Principles of Operation
Air Bag and Safety Belt Pretensioner Supplemental Restraint System (SRS)
Principles of Operation
The Supplemental Restraint System (SRS) consists of the following serviceable components:
- Driver and passenger air bag module
- Seat side air bag modules
- Safety belt buckle pretensioners
- Side air curtains
- Front and side impact sensors
- Clockspring
- Occupant Classification Sensor (OCS) system
- Belt Tension Sensor (BTS)
- Air bag warning indicator (located in the Instrument Cluster (IC) module)
- Passenger Air Bag Deactivation (PAD) indicator
- Restraints Control Module (RCM)
The RCM includes a backup power supply. This feature provides sufficient backup power to deploy the SRS in the event that the ignition circuit is lost or damaged during impact. The backup power supply will deplete its stored energy approximately one minute after power and/or ground has been removed from the RCM. The RCM performs an internal, external circuitry and component "self-test" during each startup. If an SRS fault exists, the RCM sends a message to IC module to illuminate the air bag warning indicator. In addition to the self-test at start up, the RCM continuously monitors all of its external and internal circuitry for faults.
In a frontal collision, the impact sensors located in the front of the vehicle detect the sudden deceleration and send an electrical signal to the RCM. The RCM uses the information from the impact sensors and the OCS system in the deployment determination. If the RCM determines that deployment is required, the RCM sends voltage and current to the deployable devices (squibs) causing the solid chemical propellant to undergo a rapid chemical reaction. This controlled reaction produces harmless nitrogen gas that fills the air bag(s)/side air curtains and/or activates the safety belt buckle pretensioners to remove slack from the safety belt(s).
The RCM communicates through the Data Link Connector (DLC) on the High Speed Controller Area Network (HS-CAN). The RCM also communicates over the HS-CAN with the IC module, PCM and the Occupant Classification System Module (OCSM).
Air Bag Warning Indicator
The air bag warning indicator:
- is located in the IC module.
- indicator and circuitry prove out is a function of the IC module. The IC module will prove out the air bag warning indicator by lighting the air bag warning indicator for 6 seconds and then turn off.
- will flash and/or illuminate based on the message the IC module receives from the RCM.
- will illuminate if the IC module does not receive a message from the RCM.
Air Bag Module Second Stage Deployment Check
Because the driver and passenger front air bags each have 2 deployment stages, it is possible that Stage 1 has deployed and Stage 2 has not.
If a front air bag module has deployed, it is mandatory that the front air bag module be remotely deployed using the appropriate air bag disposal procedure.
- For information on driver air bag module and/or passenger air bag module remote deployment, refer to Pyrotechnic Device Disposal Service and Repair.
Clockspring
The clockspring:
- is mounted on the steering column, behind the steering wheel.
- allows for continuous electrical connections between the driver air bag module and the RCM when the steering wheel is turned.
- allows for continuous electrical connections for any optional steering wheel-mounted controls.
Driver Air Bag Module
The driver air bag module:
- is mounted in the center of the steering wheel.
High Speed Controller Area Network (HS-CAN)
This vehicle utilizes a communication system called a HS-CAN.
For information about the HS-CAN, refer to Information Bus.
Impact Sensors
WARNING: If a vehicle has been in a crash, inspect the restraints control module (RCM) and the impact sensor (if equipped) mounting areas for deformation. If damaged, restore the mounting areas to the original production configuration. A new RCM and sensors must be installed whether or not the air bags have deployed. Failure to follow these instructions may result in serious personal injury or death in a crash.
For this vehicle line, the SRS uses 5 satellite sensors in addition to the RCM. The RCM is mounted to the center tunnel beneath the console. These vehicles are equipped with 1 front impact severity sensor (driver/center front impact sensor), located in the front-center area of the vehicle, behind the grille mounted on the hood latch bracket. The first row impact sensors are pressure sensors located in each of the front doors, the second row sensors are located near each B-pillar. Mounting orientation is critical for correct operation of all impact sensors.
Occupant Classification Sensor (OCS) System
The OCS system is found only on the front passenger seat. The OCS system is comprised of a silicone gel-filled bladder mounted between the seat cushion foam and pan, a pressure sensor that is mounted to the seat frame and an OCSM which is also mounted to the seat frame. Pressure is applied to the OCS system bladder when weight of any occupant or object in the front passenger seat is present. The pressure is then transferred through a tube, sensed by the OCS system pressure sensor, then electronically communicated to the OCSM. Based on preprogrammed set points the OCSM will inform the RCM, via a HS-CAN, if the passenger air bag module is to be deployed in the event of a deployable collision or deactivated due to the possibility a child may be in the front passenger seat. The RCM may also use this information to illuminate/not illuminate the PAD indicator. For information regarding PAD indicator operation, refer to Passenger Air Bag Deactivation Indicator.
The OCSM monitors the OCS system for faults and communicates DTCs via the DLC with the use of a scan tool.
The OCS system also incorporates a safety BTS, located at the anchor point of the passenger safety belt, that identifies the presence of a cinched child safety seat in the front outboard passenger seat. This is accomplished by a variable voltage signal provided by the BTS to the OCSM. The OCSM will then communicate the status to the RCM, automatically deactivating the passenger air bag module. Refer to Safety Belt Tension Sensor.
