1-7
1.5 SYSTEM OPERATING CONTROLS AND
COMPONENTS
a. Temperature Controller (Thermostat)
The Temperature Controller is a thermostat that
senses and controls the vehicle interi or air temperature.
The desired interior temperature, setpoint of the
controller is manually set with the temperature selector
which is located in the electrical control panel. (See
Figure 1-3.) The controller’s temperature sensor
monitors the bus’ interior temperature at the return air
sectionof theunitandcontrols theoperatingfunctionof
the system to maintain setpoint.
The controller regulates the operation cycles of the
unit with four interior relays which activate the cool or
heatandunloadedorloadedfunctionsofthesystem
relative to deviations from the thermostat setpoint.One
relay controls the operation of Unloader Valve #1
(UV1). Another relay controls the function of Unloader
Valve #2 (UV2). Another activates the cool cycle and
another relay activatesthe heat cycle. Refer to Section2
for description of operational control sequences for the
various operational modes of operation.
b. Manual Switches
A/C Switch (ACS)
The A/C Switch (ACS) selects the Cooling (Air
Conditioni ng)orHea tingmodeofoperation.Thisswitch
is located on the driver’s control panel. The switch is
placed in the COOL position to energize A/C Relay
(ACR) for cooling mode of operation and i n t he HEAT
position to energi ze the Heat Relay (HR) for heating
mode.IntheCOOLor HEATposition,andwiththeRun
ControlSwitches(RCS1 and RCS2) in the ON position,
24-voltsisalsosuppliedtoterminalboardTC,terminal
no. 8.
Reheat/Cycle Switch (RHCS)
TheReheat/CycleSwitch(RHCS)allowsoperatorto
selecteitherreheatorcyclingclutchcontrolaction.When
placed in the CYCLING position, the unit cycles in the
cooling mode of operation. When in the REHEAT
position, unit cycles from cooling only mode to cooling
with reheat mode.
Run Control Switch (RCS)
One Run Control Switch (RCS1) is located in the
electrical control panel in the return ai r opening of the
unit and the other (RCS2) is located on top of the
electricalpanel,whichisaccessedthroughtheevaporator
coveronthebusroof. Theseswitches,whenplacedinthe
OFF position, will prevent the unit from starting when
servicing the unit and when placed in the ON position,
allowspowertobeappliedtotheTerminalBlock(TC)for
operation of the control system for heating, cooling or
cooling with reheat modes of operation.
c. Thermal Switches
Low Ambient Thermostat (LATH)
TheLowAmbientThermostat(LATH)monitorsthe
vehicles outside t emperature. The switch opens a set of
contacts at 45 5_F (7.2 0.35 _C) and closes at 55
5_F(12.80.35_C).Whentheoutsidet emperatureis
belowtheopensettingof theswitch,theswitchopensthe
circuit to disengage the clutch and stop the compressor
andstopthecondenserfans.
d. Pressure Switches
Condenser Fan Speed Switch (CFS)
The Condenser Fan Speed Switch (CFS) senses
refrigerantdischargepressureandcontrolcondenserfan
speed. If refrigerant discharge pressure rises to the CFS
cutout setting, the switch will open to de-energize the
Condenser Speed Relay (CSR); thi s will close a set of
normally closedCSR1contactsandcausetheCondenser
FanMotors (CM1, CM2 and CM3) to runa t highspeed.
When the pressure drops to the CFS cut-in setting, the
switch will close to energize CSR, which will open the
CSR1 contacts and cause the Condenser Fan Motors
(CM1, CM2 and CM3) t o run at low speed. Refer to
paragraph 1.2 for switch settings.
Unloader Pressure Switch #2(UPS2)
The Unloader Pressure Switch #2 (UPS2) controls
unloader operation during the cooling mode of
operation. The switch closes on a drop in evaporator
suction pressure to energize Unloader Valve #1 (UV1).
Energizing UV1 will place the compressor in f our
cylinder ( semi-loaded) operation. As pressure rises, the
switch will re-open. Refer to 1.2 for switch settings.
e. Relays
Fault Relay (FR)
The Fault Relay (FR) is located in the electrical
control panel. The FR relay is energized during initial
start-up,throughtheclosedcontactsoftheHighandLow
PressureSwitches(HPSandLPS),toactivatethecooling
circuit components. If the High or Low Pressure Switch
(HPS or LPS) opens during normal operation due to
unsafe operating conditions, the Fault Relay (FR) will
de-energize and open a set of normally open contacts t o
deactivate the cooling circuit control components.
Another set of normally closed FR contacts will close to
energize therelay A/CStop Relay (ACSR), whichcloses
a set of normally open contacts to light the stop light.
A/C Stop Relay (ACSR)
The A/C Stop Relay (ACSR) is located in the
electrical control panel. The ACSR is a time delay relay
which illuminates the stop light when energized by the
Fault Relay (FR).
Torestarttheunitandturnthestoplightoff,theA/C
Switch (ACS) must be toggled to OFF and then to
COOL.
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