Jeep 1997 Grand Cherokee Service Manual page 1303

ZG
DESCRIPTION AND OPERATION (Continued)
Module (BCM) receives the A/C select signal on cir-
cuit C90. After receiving the input, the BCM signals
the Powertrain Control Module (PCM) on the CCD
bus.
The A/C low pressure and high pressure switches
are wired in series and connect to ground on circuit
Z1. Circuit C3 from the PCM connects to the low
pressure switch. Circuit C21 connects the low pres-
sure switch to the high pressure switch. The high
pressure switch connects circuit C21 to ground cir-
cuit Z1. If the A/C low pressure and high pressure
switches are closed, the PCM senses the A/C request
signal on circuit C3.
After sensing the A/C request signal, the PCM sup-
plies ground for the coil side of A/C compressor clutch
relay on circuit C13. Circuit F99 from fuse 18 in the
PDC powers the coil side of the relay.
When the PCM grounds the A/C compressor clutch
relay, the contacts close and connect circuit F250
from fuse 21 in the PDC to circuit C2. Circuit C2
supplies power to the case grounded A/C compressor
clutch.
The A/C compressor clutch has a built-in diode.
The diode controls the induced voltage that results
from the magnetic field collapsing when the clutch
disengages. The diode provides a current path to pro-
tect other components and systems.
HELPFUL INFORMATION
Circuit A900 from fuse 3 in the PDC powers circuit
F250 through PDC fuse 21.
BLEND AIR DOOR MOTOR ACTUATOR—MANUAL
A/C-HEATER
The A/C-Heater control head contains a blend door
position sensor. The sensor is a variable resistor that
provides the blend door position input to the blend
door motor actuator on circuit C36.
Circuit F71 from fuse 12 in the junction block pow-
ers the actuator when the ignition switch is in the
RUN position. Circuit C34 splices to connect the
blend door actuator to ground circuit Z1.
AUTOMATIC TEMPERATURE CONTROL (ATC)
Several fuses supply power for the Automatic Tem-
perature Control (ATC) system. When the ignition
switch is in the RUN position, it connects circuit A1
from fuse 8 in the Power Distribution Center (PDC)
to circuit A22. Circuit A22 powers circuit F71
through fuse 12 in the junction block. Circuit F71
connects to the ATC module and the recirculation
door motor.
Circuit A7 from fuse 15 in the PDC powers circuit
F60 through fuse 20 in the junction block. Circuit
F60 supplies power to the ATC module.
8W - 42 AIR CONDITIONING/HEATER
When the ignition switch is in the START or RUN
position, it connects circuit A1 from PDC fuse 8 to
circuit A21. Circuit A21 powers circuit F99 through
fuse 18 in the PDC. Circuit F99 powers the coil side
of the A/C compressor clutch relay.
Circuit A19 from fuse 7 in the PDC connects to the
blower power module and to the coil and contact
sides of the high speed blower motor relay.
AUTOMATIC TEMPERATURE CONTROL (ATC)
MODULE
Circuit F71 supplies battery voltage to the Auto-
matic Temperature Control (ATC) module when the
ignition switch is in the RUN position. Circuit F60
from fuse 20 in the junction block connects to the
ATC module. Circuit F60 is HOT at all times. Circuit
Z4 provides ground for the ATC module.
Circuit E2 from the headlamp dimmer switch con-
nects to the ATC module.
The ATC module communicates with other vehicle
modules and controllers on the CCD bus. Circuits D1
and D2 for the CCD Bus connect to the ATC module.
AMBIENT TEMPERATURE SENSOR
The ambient temperature sensor is a variable
resistor. Circuit C8 provides the ambient tempera-
ture sensor signal to the ATC module. Circuit D41
provides ground for the sensor. Circuit D41 connects
to the ATC module.
IN-CAR TEMPERATURE SENSOR
The in-car temperature sensor is a variable resis-
tor. Circuit C10 provides the in-car temperature sen-
sor signal to the ATC module. Circuit D41 provides
ground for the sensor. Circuit D41 connects to the
ATC module.
SOLAR SENSOR
The solar sensor is a variable resistor. Circuit C47
from the ATC module connects to the solar sensor.
Circuit D41 provides ground for the sensor. Circuit
D41 connects to the ATC module.
A/C OPERATION—AUTOMATIC TEMPERATURE
CONTROL
When the A/C select switch in the Automatic Tem-
perature Control (ATC) control head closes circuit
C90 provides the A/C select signal to the Body Con-
trol Module (BCM). After receiving the input, the
BCM signals the Powertrain Control Module (PCM)
on the CCD bus.
The A/C low pressure and high pressure switches
are wired in series and connect to ground on circuit
Z1. Circuit C3 from the PCM connects to the low
pressure switch. Circuit C21 connects the low pres-
sure switch to the high pressure switch. The high
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