Lethal voltage levels are used in operating the FDECU. Be sure to observe
extreme caution when accessing pc board with power connected. Injury or death
can occur if personnel contact any electrical terminal while the unit is connected to
4.10 TEMPERATURE POTENTIOMETER SETTINGS. See figure 4-9. The temperature potentiometers R19 through
R24 affect the operation of the defrost cycle as well as outside fan and/or compressor cycling. If you suspect the
potentiometers have been tampered with or simply want to confirm their setting, adjust them as follows.
a. With power applied to the FDECU and no operating mode selected, put the pc board into the maintenance mode
by placing switch number 8, on the dual inline parallel (DIP) switch S3, down (ON). Use a non- conducting probe
such as a plastic or wooden toothpick to move the DIP switch. At this point no light emitting diodes (LED's)
should be on.
b. Turn each potentiometer fully counterclockwise. When the first potentiometer is turned, LED D30 will come on,
and will stay on indicating that the pc board is in the potentiometer adjustment mode. LED's D31 through D35 will
flash as you turn each potentiometer indicating, in binary code, the set point. The LED's will automatically switch
to whichever potentiometer is being adjusted.
Each potentiometer adjustment is directly related to the temperature set point for
the item being controlled. Each potentiometer however has a different start point
temperature (fully counter clockwise) and temperature adjustment range. When
adjusting the potentiometers, it is important to note that the numeric value
indicated on the LED's is actually a change, or increase, of the start point
temperature and must be added to that start point temperature to find the actual
set point temperature.
The following is each potentiometer start point
temperature/temperature adjustment range; R19 start point 10 range 0-26, R20 start
point 70 range 0-21, R21 start point 10 range 0-10, R22 start point 30 range 0-21,
R23 start point 12 range 0-12, R24 start point 70 range 0-20.
c. By turning each potentiometer clockwise you can observe the binary display on LED's D31 through D35 as they
light to indicate the set point. See table 4-3 to convert the binary code to a numeric value. The set point will start
at zero and go up as you adjust clockwise. Adjust each potentiometer set point as follows; R19 to 13, R20 to 21
(fully clockwise), R21 to 0 (fully counterclockwise), R22 to 21 (fully clockwise), R23 to 6, and R24 to 10.
d. Place DIP switch S3 switch number 8 back to the up (off) position. Be sure that all DIP switches are in the up (off)
position. Failure to put the DIP switches in the off mode will prevent the safety circuits from protecting the
components and could cause equipment damage.
4.11 THERMISTOR TEMPERATURE SENSORS TESTING. See figure 4-10. The thermistor temperature sensors
provide the primary input signals for the pc board. If any one of the sensors is defective or is not getting a signal to the pc
board, it will cause erratic and unpredictable operation of the FDECU. To ensure accurate diagnostic troubleshooting of
the pc board and system, it will be necessary to confirm that the thermistors are functioning properly. The following
procedures will test the thermistors at the pc board. This will test the interconnecting wiring as well as the thermistor itself.
If an open circuit failure is indicated however, it will be necessary to disconnect the thermistor from the wiring and test it
directly to determine if the thermistor or the wiring is defective.