Page 6
Defrost Control Timing Pins
Each timing pin selection provides a different accumu-
lated compressor run period during one thermostat run
cycle. This time period must occur before a defrost cycle
is initiated. The defrost interval can be adjusted to 30, 60
or 90 minutes. See figure 9. The defrost period is a maxi-
mum of 14 minutes and cannot be adjusted. If no timing
is selected, the control defaults to 90 minutes.
DEFROST CONTROL BOARD
FIGURE 9
24V TERMINAL
STRIP
CONNECTIONS
DIAGNOSTIC
LEDs
PRESSURE SWITCH
SAFETY CIRCUIT
CONNECTIONS
DEFROST
INTERVAL
TIMING PINS
AMBIENT
THERMISTOR
CONNECTION
NOTE− There is an internal jumper between
inside PS1 and PS2 terminals.
SERVICE LIGHT
CONNECTION
NOTE − COMPONENT LOCATIONS WILL VARY
WITH BOARD MANUFACTURER
A TEST option is provided for troubleshooting. When the
jumper is placed across the TEST pins, the timing of all
functions is reduced by a factor of 128. For example, a 30
minute interval during TEST is 14 seconds and the 14 min-
ute defrost is reduced to 6.5 seconds.
The TEST mode may be started at anytime. If the jumper is
in the TEST position at power−up or for longer than five min-
utes, the control will ignore the TEST selection and will de-
fault to a 90 minute interval. In order to test defrost cycle,
defrost thermostat must be closed or jumpered. Once de-
frost is initiated, remove jumper immediately. Failure to re-
move jumper will reduce defrost cycle to seconds.
Time-Delay
A 5-minute timed-off delay protects the compressor from
short-cycling when there is an interruption in power to the
unit or when a pressure switch resets.
Pressure Switch Safety Circuits
The defrost control incorporates a safety circuit that allows
the application of an additional pressure switch. The unit’s
high pressure switch (S4) is factory-wired into this circuit.
See figure 9. PS1 and PS2 terminals are wired in series
with a jumper internal to the control board. This feature is
available on all units.
During one demand cycle, the defrost control will lock out
the unit on the third instance that the unit goes off on any
auto−reset pressure switch wired to this circuit. In addition,
the diagnostic LEDs will indicate a locked out pressure
switch after the third occurrence of an open pressure
switch. See table 4. The unit will remain locked out until 24
volt power is broken to terminal R" on the defrost board
and then remade.
The PS2 safety circuit terminals are connected to the com-
pressor thermostat. An optional loss of charge switch may
be field-installed by connecting it in series with the other
switches. See unit wiring diagram.
Diagnostic LEDs
The defrost board uses two LEDs for diagnostics. The
LEDs flash a specific sequence according to the condi-
tion.
TABLE 4
DEFROST CONTROL BOARD DIAGNOSTIC LED
MODE
LED 1
LED 2
Normal Operation/
Power to board
Flash together with
LED 2
Flash together with
LED 1
Time Delay
To Protect Compressor
Alternating Flashes
with LED 2
Alternating Flashes
with LED 1
Pressure Switch Open
Off
On
Pressure Switch Lockout
On
Off
Board Malfunction
On
On
Ambient Thermistor & Service Light Connection
The defrost control board provides terminal connections
for the ambient thermistor and a service light. The thermis-
tor compensates for changes in ambient temperature
which might cause thermostat droop. The service light
thermostat provides a signal which activates the room
thermostat service light during periods of inefficient opera-
tion.
