Allied 4SCU18LT Series, Installation And Maintenance Instructions Manual

Page: 17 / 25

OPERATION
Outdoor unit and indoor blower cycle on demand from the
room thermostat. When the thermostat blower switch is
moved to the ON position, the indoor blower operates
continuously.
System Diagnostic Module
4SCU18LT units contain a diagnostic module for
troubleshooting air conditioning system failures. By
monitoring and analyzing data from the compressor and
thermostat demand, the module can accurately detect the
cause of electrical and system related failure without any
sensors. If a system problem occurs, a flashing LED
indicator communicates the failure code.
LED Description
POWER LED (Green) indicates voltage is present at the
power connection of the module.
ALERT LET (Yellow) communicates an abnormal system
condition through a unique flash code. The ALERT LED will
flash a number of times consecutively, pause, and then
repeat the process. The number of consecutive flashes
correlates to a particular abnormal condition.
TRIP LED (Red) indicates there is a demand signal from
the thermostat but no current to the compressor is detected
by the module. The TRIP LED typically indicates the
compressor protector is open or may indicate missing supply
power to the compressor.
Interpreting the Diagnostic LEDs
When an abnormal system condition occurs, the diagnostic
module displays the appropriate ALERT and/or TRIP LED.
The yellow ALERT LED will flash a number of times
consecutively, pause, and then repeat the process. To
identify a flash code number, count the number of
consecutive flashes. Refer to Table 9 on page 18 and Table
10 on page 19 for information on the flash codes.
Every time the module powers up, the last ALERT LED flash
code that occurred prior to shutdown is displayed for 60
seconds. The module will continue to display the previous
flash code until the condition returns to normal or 24 VAC is
removed from the module. TRIP and ALERT LEDs flashing
at the same time means control circuit voltage is too low for
operation.
24 VAC Power Wiring
The diagnoctic module requires a constant nominal 24VAC
power supply. The wiring to the module’s R and C terminals
must be directly from the indoor unit or thermostat.
Thermostat Demand Wiring
The diagnostic module requires a thermostat demand signal
to operate properly. The thermostat demand signal input,
labeled Y on the module, should always be connected to
the compressor contactor coil so that when the coil is
energized, the demand signal input is 24VAC. When the
Coil is not energized, the demand signal input should be
less than 0.5VAC.
Miswired Module Codes
Depending on the system configuration, some ALERT flash
codes may not be active. The presence of safety switches
affects how the system alerts are displayed by the module.
Miswiring the diagnostic module will cause false LED codes.
Table 11 on page 19 describes LED opeation when the
module is miswired and what troubleshooting action is
required to correct the problem.
506254-01 Page 17 of 25 Issue 0912