Boost Pump - Carrier CAPRI-280 Operation And Service Manual

2.3.1.2 Cooling/Reheat
Cooling and reheat require various combinations of
compressor unloading. Cooling is accomplished by
energizing the compressor and condenser fans,
opening the liquid solenoid valve and closing the
heating valve. Reheat opens the heat valve while
cooling. This allows for reduced capacity around
setpoint and de-humidification.
2.3.1.3 Heating
During heating, the liquid solenoid is closed to stop
cooling and the compressor and condenser fans will
shut down. The fan speed will vary based on the
temperature difference from setpoint. Heating will
not start until the engine coolant switch (ECS) closes.
The ECS is located on the block of the vehicle and is
provided by the OEM. It senses the engine coolant
temperature. The switch closes at 105F on
temperature rise. The switch prevents the circulation
of cooler air throughout the vehicle during initial
start-up.
2.3.2

Boost Pump

When the unit is in heat the boost pump relay is
energized. This signal is read by the bus multiplex
system to activate the boost pump.
2.3.3 Compressor Unloader Control
The unloader outputs control the capacity of the
compressor by energizing or de-energizing unloader
solenoid valves. Energizing a valve solenoid
de-activates a pair of compressor cylinders. The
Model 05G compressor has six cylinders. Four
cylinders are equipped with two sets of unloader
valves (UV1 and UV2), each controlling two
cylinders; this allows the compressor to be operated
with two, four or six cylinders. When the compressor is
off, the unloaders are de-energized immediately.
Whenever the compressor is changed from off to on,
the unloaders are forced energized for fifteen
seconds. After fifteen seconds, one unloader may be
de-energized, if required. Any subsequent changes
between energizing and de-energizing the unloaders
must be staged with a two second delay. Only one
unloader may change state at a time when staging is
required.
a. Suction Pressure
In addition to temperature control, the electric
unloaders will be used to prevent coil frosting:
1. Compressor Unloader UV1 Relay. When the
suction pressure decreases below 26 psig, the first
unloader is energized unloading the first
compressor cylinder bank (two cylinders); this
output will remain energized until the pressure
increases to above 34 psig.
2. Compressor Unloader UV2 Relay. When the
suction pressure decreases below 23 psig, the
second unloader is energized unloading the
T-283
second compressor cylinder bank (two cylinders);
this output will remain energized until the
pressure increases to above 31 psig.
b. Discharge Pressure
Head Pressure is also controlled by the unloaders:
1. Compressor Unloader UV1 Relay. When the
discharge pressure increases above 275 psig, the
first compressor unloader is energized; this
output will remain energized until the pressure
decreases below 220 psig. Staging is ignored for
energizing the unloader due to discharge
pressure overrides.
2. Compressor Unloader UV2 Relay. When the
discharge pressure increases above 285 psig, the
second unloader is energized; this output will
remain energized until the pressure decreases
below 225 psig.
2.3.4 Evaporator Fan Speed Selection
Each air conditioning unit is equipped with six two
speed fan motors. Temperature control is the primary
method of determining the fan speed selection. The
following table indicates relay operational status for
the various fan motor states.
Table 2-1. Main Evaporator Fan Speed
Relay Operation
STATE HIGH
SPEED
RELAY
Off Off
Low Off
High On
The evaporator fans will start in low speed and run in
high speed for cool and reheat modes. During heat
mode the fans will run in either high or low speed (see
figure 2-1).
Exceptions to the above are as follows:
a. In the event that the coolant temperature switch
is open, the evaporator fans are kept off during
heating.
2.3.5 Condenser Fan Speed Control
The condenser fans are energized when the
compressor clutch output is energized. The fans will
also be activated if a high pressure alarm has been
activated and operation has not been locked out
(refer to table 3-3).
2.3.6 Compressor Clutch Control
A belt driven electric clutch is employed to transmit
engine power to the air conditioning compressor.
De-energizing the clutch electric coil disengages the
clutch and removes power from the compressor. The
clutch will be engaged when in cooling. The clutch will
2-3
EVAP FAN
RELAYS
Off
On
On