The OCS system is also used for operation of the passenger Belt-Minder(R). For information on the passenger Belt-Minder(R) feature, refer to Seat Belt Systems. To deactivate or reactivate the passenger Belt-Minder(R) feature, refer to Instrument Panel, Gauges and Warning Indicators or the Owner's Literature.
The OCS system components (seat cushion foam pad, bladder with pressure sensor, seat wiring harness and OCSM) are calibrated to each other. OCS system components are not to be installed or serviced separately.
Passenger Air Bag Deactivation (PAD) Indicator
The PAD indicator is a visual indicator used to inform the front seat occupants of the passenger air bag deactivation state. The PAD indicator is a stand-alone lamp installed into the vehicle instrument panel in a position visible to each front seat occupant.
The RCM controls the state of the PAD indicator through a direct hardwire connection, based on information provided by the OCS system. The PAD indicator is lit to indicate the passenger air bag module is disabled. An exemption to this is when the front passenger seat is determined to be empty and the passenger safety belt buckle is unbuckled, and therefore indication of a deactivated passenger air bag module is not necessary. In all other cases, the PAD indicator is unlit when the passenger air bag module is enabled.
When the ignition is in the ON position, the PAD indicator prove-out period is initiated by the RCM. The RCM briefly activates the PAD indicator to verify to the front occupants correct functional operation of the PAD indicator.
The following table indicates the passenger air bag status and the PAD indicator status based on the size of the front outboard passenger occupant.
Passenger Air Bag Module
The passenger air bag module:
- is mounted in the passenger side of the instrument panel.
Restraints Control Module (RCM)
WARNING: If a vehicle has been in a crash, inspect the restraints control module (RCM) and the impact sensor (if equipped) mounting areas for deformation. If damaged, restore the mounting areas to the original production configuration. A new RCM and sensors must be installed whether or not the air bags have deployed. Failure to follow these instructions may result in serious personal injury or death in a crash.
WARNING: Always tighten the fasteners of the restraints control module (RCM) and impact sensor (if equipped) to the specified torque. Failure to do so may result in incorrect restraint system operation, which increases the risk of personal injury or death in a crash.
NOTICE: Prior to removal of the Restraints Control Module (RCM), it is necessary to upload module configuration information to the scan tool. This information needs to be downloaded into the new RCM module once installed. Refer to Information Bus. Failure to follow these instructions may result in incorrect operation of the SRS and may cause system failure.
NOTE: This vehicle may be equipped with the SYNC(R) V2 feature which contains the 911 assist option. Refer to the Owner's Literature.
NOTE: Carrying out Programmable Module Installation (PMI) will not enable the 911 assist option that is disabled. The RCM and Accessory Protocol Interface Module (APIM) must be configured correctly to fully support 911 assist functionality.
The RCM carries out the following functions:
- deploys the air bag(s)/side air curtains in the event of a deployable collision.
- activates the safety belt buckle pretensioners to remove slack from the safety belt in the event of a deployable collision.
- monitors the SRS for faults.
- sends a message to the IC module to illuminate the air bag warning indicator if a fault is detected.
- communicates DTCs through the DLC.
The RCM monitors the SRS for possible faults. If a fault is detected, the RCM will request illumination of the air bag warning indicator.
The RCM includes a backup power supply. This feature provides sufficient backup power to deploy the air bags in the event that the ignition circuit is lost or damaged during impact. The backup power supply will deplete its stored energy approximately one minute after power and/or ground has been removed from the RCM.
For information on the air bag warning indicator and prove out, refer to the Prove Out Procedure.
Safety Belt Buckle Switches
The safety belt buckle switches indicate to the RCM whether the safety belts are connected or disconnected. The RCM uses this information in determining the deployment of the dual-stage driver and passenger air bag modules.
Safety Belt Pretensioners
The safety belt buckle pretensioners remove excess slack from the safety belt webbing. The pretensioners are activated by the RCM when the module detects a collision exceeding a programmed limit.
Side Air Curtain Module
Side air curtains require a specific headliner. The word AIRBAG will appear on the headliner where it meets each B-pillar trim panel.
The side air curtain module:
- is mounted above the headliner.
- attaches from the A-pillar frame to the C-pillar frame.
- cannot be interchanged from side-to-side.
Seat Track Position Sensor
The seat track position sensor is mounted to a bracket attached to the inboard side of the driver seat track. The seat track position sensor informs the RCM of the driver seat position. The RCM uses this information in determining the deployment of the dual-stage driver air bag module.
Side Air Bag Module
The side air bag module:
- will deploy upon receiving a flow of current from the RCM, initiated by the side impact sensor and internal RCM circuitry.
- is mounted in the driver and passenger seat backrest.
- is used in conjunction with a side air curtain.
Secondary Air Bag Warning (Chime)
The secondary air bag warning chime is an audible tone controlled by the IC module. If a fault is detected with the air bag warning indicator, a DTC will be stored in memory of the IC module.
Upon receiving the message from the RCM that a SRS fault has been detected, the IC module will sound the secondary air bag warning chime in a pattern of 5 sets of 5 beeps if a air bag warning indicator fault is present.