SIR System Description and Operation
SIR System Overview
The supplemental inflatable restraint (SIR) system supplements
the protection offered by the seat belts. The SIR system contains
an inflatable restraint sensing and diagnostic module (SDM), air
bags, seat belt pretensioners (anchor and retractor), and impact
sensors. The SDM determines the severity of a collision with the
assistance of side impact sensors located at strategic points on
the vehicle. When the SDM detects a collision, the SDM will process
the information provided by the sensors to further support air bag
or pretensioner deployment. The SDM will deploy the air bags and
pretensioners if it detects a collision of sufficient force. If the
force of the impact is not sufficient to warrant air bag
deployment, the SDM may still deploy the seat belt pretensioners.
The SDM contains a sensing device that converts vehicle velocity
changes to an electrical signal. The SDM compares these signals to
values stored in memory. If the signals exceed a stored value, the
SDM will determine the severity of the impact and either cause
current to flow through the frontal deployment loops deploying the
frontal air bags and pretensioners, or it will deploy the
pretensioners only. The SDM continuously monitors the deployment
loops for malfunctions and illuminates the SIR system AIR BAG
indicator if a fault is detected.The SDM performs continuous
diagnostic monitoring of the SIR system electrical components. Upon
detection of a circuit malfunction, the SDM will set a DTC and
inform the driver by illuminating the SIR system AIR BAG indicator.
The steering column and knee bolsters are designed to absorb energy
and compress during frontal collisions in order to limit leg
movement and decrease the chance of injury to the driver and
passenger.
SIR System AIR BAG Indicator
The SIR system AIR BAG indicator, located in the instrument
cluster, is used to notify the driver of SIR system malfunctions
and to verify that the inflatable restraint sensing and diagnostic
module (SDM) is communicating with the instrument cluster. When the
ignition is turned ON, the SDM is supplied with ignition positive
voltage. The instrument cluster will momentarily turn on the SIR
system AIR BAG indicator. While the indicator is on, the SDM
conducts tests on all SIR system components and circuits. If no
malfunctions are detected the SDM will communicate with the
instrument cluster through the serial data circuit and command the
SIR system AIR BAG indicator OFF. The SDM provides continuous
monitoring of the air bag circuits by conducting a sequence of
checks. If a malfunction is detected the SDM will store a
diagnostic trouble code (DTC) and command the instrument cluster to
illuminate the SIR system AIR BAG indicator via serial data. The
presence of a SIR system malfunction could result in non-deployment
of the air bags or deployment in conditions less severe than
intended. The SIR system AIR BAG indicator will remain ON until the
malfunction has been repaired.
Inflatable Restraint Sensing and Diagnostic Module (SDM)
The inflatable restraint sensing and diagnostic module (SDM) is
a microprocessor and the control center for the supplemental
inflatable restraint (SIR) system. The SDM contains internal
sensors along with external impact sensors, mounted at strategic
locations on the vehicle. In the event of a collision, the SDM
compares the signals from the internal and external impact sensors
to a value stored in memory. When the generated signals exceed the
stored value, the SDM will cause current to flow through the
appropriate deployment loops to deploy the air bags. The SDM
records the SIR system status when a deployment occurs and
illuminates the SIR system AIR BAG indicator located in the
instrument cluster. The SDM performs continuous diagnostic
monitoring of the SIR system electrical components and circuitry
when the ignition is turned ON. If the SDM detects a malfunction, a
DTC will be stored and the SDM will request the instrument cluster
to illuminate the SIR system AIR BAG indicator, notifying the
driver that a malfunction exists. In the event that ignition
positive voltage is lost during a collision, the SDM maintains a
23-volt loop reserve for deployment of the air bags. It is
important when disabling the SIR system for servicing or rescue
operations to allow the 23-volt loop reserve to dissipate, which
could take up to 1 minute.
Air Bags
This vehicle contains 6 air bags. The 6 air bags are located in
the driver steering wheel (dual inflators), passenger instrument
panel (passenger side) (dual inflators), driver seat side
(B-pillar), passenger seat side (B-pillar), left roof rail, and
right roof rail. To view the locations of the air bags refer to
SIR Identification Views . Air bags contain a housing,
inflatable air bag, two initiating devices (if dual inflator),
canister of gas generating material and, in some cases, stored
compressed gas. The deployment loops supply current to deploy the
air bags. The driver steering wheel and passenger instrument panel
air bags have two stages of deployment, which varies the amount of
restraint to the occupant according to the collision severity. For
moderate frontal collisions the air bags deploy at less than full
deployment which consists of stage 1 of the air bag. For more
severe frontal collisions a full deployment is initiated which
consists of stage 1 and stage 2 of the air bag. The current passing
through the air bags ignite the material in the canister producing
a rapid generation of gas and is some cases, the release of
compressed gas. The gas produced from this reaction rapidly
inflates the air bag. Once the air bag is inflated it quickly
deflates through the air bag vent holes and/or the bag fabric. A
shorting bar (if equipped) is located in the connector.
Seat Belt Pretensioners (Anchor and Retractor)
The seat belt pretensioners (driver and passenger) consist of a
housing, seat belt retractor (located in the B-pillar), seat belt
anchor (located on the floor), seat belt webbing, an initiator, and
a canister of gas generating materials. To view the locations of
the seat belt pretentioners refer to
SIR Identification Views . The initiator is part of the seat
belt pretensioner deployment loop. When the vehicle is involved in
a collision of sufficient force, the SDM causes current to flow
through the seat belt deployment loops to the initiator. Current
passing through the initiator ignites the material in the canister
producing a rapid generation of gas. The gas produced from this
reaction deploys the seat belt pretensioners which removes all of
the slack in the seat belts. Depending on the severity of the
collision, the seat belt pretensioners may deploy without the
frontal air bags deploying, or they will deploy immediately before
the frontal air bags deploy. A shorting bar (if equipped) is
located in the connector.
Impact Sensors
This vehicle contains 3 impact sensors. The 3 impact sensors are
located in the front of the vehicle, and 2 in the B-pillars (left
and right). To view the locations of the impact sensors refer to
SIR Identification Views . The impact sensors contain a sensing
device which monitors vehicle acceleration and velocity changes to
detect side collisions that are severe enough to warrant air bag
deployment. The impact sensors are not part of the deployment loop,
but instead provide input to the SDM. The SDM contains a
microprocessor that performs calculations using the measured
accelerations and compares these calculations to a value stored in
memory. When the generated calculations exceed the stored value,
the SDM will cause current to flow through the deployment loops
deploying the appropriate air bags.
Passenger Air Bag Disable Switch
The passenger air bag disable switch is used to turn the
passenger frontal air bag on or off. The customer can decide to
disable the passenger air bag by turning this switch on. When the
SDM sees a ground circuit from this switch the passenger air bag is
turned off. The vehicle has a PASSENGER AIR BAG ON/OFF indicator to
tell the customer when the passenger air bag is on or off based on
the disable switch position.
Seat Belt Indicators
The seat belt indicators are controlled through the inflatable
restraint sensing and diagnostic module (SDM). For further
information on seat belt indicators refer to
Seat Belt System Description and Operation .
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