Temperature Sensors; Low Ambient Control; Frame Heater - Dometic NDA1402 Service Manual

5.2 temperature Sensors

The NDA/NDE 1402 has two sets of thermistors. One
for the freezer and the other for fresh food compartment.
The display range is from - 9° F to 41° F in the freezer
and from - 9° F to 59° F in the fresh food side. For tem-
peratures below and above these ranges, the display will
show "Lo" and "hi" respectively.
During normal operation, the temperatures in degrees
Fahrenheit in the compartments are shown on the display,
freezer to the left and fresh food to the right. However,
the shown temperatures are not the ones actually mea-
sured. Instead they are calculated as the most probable
temperatures of the food that is stored in the respective
compartment. Therefore these temperatures should not
be compared with temperatures measured with ordinary
thermometers placed inside the compartments. When the
power is switched "OFF" and then "ON" again, the tem-
perature filters will forget historical values and the calcu-
lation restarts from the beginning. In this case the shown
values will be about 2° F above the real air temperatures
at the sensors.
Each thermistor harness consists of two thermistor sen-
sors. One each for "air" and the other for "surface" for the
defrosting process.
Disconnect the thermistor harness from the P2, 4-pin ter-
minal (freezer) or P3, 4-pin terminal (fresh food) on the
lower circuit board. Place the sensing tip in a glass of ice
water (more ice than water), approximately 33° F to 35°
F. Wait 8 to 10 minutes. You should get a reading of ap-
proximately 5,500 Ohms. Always test from the wire side of
the 4-pin connector with the meter leads as not to create a
connection problem at either the P2 or P3 connector.
note:
A disconnected or faulty thermistor will result in an
error code displayed on the front display panel. See sec-
tion 11 for trouble shooting error codes.
Air sensor 4-pin connector
Fresh food air thermistor:
Remove the P3 connector from the lower control
board. Check the resistance between pins 1 and 2 on
the P3 connector.
Fresh food surface thermistor:
Remove the P3 connector from the lower control
board. Check the resistance between pins 3 and 4 on
the P3 connector.
Freezer air thermistor:
Remove the P2 connector from the lower control
board. Check the resistance between pins 1 and 2 on
the P2 connector.
Freezer surface thermistor:
Remove the P2 connector from the lower control
board. Check the resistance between pins 3 and 4 on
the P2 connector.
note:
See page 49 for thermistor values at varying tem-
peratures.

5.3 low ambient control

The automatic Low Ambient Control ensures trouble free
operation in low ambient temperatures (e.g. Below 50° F).
In colder weather, the temperature inside the fresh food
compartment tends to hold the temperature inside for a
very much longer period of time, with very long periods
in between ON/OFF cycling of the heat source; this is OK
for any food product inside the fresh food compartment,
but is not OK for the freezer compartment. The NDA/
NDE1402 have active thermostatic control of both com-
partments (however, you can only set the temperature
level for the fresh food thermostat). If the temperature in
the freezer is not low enough, the cooling is switched on.
If the fresh food compartment then becomes too cold, the
defrosting element will be switched on to keep it at the
correct temperature.

5.4 frame Heater

The frame heater is located behind the metal frame be-
tween the doors. The heater helps to reduce sweating on
the metal frame during periods of high humidity. The heat-
er is on all the time except during the defrosting phases
to reduce maximum current draw from the DC supply. To
test the heater, remove the white wire from terminal (J4)
Surface
and measure the resistance between the wire removed
sensor
and chassis ground. You should have a resistance read-
ing of 24 Ohms +/- 10%.
note:
Some units may have a Light blue wire on J4.
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