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System Operation
IMPORTANT
Some scroll compressor have internal vacuum protector
that will unload scrolls when suction pressure goes
below 20 psig. A hissing sound will be heard when the
compressor is running unloaded. Protector will reset
when low pressure in system is raised above 40 psig. DO
NOT REPLACE COMPRESSOR.
This section addresses:
S Unit components (sensors, temperature switch,
pressure switches and demand defrost control)
S Second−stage operation
UNIT COMPONENTS
Demand Defrost Control (A108)
The demand defrost control measures differential
temperatures 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 demand defrost
control’s: components are shown in figure 25.
S Demand defrost control connections, jumpers and
LED locations are shown in figure 25.
S Demand defrost control connections, jumpers and
LED descriptions are listed on table 9.
S Demand defrost control status, fault and lockout LEDs
are listed in table 10.
24V TERMINAL STRIP
CONNECTIONS (P2)
DIAGNOSTIC LEDS
(DS1 AND DS2)
PRESSURE SWITCH
CIRCUIT
CONNECTIONS
TEST PINS (P1)
Note − Component locations vary by board manufacturer.
SENSOR PLUG IN
(COIL AND
AMBIENT
SENSORS) (P4)
REVERSING VALVE
(O OUT)
DELAY PINS
(P5)
LOW AMBIENT
THERMOSTAT PINS
(P3)
DEFROST TERMINATION
PIN SETTINGS (P1)
Figure 25. Demand Defrost Control (A108)