Control System Diagnostics - Scotsman NDE754 Service Manual

NDE754

CONTROL SYSTEM DIAGNOSTICS

The control system consists of:
·
Control Board
·
Water Sensor
·
Ice Sensors
·
High Pressure Cut Out
·
Low Pressure Cut Out
If the unit is OFF, check the control board:
1. Is the Power OK light on? If not check power to
the unit. If it has power, and the Power OK light is
NOT on, check the high pressure and low pressure
cut outs. If they are both closed, replace the board.
If the Power OK light is ON, go to the next step.
2. Is the Water OK light on? If it is, go to the next
step. If not, check the water level in the reservoir. If
there is water in the reservoir, check that the water
sensor is plugged in. To check the water sensor:
A. Unplug water sensor.
B. Pull water sensor from reservoir.
C. Place one ohmmeter lead on the sensor's plug
and the other on the sensor's tip. The meter should
show nearly zero resistance. If it reads infinite
resistance, check the tip for corrosion. If it is clean
and still reads open, replace the sensor.
OR connect a copper wire to the wire where the
water sensor plugs into and place the other end in
the water. The water OK light should go ON. If it
does not, replace the control board.
3. Ice sensor check. Is the Bin Full light Off? If it is
OFF and the Service light is Off, and the unit is not
running, replace the control board.
If it is OFF and the auger motor is running but the
compressor is not, check the compressor contactor
coil.
If it is on, the ice sensors may be blocked. Remove
them and check for mineral scale. Scotsman's test
box can also be used to determine if the ice
sensors or board are defective.
Using the tester:
A. Disconnect the ice sensors at the connection by
the ice chute. Connect the LED and PHOTO
TRANS wires to the control board's wires.
B. With the On - Off (mode) switch in either
position, move the Bin Full switch on the tester to
Bin Full - the tester's light will blink and after a few
seconds the bin full light on the control board will
come on. If not, replace the board.
Move the Bin switch on the tester to Bin Empty.
The light on the tester will go out, and after a few
seconds the Bin Full light on the board will go out.
If master switch is ON, the unit should start.
4. High pressure cut out check.
Disconnect electrical power.
Pull the wires off the high pressure cut out.
Use an ohmmeter to determine if the switch is
OPEN, If it is, check the discharge pressure. If the
discharge pressure is less than 300 PSIG, replace
the high pressure cut out.
If the high pressure cut out is open and the system
has high discharge pressure, check for the cause.
5. Low pressure cut out check.
Disconnect electrical power.
Pull the wires off the low pressure cut out.
Use an ohmmeter to determine if the switch is
OPEN, If it is, check the suction pressure. If the
suction pressure is greater than 35 PSIG, replace
the low pressure cut out.
If the low pressure cut out is open and the system
has low suction pressure, check for the cause.
March 2000
Page 20