2-2
Speed Relay (CSR) opens a set of normally closed
contacts to switch to low speed condenser fan operation.
At this point, the thermostat also applies power to plug
J1, pin no.6 and pin no. 5 to energize Unloader Valves #1
and #2 (UV1 and UV2), which unloads two banks of
compressor cylinders . The compressor now operates in
unloaded (2-cylinder) cool mode.
When the vehicle’s interior temperature rises to
+3˚F above the thermostat setpoint, the thermostat
switches from high speed unloaded (2-cylinder) cool
operation to high speed semi-loaded (4-cylinder) cool
operation. In the semi-loaded cool mode (See Figures 2-1
and 2-6.), the thermostat removes power from plug J1,
pin no. 6., which de-energizes Unloader Valve #1 (UV1)
to load two additional compressor cylinders. The
compressor now operates in semi-loaded (4-cylinder)
cool mode. However, Unloader Pressure Switch (UPS2)
can override the thermostat demand for semi-loaded
operation by closing a set of contacts to energize
Unloader Valve UV1, unloading a bank of compressor
cylinders when refrigerant suction pressure drops below
its setpoint.
When the vehicle interior temperature rises to +5˚F
above the thermostat setpoint, the thermostat switches
from high speed semi-loaded cool operation to high
speed fully loaded cool operation. In the high speed fully
loaded cool mode (See Figures 2-1 and 2-7), the
thermostat removes power from plug J1, pin no. 5, which
de-energizes Unloader Valve #2 (UV2) to load the two
remaining compressor cylinders. The compressor now
operates in fully loaded (6-cylinder) cool mode.
On a falling temperature, the controller sequence
changes in reverse to the above. See Figure 2-1 for
controller sequence on falling temperature.
2.3.2 Cooling (Air Conditioning) Cycle Operation --- Re-
heat Control (See Figure 2-2.)
When the vehicle interior temperature falls to -1˚F
below the thermostat setpoint and the Reheat/Cycle
Switch (RHCS) is set to the REHEAT position, the
thermostat switches from cooling only operation to
cooling with reheat. (See Figures 2-2 and 2-8.) Under
normal conditions (i.e., with ACR, LATH, FTS, CFS and
LVCO contacts closed), and with the ACS switch in the
COOL position and the RHCS switch in the REHEAT
position, the cooling circuit is always energized (i.e.,
clutch relay and condenser fans are energized). In this
mode, the thermostat cycles the reheat control circuit to
maintain vehicle interior temperature. When reheat is
required on a falling temperature, at -1˚F below the
setpoint, the thermostat removes power from plug J1, pin
no. 3, which de-energizes Heat Relay #2 (HR2) to close
normally closed HR2 contacts and energize Reheat
Coolant Valve (RCV) solenoid and Boost Pump Relay
(BPR); this activates the reheat control circuit.
Energizing the Boost Pump Relay closes normally open
BPR contacts to start the Boost Pump Motor (BPM).
Energizing the reheat coolant valve solenoid coil opens
the valve allowing flow of engine coolant to the heater
coil for heating.
On a rising temperature to +1˚F above the
thermostat setpoint, the thermostat switches from high
speed cool with reheat to high speed cool only mode.
Figure 2-1. Temperature Controller Sequence
During Cooling Mode
FALLING
TEMPERATURE
RISING
TEMPERATURE
1˚F ABOVE
SETPOINT
SETPOINT
SETPOINT
3˚F ABOVE
SETPOINT
HIGH SPEED VENT
HIGH SPEED
FULLY LOADED
COOL
5˚F ABOVE
SETPOINT
3˚F ABOVE
SETPOINT
1˚F ABOVE
SETPOINT
-1˚F BELOW
SETPOINT
HIGH SPEED,
SEMI-LOADED
COOL
HIGH SPEED
UNLOADED
COOL
Figure 2-2. Temperature Controller Sequence
During Cooling with Reheat Mode
FALLING
TEMPERATURE
1˚F ABOVE
SETPOINT
SETPOINT
SETPOINT
3˚F ABOVE
SETPOINT
HIGH SPEED
FULLY LOADED
COOL
5˚F ABOVE
SETPOINT
3˚F ABOVE
SETPOINT
1˚F ABOVE
SETPOINT
HIGH SPEED,
SEMI-LOADED
COOL
HIGH SPEED
UNLOADED
COOL
RISING
TEMPERATURE
HIGH SPEED
UNLOADED
COOL WITH
REHEAT
-1˚F BELOW
SETPOINT