Jeep Cherokee (XJ): Description and operation
DESCRIPTION A dual front airbag system is standard factory-installed
safety equipment on this model. The primary
passenger restraints in this vehicle are the standard
equipment factory-installed seat belts, which require
active use by the vehicle occupants. The airbag system
is a supplemental passive restraint that was
designed and is intended to enhance the protection
for the front seat occupants of the vehicle only when
used in conjunction with the seat belts. See the owner's
manual in the vehicle glove box for more information
on the features, use and operation of all of
the factory-installed passenger restraints, including
the airbag system.
NOTE: This group covers both Left-Hand Drive
(LHD) and Right-Hand Drive (RHD) versions of this
model. Whenever required and feasible, the RHD
versions of affected vehicle components have been
constructed as mirror-image of the LHD versions.
While most of the illustrations used in this group
represent only the LHD version, the diagnostic and
service procedures outlined can generally be
applied to either version. Exceptions to this rule
have been clearly identified as LHD or RHD, if a
special illustration or procedure is required.
The dual front airbag system consists of the following
components: This group provides complete service information
for the ACM, both airbag modules, and the clockspring.
Complete service information for the other
airbag system components can be located as follows: See the proper Diagnostic Procedures manual to
test or diagnose a problem with any component of
the airbag system. OPERATION The airbag system electrical circuits are continuously
monitored and controlled by a microprocessor
and software contained within the Airbag Control Module (ACM). The ACM also
contains an impact
sensor and a safing sensor, which are monitored by
the ACM to determine when an impact occurs that is
severe enough to require airbag system protection.
When a frontal impact is severe enough, the ACM
signals the inflator units of both airbag modules to
deploy the airbags.
An airbag indicator lamp in the instrument cluster
lights for about seven seconds as a bulb test, each
time the ignition switch is turned to the On or Start
positions. Following the bulb test, the airbag indicator
lamp is turned on or off by the ACM to indicate
the status of the airbag system. If the airbag indicator
lamp comes on at any time other than during the
bulb test, it indicates that there is a problem in the
airbag system circuits. Such a problem may cause
the airbags not to deploy when required, or to deploy
when not required.
The driver side airbag module includes an inflatable
airbag and an inflator unit behind a trim cover
in the hub area of the steering wheel. The passenger
side airbag module includes a second inflatable airbag
and an inflator unit behind an airbag door in the
instrument panel above the glove box.
During a frontal vehicle impact, the knee blockers
work in concert with properly adjusted seat belts to
restrain the driver and front seat passenger in the
proper position for an airbag deployment. The knee
blockers also work to absorb and distribute the crash
energy from the driver and front seat passenger to
the structure of the instrument panel. The driver
side knee blocker is a stamped metal reinforcement
located behind the instrument panel steering column
opening cover. The passenger side knee blocker is
integral to the glove box door.
Following are general descriptions of the major
components in the airbag system. WARNING: DESCRIPTION The driver side airbag module protective trim cover
is the most visible part of the driver side airbag system.
The driver side airbag module is mounted
directly to the steering wheel. Located under the airbag
module trim cover are the horn switch, the
folded airbag cushion, and the airbag cushion supporting
components. The resistive membrane-type
horn switch is secured with heat stakes to the inside
surface of the airbag module trim cover, between the
trim cover and the folded airbag cushion.
The driver side airbag module cannot be repaired,
and must be replaced if deployed or in any way damaged.
The driver side airbag module trim cover and
the horn switch are available as a unit for service
replacement. OPERATION The driver side airbag module includes a stamped
metal housing to which the cushion and an inflator unit are attached and sealed.
The conventional pyrotechnic-
type inflator assembly is mounted to studs on
the back of the airbag module housing. The inflator
seals the hole in the airbag cushion so it can discharge
the gas it produces directly into the cushion
when supplied with the proper electrical signal. Following
an airbag deployment, the airbag cushion
quickly deflates by venting this gas towards the
instrument panel through the porous fabric material
used on the steering wheel side of the airbag cushion.
The protective trim cover is fitted to the front of
the airbag module and forms a decorative cover in
the center of the steering wheel. The inside of the
trim cover has locking blocks molded into it that
engage a lip on the airbag module metal housing.
Two stamped metal retainers then fit over the inflator
mounting studs on the back of the airbag module
housing and are engaged in slots on the inside of the
cover, securely locking the trim cover into place. The
trim cover will split at predetermined breakout lines,
then fold back out of the way along with the horn
switch upon airbag deployment. DESCRIPTION The passenger side airbag door on the instrument
panel above the glove box is the most visible part of
the passenger side airbag system. Located under the
airbag door are the passenger side airbag cushion
and the airbag cushion supporting components.
The passenger side airbag module includes an
extruded aluminum housing within which the cushion
and inflator are mounted and sealed. Two
stamped metal brackets, one on each end of the housing,
enclose the cushion and inflator and also serve
as the mounting brackets for the module. The two
mounting brackets at the top front of the airbag module
are secured with screws to the top of the instrument
panel structural support beneath the
instrument panel top cover. The two mounting brackets
at the bottom front of the airbag module are
secured with screws to the instrument panel structural
support over the glove box.
