Microprocessor Board Control Models - Maytag 111990" Automatic Dryers Service Manual

MICROPROCESSOR BOARD
CONTROL MODELS
The microprocessor board receives
input information from the various com-
ponents involved with the operation of
the dryer; such as the dryness sensor
and the thermistor. It uses this infor-
mation to determine the necessary
control for the completion of the cycle.
One of the ways it accomplishes this is
through the use of control relays. A
control relay (S.P.S.T.) is placed in the
motor circuit for both the gas and elec-
tric dryer. Another relay (S.P.S.T.) is
placed in the heat circuit for the electric
dryer only. With these relays the
microprocessor board is capable of
using a low voltage circuit to control a
high voltage circuit. We can call these
two circuits the control circuit and the
controlled circuit. NOTE: The control
for the gas dryer's gas valve circuit is
different and is covered later.
The control circuit for the drive motor
consists of the microprocessor board
and the Motor Relay's coil. The con-
trolled circuit for the drive motor con-
sists of the Motor Relay's switch
contacts, thermal fuse and cut-off drive ,
motor and door switch. The micro-
processor board starts the drive motor
by sending 24 VDC to the Motor
Relay's coil. When energized, the coil
creates a magnetic field that closes the
CAUTION
switch contacts in the relay and pro-
vides a path for voltage to the motor.
When the microprocessor board deter-
mines it needs to stop the motor, it
removes the 24 VDC to the relay's coil.
The switch returns to its normally open
position and the path for voltage to the
motor is opened.
The control circuit and controlled circuit
for the heating element on the electric
dryer operate on the same principle as
the motor relay's circuits. When the
microprocessor board wants to ener-
gize the heating element it sends 24
VDC to the coil in the Heater Relay and
a path for voltage is completed through
the switch contacts to the heating
element. When the 24 VDC are
removed from the coil the switch con-
tacts open and the path for voltage to
the heating element is opened.
TROUBLESHOOTING
When trying to determine what is
causing the problem (drive motor will
not run or heating element will not
come on), separate the two circuits to
isolate the problem area. Start with the
control circuit (it has to function before
the controlled circuit will work). Deter-
mine what is involved in this circuit and
proceed to check the components. If
these components are found to be func-
tioning correctly, focus on the con-
trolled circuit. Determine what is
involved and proceed in checking the
components.
Always disconnect power supply before making any continuity checks or resist-
ance readings.
ED/GD401 1090
SECTION 2. ELECTRICAL - MECHANICAL TROUBLESHOOTING
2-7