Unit Description
24
Pilot Solenoid Relay 3 (PS3)
When the Remote Fan Relay 3 is energized,
battery voltage energizes the liquid line solenoid
for the remote evaporator. It is controlled by the
second compartment thermostat.
Defrost Timer
The Defrost Timer automatically initiates the
Defrost Cycle at the preset time intervals. The
time interval can be set with the Red Round
Selector and the A and B Switches.
The Defrost Timer is powered directly by truck
battery, this means that the Defrost Timer is
always counting (while the Defrost Termination
Switch is closed) even when the unit is turned off.
Clutch Timer
The clutch timer is located on the P.C. Board. It
delays stand-by compressor clutch activation for
12 to 15 seconds when the stand-by electric motor
is first started. The clutch timer LED indicator
(P.C. Board) will be lit during this period of time.
EFR Selector
The EFR selector allows the user to choose if the
evaporator fan keeps running or not in Null mode.
If the jumper is between 1 and 2 the evaporator
fan will run in Null mode. If the jumper is
between 2 and 3 the fan will not run in the Null
mode.
Unit Operation
These units cycle between Cool and Null to
maintain the box temperature at the thermostat
setpoint. Heat is available as an option. The
operating modes are: Cool, Null, Heat (optional)
and Defrost.
The thermostat controls the operation of the unit
by energizing and de-energizing the Power Relay
(PR). When PR is energized it energizes the
evaporator fans, the condenser fans, and the
compressor clutch (or the compressor motor
contactor during electric stand-by operation). On
TC units PR is energized by either thermostat and
is de-energized when neither thermostat is calling
for cool.
The condenser fan is also controlled by the
condenser fan pressure switch (CFPS) on R-134a
units. This normally open switch monitors the
compressor discharge pressure. When the
discharge pressure rises to 180 psi (1241 kPa) the
switch closes and energizes the condenser fan.
When the discharge pressure drops below 130 psi
(896 kpa) the switch opens and de-energizes the
condenser fan.
Thermostat Operation
Cool
The thermostat energizes the (PR) at box
temperatures higher than 5.4 F (3.0 C) above
setpoint. The thermostat keeps PR energized
while the box temperature is higher than setpoint.
The fans and the compressor run and the unit
cools.
Null
The thermostat de-energizes PR at box
temperatures lower than setpoint. The thermostat
keeps PR de-energized while the box temperature
is lower than 5.4 F (3.0 C) above the setpoint
temperature. When PR is de-energized and the
unit does not operate. On TC units PR is
de-energized only when the condition is required
from both thermostats.
Temperature
Drop
Temperature
Rise
3.0 C (5.4 F)
Above Setpoint
Setpoint
NULL
COOL
Figure 4: Thermostat Algorithm
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