ber of commutator pulses that correspond to
the desired door position. To maintain accu-
racy, the system recalibrates itself periodi-
cally at known zero and full travel conditions.
c
When a mode change is requested through
the MODE switch, the ATC drives the Rear
Mode Door Actuator for 10 seconds. The
Panel and the Floor/Mix/Defrost mode set-
tings direct rear air flow to the rear panel or
rear floor air outlets respectively. There is no
Bi-Level position for the rear mode door.
When the MODE switch setting is changed to
Bi-Level, the rear mode door remains in the
last known position (panel or floor). Unlike
the front door actuators, the Automatic Tem-
perature Control will not set Diagnostic
Trouble Codes for faults pertaining to the
Rear Mode Door or its electrical circuits.
•
uses Ambient Temperature Sensor data received
from the PCI Bus to display outside tempera-
ture.
c
This signal is filtered to compensate for hot
engine heating effects which can cause erro-
neous temperature display values.
The Rear Booster Fan Rear Control Switch:
•
provides desired rear booster fan speed input to
the ATC Module.
The Dual-Zone ATC system uses:
•
two, two-wire electric blend door actuators.
•
one, two-wire electric front mode door actuator.
•
one, two-wire electric recirculation door actua-
tor.
•
one, two-wire electric rear mode door actuator.
System Relays
•
The Integrated Power Module (IPM) houses and
provides power to the A/C Clutch Relay, EBL
Relay, Front Blower Motor Relay, and Rear
Booster Fan Relay.
Ambient Temperature Sensor
•
The ambient air temperature is monitored by the
NGC and displayed by the ATC. The NGC re-
ceives a hardwire input from the ambient tem-
perature sensor (ATS). The ATS is a variable
resistor that operates on a 5-volt reference signal
circuit hardwired from the NGC. The resistance
in the ATS changes as the outside temperature
rises or falls. The NGC senses the change in
reference voltage through the ATS resistor.
Based on the resistance of the ATS, the NGC is
programmed to correspond to a specific temper-
ature. The NGC stores and filters the ambient
temperature data and transmits this data to the
ATC via the PCI Bus. The ATS cannot be ad-
justed or repaired and, if faulty or damaged, it
must be replaced.
3.11.1.3
SYSTEM REVISIONS
Revisions to the HVAC system include:
•
moving the evaporator temperature sensor from
the expansion valve to the top of the HVAC
housing near the evaporator.
•
a new Manual Temperature Control (MTC) sys-
tem (available in certain models). Refer to 3.11.2
Manual Temperature Control (MTC) in this sec-
tion for additional information.
3.11.1.4
SYSTEM DIAGNOSTICS
Fault detection is through active and stored Diag-
nostic Trouble Codes (DTCs)
•
DTCs are displayed by the DRBIII
t.
•
Active DTCs are those which currently exist in
the system. The condition causing the fault must
be repaired in order to clear this type of DTC.
•
Stored DTCs are those that occurred in the
system since the ATC received the last
9clear
diagnostic info
9 message.
The AC Cooldown Test:
•
is actuated with the DRBIII
t.
•
checks A/C system performance based on evapo-
rator temperature sensor input.
•
will not run:
c
if ambient temperature is below 12°C (53°F).
c
if A/C pressure is below 50 psi or above 300
psi
c
if the evaporator temperature sensor/circuit
is open.
c
if the evaporator temperature sensor/circuit
is shorted.
•
will pass if the evaporator temperature drops
either 6.7°C (20°F) within two minutes at ambi-
ent temperatures of 21°C (70°F) or above, or if
the evaporator temperature drops 9.4°C (15°F)
within two minutes at ambient temperatures
below 21°C (70°F).
•
messages display on the DRBIII
t after running
this test.
c
These messages will clear after paging back
out of this test. Therefore, it is important to
note all of the AC Cooldown test messages
before doing so.
•
will cause the DELAY VF segment and Snow-
flake LED on the ATC to flash. If the test fails
the VF segment and LED will continue to flash
21
GENERAL INFORMATION