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3 − Defrost System (CMC1)
The demand defrost control measures differential tem-
peratures to detect when the system is performing poorly
because of ice build−up on the outdoor coil. The controller
self−calibrates" when the defrost system starts and after
each system defrost cycle. The defrost control board com-
ponents are shown in figure 4.
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 state (Off, On, Flashing) of two LEDs on the defrost
board (DS1 [Red] and DS2 [Green]) indicate diagnostics
conditions that are described in table 2.
24V TERMINAL
STRIP
CONNECTIONS
DIAGNOSTIC
LEDS
PRESSURE
SWITCH CIRCUIT
CONNECTIONS
TEST PINS
NOTE − Component Locations Vary by Board Manufacturer.
Y2 not used on 14HPX
SENSOR
PLUG IN
(COIL & AM-
BIENT
SENSORS)
REVERSING
VALVE
DELAY
PINS
LOW
AMBIENT
THERMOSTAT
PINS
DEFROST
TERMINATION
PIN SETTINGS
FIGURE 4
Defrost Control Pressure Switch Connections
The unit’s automatic reset pressure switches (LO PS − S87
and HI PS − S4) are factory−wired into the defrost board on
the LO−PS and HI−PS terminals, respectively.
(OPTIONAL) Low Pressure Switch (LO−PS)
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 ignored under the fol-
lowing conditions:
S
during the defrost cycle and 90 seconds after the ter-
mination of defrost
S
when the average ambient sensor temperature is be-
low 15° F (−9°C)
S
for 90 seconds following the start up of the compressor
S
during "test" mode
High Pressure Switch (HI−PS)
When the high pressure
switch trips, the defrost control will cycle off the compres-
sor, and the strike counter in the control will count one
strike.
Defrost Control Pressure Switch Settings
High Pressure
(auto reset) − trip at 590 psig; reset at 418.
Low Pressure
(auto reset) − trip at 25 psig; reset at 40 psig.
5−Strike Lockout Feature
The internal control logic of the control counts the pressure
switch trips only while the Y1 (Input) line is active. If a pres-
sure switch opens and closes four times during a Y1 (In-
put), 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 condition.
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 and 2 seconds. All timer
functions (run times) will also be reset.
If a pressure switch opens while the Y1 Out line is engaged,
a 5−minute short cycle will occur after the switch closes.
Defrost System Sensors
Sensors connect to the defrost control through a field-re-
placeable harness assembly that plugs into the board.
Through the sensors, the control detects outdoor ambient
and coil temperature fault conditions. As the detected tem-
perature changes, the resistance across the sensor
changes. Figure 5 shows how the resistance varies as the
temperature changes for both type of sensors. Sensor re-
sistance values can be checked by ohming across pins
shown in table 1.
TABLE 1
Sensor
Temperature
Range °F (°C)
Resistance values
range (ohms)
Pins/Wire
Color
Outdoor
(Ambient)
−35 (−37) to 120
(48)
280,000 to 3750
3 & 4
(Black)
Coil
−35 (−37) to 120
(48)
280,000 to 3750
5 & 6
(Brown)
Discharge (if
applicable)
24 (−4) to 350
(176)
41,000 to 103
1 & 2
(Yellow)
Note: Sensor resistance decreases as sensed temperature increases
(see figure5).
NOTE − When checking the ohms across a sensor, be
aware that a sensor showing a resistance value that is not
within the range shown in table 1, 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
needs to be replaced.
Coil Sensor
The coil temperature sensor (shown in fig-
ure 6) considers outdoor temperatures below −35°F
(−37°C) or above 120°F (48°C) as a fault. If the coil temper-
ature sensor is detected as being open, shorted or out of
the temperature range of the sensor, the board will not per-
form demand or time/temperature defrost operation and
will display the appropriate fault code. Heating and cooling
operation will be allowed in this fault condition.
