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a. Temperature Control
The unloaders are operaed as follows for temperature
control:
1 Compressor Unloader UV1. When return air temper-
ature falls to less than 2°F (1.1°C) above set point
unloader UV1 is energized. If temperature rises to
greater than 3°F (1.7°C) above set point, UV1 will be
de--energized to place the compressor at 100% ca-
pacity.
2 Compressor Unloader UV2. When return air temper-
ature falls to less than 1°F (0.6°C) above set point
unloader UV2 is energized. If temperature rises to
greater than 2°F (1.1°C) above set point, UV2 will be
de--energized to place the compressor at 66% ca-
pacity.
b. Suction Pressure
The unloaders are used to control suction pressure and
thereby prevent coil frosting:
1 Compressor Unloader UV1. When the suction pres-
sure decreases below 26 psig (1.77 bar), unloader
UV1 is energized unloading a cylinder bank (two cyl-
inders); this output will remain energized until the
pressure increases to above 34 psig (2.31 bar).
2 Compressor Unloader UV2. When suction pressure
decreases below 23 psig (1.56 bar) unloader UV2 is
energized unloading the second compressor cylin-
der bank; this output will remain energized until the
pressure increases to above 31 psig (2.11 bar).
c. Discharge Pressure
Discharge pressure is also controlled by the unloaders:
1 Compressor Unloader UV2. When the discharge
pressure increases above 320 psig, unloader UV2 is
energized; this output will remain energized until the
pressure decreases below 250 psig.
2.3.5 Condenser Fan Control
The condenser fans are energized when the
compressor clutch output is energized. The fans are
started in low speed and will remain in low speed until
the discharge pressure increases to 275 psig (R--134a).
The fans will remain in high speed until discharge
pressure decreases below 220 psig (R--134a).
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 and disengaged
when the system is off, vent, or during high and low
pressure conditions.
The clutch coil is prevented from engagement when the
ambient temperature is below ambient lockout setpoint.
The clutch coil will be de--energized if the discharge
pressure rises to the cutout setting of the compressor
mounted high pressure switch. The clutch coil will
energize when the discharge pressure falls to the reset
point of the compressor mounted high pressure switch.
2.3.7 Liquid Line Solenoid Control
The liquid line solenoid is energized (open) when the
compressor clutch is energized and de--energized
(closed) when the clutch is not.
T--336
2.4 SEQUENCE OF OPERATION
Operation begins when the A/C mode switch (ACMS) is
placed in the HIGH or LOW position. This provides:
S
a signal the vehicle to apply 24V power from the
vechcle alternator to the ALT+ terminal (Refer to wiring
schematic, Section 5.)
S
energizes the power realy (PWRR) to provide
power to the speed contacts of the ACMS, power to
energize the A/C relay (ACR) and power to the
thermostat.
If the ACMS is in the LOW speed postiion, power flows
through evaporator fan motor EM1, the evaporator
speed relay ESR1 normally closed contacts and
evaporator fan motor EM2 to energize the fans in series
(low speed).
If the ACMS is in the HIGH position, ESR1 & ESR2 are
both energized. With these realys energized, power
flows through EM1 and the ESR1 normally open contact
(now closed) direct to ground and through the ESR2
eveporator fan relay contact (now closed) and EM2
direct to ground, placing both fans in high speed.
With the evaporator fans in operation, the system is in
the VENT mode.
Before the system can enter the COOL mode, the high
pressure switch (HPS) and low pressure switch (LPS)
must be closed. With these switches closed, the fault
relay (FR) is energized. Energizing FR closes a set of
normally open contacts to allow a potential circuit to the
clutch (CL) and opens a set of normally closed contacts
to the AC stop realys ACR1 & ACR2 and STOP light.
Also, the ambient temperature must be sufficiently high
enough to close the low ambient temperature
thermostat (LATH).
If the thermostat is calling for cooling, power flows
through the normally open ACR contacts (previously
closed) and LATH (or jumper) and the following
sequence occures:
S
If condenser fan speed switch CFS2 is closed, the
condenser speed relay (CSR) is energized and power
for the fans flows through its normally open contact (now
closed) to place the fans in high speed. If condenser fan
speed switch CFS2 is open, CSR is de--energized and
power for the fans flows through a resistor (RES) to
place the fans in low speed.
S
condenser fan relay CFR1 is energized. Energizing
CFR1 provides power to two condenser fan motors (CF)
S
if CFS1 is colsed, relays CFR2 & CFR3 are
energized. Energizing CFR2 & CFR3 provides power to
two (DC12174) or three (DC12175) additional
condenser fan motors
S
the liquid line solenooid valve (LLS) is energized
S
the clutch relay (CR) is energized. Energizing relay
CR closes a set of contacts to energize the compressor
clutch.
The system is now operating in the COOL mode. The
thermostat will open and close (energizing and
de--energizing) the compressor, condenser fans and
LLS as required to maintain desired temperature.
2--2
05/09
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