
open at a temperature above the preset temperature
specifi cations of the electronic control board. From the
output terminal of the high limit thermostat, line 1 is
connected to pin 3 of the eight pin plug, one side of the
holding coil of the gas valve, the secondary coil of the
gas valve and the sensor that is mounted on the burner
chamber.
The connection to pin 3 allows the control to monitor
the contacts of the high limit thermostat and An error
code will show if contacts open above a preset number
of times in a cycle.
The holding coil, secondary coil, booster coil, sensor
and igniter circuits interact with one another to assure
safe operation of the dryer gas burner.
The gas valve has two chambers in series, both must be
opened before gas will fl ow into the burner. The solenoid
that controls the gas fl ow through the fi rst chamber has
two coils: the booster coil and the holding coil. The
solenoid that controls the second chamber has one coil,
the secondary coil. (See Figure 3-14)
The other side of the holding coil, booster coil and igniter
are connected to neutral through the second centrifugal
switch in the motor (closed when the motor is running).
When power is applied across these circuits, current
fl ows through the holding coil, but the holding coil does
not have enough magnetic force to open the solenoid by
itself. At the same time, current fl ows through sensor
contacts providing power to the booster coil and the
igniter. When current fl ows through both the holding and
booster coils, the fi rst chamber opens. The contacts of
the sensor are in parallel with the secondary coil. As
long as the contacts of the sensor remain closed, current
fl ow bypasses the secondary coil, and gas is prevented
from fl owing through the second chamber of the valve to
the burner.
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It is necessary to raise the temperature above 1100° F
to ignite gas. As current fl ows through the igniter, the
temperature of igniter raises from room temperature to
approximately 1800° F within 30 seconds. The contacts
of the sensor are heat sensitive and set to open above
the ignition temperature of gas. When the sensor
contacts open, current fl ows through the secondary coil,
opening the second chamber, allowing gas to the burner,
and is ignited by the heat of the igniter. When the
contacts of the sensor are open, the parallel circuit
formed by the igniter and the booster coil are in series
with the secondary coil which lowers the current fl ow
through the igniter and booster coil. Since it takes less
magnetic force to hold a solenoid open than it does to
open it, the fi rst solenoid remains open when current
through the booster coil is reduced. The reduction of
current fl ow through the igniter reduces heat from the
igniter but the sensor contacts are held open by the heat
of the burner fl ame.
Drying Time:
The amount of drying time is determined in one of two
ways:
1) A fixed amount of drying time may be set by
turning the SELECTOR knob to one of the fixed
drying times.
2) The AUTO DRY cycles a variable amount of time by
the size of the load, the amount of moisture in the
clothes and the dryness setting selected.
In the AUTO DRY cycles, the electronic control reads
the capacitance between the two sensor bars located in
the drum.
Figure 3-14. Gas Valve Operation