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Defrost Control Board
24V TERMINAL
STRIP
CONNECTIONS
DIAGNOSTIC
LEDS
PRESSURE
SWITCH CIRCUIT
CONNECTIONS
TEST PINS
Note − Component Locations Vary by Board Manufacture
r.
SENSOR PLUG IN
(COIL, DISCHARGE
LINE & AMBIENT
SENSORS)
REVERSING
VALVE
DELAY
PINS
LOW
AMBIENT
THERMOSTAT
PINS
DEFROST
TERMINATION
PIN SETTINGS
FIGURE 12
The control monitors ambient temperature, outdoor coil
temperature, and total run time to determine when a de-
frost cycle is required. The coil temperature probe is de-
signed with a spring clip to allow mounting to the outside
coil tubing. The location of the coil sensor is important for
proper defrost operation.
NOTE − The demand defrost board accurately measures
the performance of the system as frost accumulates on the
outdoor coil. This typically will translate into longer running
time between defrost cycles as more frost accumulates on
the outdoor coil before the board initiates defrost cycles.
Diagnostic LEDs
The defrost board uses two LEDs for diagnostics − DS1
(Red) and DS2 (Green). The LEDs flash a specific se-
quence according to the diagnostic condition. See table 5.
Low Pressure Switch (LO−PS)
The unit’s automatic reset low pressure switch (S87) is fac-
tory−wired into the defrost board on the LO−PS terminals.
When the low pressure switch trips, the defrost board will
cycle off the compressor, and the strike counter in the
board will count one strike. The low pressure switch is ig-
nored under the following conditions:
during the defrost cycle and 90 seconds after the ter-
mination of defrost
when the average ambient sensor temperature is be-
low 15° F (−9°C)
for 90 seconds following the start up of the compressor
during "test" mode
High Pressure Switch (HI−PS)
The unit’s automatic reset high pressure switch (S4) is fac-
tory−wired into the defrost board on the HI−PS terminals.
When the high pressure switch trips, the defrost board will
cycle off the compressor, and the strike counter in the
board will count one strike.
5−Strike Lockout Feature
The internal control logic of the board counts the pres-
sure switch trips only while the Y1 (Input) line is active.
If a pressure switch opens and closes four times during
a Y1 (Input), the control logic will reset the pressure
switch trip counter to zero at the end of the Y1 (Input).
If the pressure switch opens for a fifth time during the
current Y1 (Input), the control will enter a lockout condi-
tion.
The 5−strike pressure switch lockout condition can be
reset by cycling OFF the 24−volt power to the control
board or by shorting the TEST pins between 1 to 2 sec-
onds. All timer functions (run times) will also be reset.
If a pressure switch opens while the Y1 Out line is en-
gaged, a 5−minute short cycle will occur after the switch
closes.
Sensors
Sensors connect to the defrost board through a field-re-
placeable harness assembly that plugs into the board.
Through the sensors, the board detects outdoor ambient,
coil, and discharge temperature fault conditions. As the de-
tected temperature changes, the resistance across the
sensor changes. Sensor resistance values can be checked
by ohming across pins shown shown in table 4.
NOTE − When checking the ohms across a sensor, be
aware that a sensor showing a resistance value (as op-
posed to either a shorted or open circuit) that is not within
the range shown in table 4, may be performing as de-
signed. However, if a shorted or open circuit is detected,
then the sensor may be faulty and the sensor harness will
need to be replaced.
TABLE 4
Sensor Temperature / Resistance Range
Sensor
Temperature
Range °F (°C)
Resistance values
range (ohms)
Lead
Color
Outdoor
−35 (−37) to 120 (48)
280,000 to 3750
Black
Coil
−35 (−37) to 120 (48)
280,000 to 3750
Brown
Discharge (if
applicable)
24 (−4) to 350 (176)
41,000 to 103
Yellow
Note: Sensor resistance increases as sensed temperature decreases.
Ambient Sensor
The ambient sensor considers outdoor
temperatures below −35°F (−37°C) or above 120°F (48°C)
as a fault. If the ambient sensor is detected as being open,
shorted or out of the temperature range of the sensor, the
board will not perform demand defrost operation. The
board will revert to time/temperature defrost operation and
will display the appropriate fault code. Heating and cooling
operation will be allowed in this fault condition. The ambi-
ent temperature sensor is located in the PVC tube next to
the filter drier. See figure 15.
Coil Sensor
The coil temperature sensor considers out-
door temperatures below −35°F (−37°C) or above 120°F
(48°C) as a fault. If the coil temperature sensor is detected
as being open, shorted or out of the temperature range of
the sensor, the board will not perform demand or time/tem-
perature defrost operation and will display the appropriate
fault code. Heating and cooling operation will be allowed in
this fault condition. See figure 13 for location.
