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1.5
SYSTEM OPERATING CONTROLS AND
COMPONENTS
a. Electronic Thermostat (TH)
The electronic thermostat senses and controls the
vehicle interior air temperature.
The desired interior temperature set point of the
controller is manually set with the temperature
selector located on the electrical control panel.
(See Figure 1-3.) The thermostat's temperature
sensor monitors the bus interior temperature at
the return air section of the unit and controls the
operation function of the system to maintain
temperature at the desired set point.
The thermostat regulates the operation of the unit
with three interior switches (LOAD/UNLOAD,
LOW/HIGH
and
LOAD/UNLOAD switch controls the compressor
unloaders; the LOW/HIGH switch controls the
evaporator fan speed; the HEAT/COOL switch
establishes heating or cooling operating mode.
b. Manual Switches
A/C Mode Switch (ACMS)-Supplied by OEM
When the A/C Mode Switch (ACMS) is placed in
the ON position, the climate control system
responds to the mode (HEAT, COOL or VENT)
determined by the electronic thermostat.
Evaporator Speed Switch (ESS) - Optional
The evaporator speed switch (ESS), if installed, is
located on the driver's control panel. The ESS,
when in the LOW or HIGH position, overrides the
thermostat. When the switch is in the HIGH
position, the evaporator fans are operated at high
speed at all times. When in the LOW position, the
fans are operated at low speed at all times. When
in the AUTO position, evaporator fan speed is
controlled by the thermostat.
c. Thermal Switches
Water Temperature Switch (WTS)-Supplied by
OEM
The water temperature switch (WTS) is located on
the engine block of the vehicle and senses the
engine coolant temperature. The ECS is a
normally open switch which closes on a
temperature rise to complete a path to energize
the control circuits. If this switch is used, and the
water temperature is below the switch set point,
then the evaporator fan motors will not operate
when the thermostat is calling for heat.
HEAT/COOL).
The
Condenser Motor Overload (COL1 and COL2)
Each condenser fan motor is equipped with an
internal thermal protector switch. If excessively
high motor temperature exists, the COL switch will
open
to
de-energize
condenser fan relay (CFR1 or CFR2); this will stop
the corresponding condenser fan motor.
Evaporator Motor Overload (EOL1 and EOL2)
Each evaporator fan motor is equipped with an
internal thermal protector switch embedded in the
motor windings. If excessively high
temperature exists, the EOL switch will open to
de-energize the corresponding evaporator fan
relay (EFR1 or EFR2); this will stop the
corresponding evaporator fan motor.
Low Ambient Thermostat (LATH)
The low ambient thermostat (LATH) monitors
temperature outside the vehicle. The switch
opens at 45 5_F (7.3
5_F (12.9
temperature is below the open setting of the
switch, the switch opens to stop the compressor
clutch and condenser fans. The low ambient
thermostat is mounted on the condenser coil.
d. Pressure Switches
Condenser Fan Switch (CFS)
The
condenser fan
refrigerant discharge line pressure to control
condenser fan speed. If the condenser coil
discharge pressure rises to the CFS cutout
setting, the switch will open to de-energize the
condenser speed relay (CRS1); this will cause the
condenser fan motors (CM1 and CM2) to run at
high speed. When the pressure drops to the CFS
cut-in setting, the switch will close, energizing
CSR1 to run the condenser fan motors at low
speed. Refer to section 1.2 for switch settings.
Unloader Pressure Switches (UPS1, UPS2 and
UPS3)
The unloader pressure switches UPS1, UPS2 and
UPS3 control unloader operation during A/C
mode of operation.
Unloader pressure switches UPS1 and UPS2
close on pressure drop to energize unloader
valves (UV1 and UV2, respectively). Energizing
UV2 will place the compressor in four cylinder
operation.
Energizing
compressor in 2 cylinder operation. As pressure
1-7
the
corresponding
2.8 _C) and closes at 55
2.8 _C). When the outside
switch
(CFS) senses
UV1
will
place
motor
the
T-288
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