Following a passenger side airbag deployment, the
passenger side airbag module and the passenger side
airbag door must be replaced. The passenger side airbag
module cannot be repaired, and must be replaced
if deployed or in any way damaged. The passenger
side airbag door is available as a separate service
item. OPERATION The hybrid-type inflator assembly includes a small
canister of highly compressed argon gas. The inflator
seals the hole in the airbag cushion so it can discharge
the gas it produces directly into the cushion
when supplied with the proper electrical signal. Following
an airbag deployment, the airbag cushion
quickly deflates by venting this gas through the
porous fabric material used on each end panel of the
airbag cushion.
The molded plastic passenger side airbag door is
secured to extruded tabs at the top and bottom rear
of the airbag module housing by keyed openings in
the upper and lower mounting flange returns of the
airbag door. The upper and lower airbag door mounting
flanges are then secured to the instrument panel
structural support and the upper glove box opening
reinforcement with screws. The airbag door has predetermined
breakout lines concealed beneath its decorative
cover. Upon airbag deployment, the airbag
door will split at the breakout lines and the door will
fold back over the top of the instrument panel, out of
the way. DESCRIPTION The Airbag Control Module (ACM) is secured with
nuts to weld-studs on the ACM mounting bracket.
The ACM mounting bracket is secured with screws to
the floor panel transmission tunnel underneath the
center floor console and behind the park brake mechanism
in the passenger compartment of the vehicle.
The ACM contains an electronic microprocessor, an
electronic impact sensor, an electromechanical safing
sensor, and an energy storage capacitor.
The ACM cannot be repaired or adjusted and, if
damaged or faulty, it must be replaced. OPERATION The microprocessor in the ACM contains the airbag
system logic. The airbag system logic includes
On-Board Diagnostics (OBD), and the ability to communicate
with the instrument cluster circuitry over
the Chrysler Collision Detection (CCD) data bus to
control the airbag indicator lamp. The microprocessor
continuously monitors all of the airbag system electrical
circuits to determine the system readiness. If
the ACM detects a monitored system fault, it sends
messages to the instrument cluster over the CCD
data bus to turn on the airbag indicator lamp. Refer
to Instrument Cluster in the proper section of
Group 8E - Instrument Panel Systems for more information
on the airbag indicator lamp.
One electronic impact sensor is used in this airbag
system. The impact sensor is an accelerometer that
senses the rate of vehicle deceleration, which provides
verification of the direction and severity of an
impact. The impact sensor is calibrated for the specific
vehicle, and is only serviced as a unit with the ACM. A pre-programmed decision
algorithm in the
ACM microprocessor determines when the deceleration
rate as signaled by the impact sensor indicates
an impact that is severe enough to require airbag
system protection. When the programmed conditions
are met, the ACM sends an electrical signal to deploy
the airbags.
In addition to the electronic impact sensor, there is
an electromechanical sensor within the ACM called a
safing sensor. The safing sensor is a normally open
series switch located in the airbag deployment circuit
of the ACM. This sensor detects impact energy of a
lesser magnitude than the electronic impact sensor,
and must be closed in order for the airbags to deploy.
The ACM also contains an energy-storage capacitor.
This capacitor stores enough electrical energy to
deploy the airbags for up to one second following a
battery disconnect or failure during an impact. The
purpose of the capacitor is to provide airbag system
protection in a severe secondary impact, if the initial
impact has damaged or disconnected the battery, but
was not severe enough to deploy the airbags. DESCRIPTION The clockspring assembly is secured with two integral
plastic latches onto the steering column lock
housing near the top of the steering column behind
the steering wheel. The clockspring is used to maintain
a continuous electrical circuit between the fixed
clockspring wire harness on the steering column and
several electrical components that rotate with the
steering wheel. The rotating components include the
driver side airbag module, the horn switch and, if the
vehicle is so equipped, the vehicle speed control
switches.
The clockspring cannot be repaired. If the clockspring
is faulty, damaged, or if the driver side airbag
has been deployed, the clockspring must be replaced. OPERATION The clockspring assembly consists of a plastic case
which contains a flat, ribbon-like, electrically conductive
tape that winds and unwinds like a clockspring
with the steering wheel rotation. The electrically conductive
tape consists of several fine gauge copper
wire leads sandwiched between two narrow strips of
plastic film.
Like the clockspring in a timepiece, the clockspring
tape has travel limits and can be damaged by being
wound too tightly. To prevent this from occurring, the
clockspring is centered when it is installed on the
steering column. Centering the clockspring indexes
the clockspring tape to other steering components so
that it can operate within its designed travel limits.
However, if the clockspring is removed for service or
if the steering column is disconnected from the steering
gear allowing the clockspring tape to change position
relative to the other steering components, it
must be re-centered following completion of the service
or it may be damaged. Refer to Clockspring
Centering in the Adjustments section of this group
for the proper centering procedures.
Service replacement clocksprings are shipped precentered
and with a locking pin installed. This locking
pin should not be removed until the clockspring
has been installed on the steering column. If the
locking pin is removed before the clockspring is
installed on a steering column, the clockspring centering
procedure must be performed.Airbag system
Driver side airbag module
Passenger side airbag module
Airbag control module
Clockspring
Other materials:
Description and operation
Power distribution system
DESCRIPTION
This group covers the various standard and
optional power distribution components used on this
model. The power distribution system for this vehicle
consists of the following components:
Power Distribution Center (PDC)
Junction Block (JB).
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