Principles Of Operation
Air Bag and Safety Belt Pretensioner Supplemental Restraint System (SRS)
Principles of Operation
A Supplemental Restraint System (SRS) consists of a driver and passenger dual stage air bag module (containing an inflator/squib or gas generator and an air bag), safety belt pretensioners (containing an inflator/squib or gas generator), impact sensors, a Restraints Control Module (RCM), a clockspring, a deployable steering column, an air bag warning indicator, Occupant Classification System (OCS) system and a Passenger Air Bag Deactivation (PAD) indicator. These components are all interconnected by a wiring harness and powered by the vehicle battery. The RCM includes a backup power supply. This feature provides sufficient backup power to deploy the SRS components 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. If a SRS fault exists, the RCM supplies a ground to flash a coded sequence which is called a Lamp Fault Code (LFC) on the air bag warning indicator located in the Instrument Cluster (IC) module. Each LFC is flashed 3 times after which the air bag indicator will remain lit for the remainder of the ignition cycle. 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 sensor located in the front of the vehicle detects sudden deceleration and sends an electrical signal to the RCM. The RCM uses the information from the impact sensor in the deployment determination. If the RCM determines that deployment is required, the RCM sends voltage and current to the squib(s) causing the solid chemical propellant to undergo a rapid chemical reaction. This controlled reaction produces harmless nitrogen gas that fills the air bag(s) and/or activates the safety belt pretensioners to remove slack from the safety belt(s).
The RCM communicates through the Data Link Connector (DLC) the current and historical DTCs.
Air Bag Warning Indicator
The air bag warning indicator:
- is located in the IC module.
- prove out is a function of the RCM. The RCM will prove out the air bag warning indicator by illuminating it for 6 seconds and then turn off.
- will flash/illuminate based on the SRS fault.
- will illuminate continuously if the RCM is disconnected.
Air Bag Module Second Stage Deployment
Because the driver and passenger front air bags each have 2 deployment stages, it is possible that Stage 1 has deployed and the 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 to make sure that the second stage has been deployed.
- 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.
Deployable Steering Column
The deployable steering column includes a deployable device that, once deployed, reduces the amount of force necessary to collapse the steering column during a crash event. The deployable device is activated by the RCM depending on when RCM the module detects a crash event force exceeding a programmed limit and the position of the driver seat. After deployment, a new steering column must be installed.
Driver Air Bag Module
The driver air bag module:
- is a dual-stage air bag, deploying at 1 of 2 different rates depending upon impact severity.
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 these vehicles, the SRS employs 5 impact sensors. One of the sensors is integral to the RCM and is not separately serviceable. The RCM is mounted on the center tunnel under the instrument panel. There are 2 front impact severity sensors located in the front-center of the vehicle, behind the grille. In addition, there are 2 side impact sensors. The side impact sensors are located at the base of each B-pillar below the safety belt retractor and pretensioner. Mounting orientation is critical for correct operation of all impact sensors.
Loops/Squibs
All deployable devices contain an initiating device called a squib. The squib is part of the deployment loop. Air bag/safety canopy modules can contain more than one squib, some vehicles may have up to 4 squibs in one air bag module. Squibs are often referred to as loops during the diagnostic process.
Occupant Classification System (OCS) System
The OCS system is found only on the front passenger seat. The front passenger seat OCS system is comprised of a silicone gel-filled bladder mounted in the seat cushion, a pressure sensor that is mounted to the seat frame and an Occupant Classification System Module (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, is 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 dedicated High Speed Controller Area Network (HS-CAN), of the necessary information. The RCM uses this information in determining if the passenger air bag module is to be deployed in the event of a deployable collision. 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 on-demand and continuous DTCs to the RCM.
When an OCS system fault is present, the SRS defaults the passenger air bag module to ON (activated) regardless of the size of the occupant in the front passenger seat.
The OCS system components (seat cushion foam pad, bladder with pressure sensor and OCSM) are calibrated to each other and are serviced as an assembly. OCS system components are not to be installed separately.
The OCS system also interprets a variable voltage signal provided by the Belt Tension Sensor (BTS) to identify the presence of a correctly fastened child safety seat in the front passenger seat. The OCSM then communicates with the RCM, automatically deactivating the passenger air bag module. Refer to Belt Tension Sensor (BTS). Restraint System Diagnostic Service Tool Warning
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 Safety Belt System Testing and Inspection. To deactivate or reactivate the passenger Belt-Minder(R) feature, refer to Instrumentation, Message Center, and Warning Chimes Service and Repair or the Owner's Literature.
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 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 switch is in the ON position, the PAD indicator prove-out period is initiated by the RCM. The RCM briefly activates the PAD indicator to prove-out the indicator function and verify to the front occupants correct functional operation of the PAD indicator.
When an OCS system fault is present, the SRS defaults the passenger air bag module to enabled regardless of the size of the occupant in the front outboard passenger seat. The PAD indicator will be unlit. For additional information on the OCS system, refer to Occupant Classification Sensor.
The following table indicates the passenger air bag status and the PAD indicator status based the size of the front outboard passenger occupant.
Passenger Air Bag and Passenger Air Bag Deactivation (PAD) Indicator Status
Passenger Air Bag Module
NOTE: References to the passenger air bag module must not be confused with the passenger seat side air bag components of the SRS.
The passenger air bag module:
- is a dual-stage air bag, deploying at 1 of 2 different rates depending upon impact severity.
- is installed as an assembly.
- 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.
NOTE: When installing a new RCM, always make sure the correct RCM is being installed. If an incorrect RCM is installed, erroneous DTCs will result.
The RCM carries out the following functions:
- Signals the inflators to deploy the air bags in the event of a deployable crash.
- Activates the steering column squib to help reduce the force needed to collapse the steering column.
- Activates the safety belt retractor pretensioners to remove slack from the safety belt.
- Monitors the air bag SRS for faults.
- Illuminates the air bag warning indicator if a fault is detected.
- Receives information from the front and side impact sensors.
- Sends safety belt buckle status to the IC module for safety belt indicator and Belt-Minder(R).
- Flashes the air bag warning indicator to indicate the Lamp Fault Code (LFC) detected.
- Communicates with the OCSM on a dedicated HS-CAN.
- Signals the Lighting Control Module (LCM) to activate a chime if a fault is detected.
- Communicates through the Data Link Connector (DLC) the current or historical DTCs.
- Contains an internal sensor which is not serviced separately.
The RCM monitors the SRS for possible faults. If a fault is detected while the ignition is in the ON position, the RCM will illuminate the air bag warning indicator located in the IC module and signal the LCM to activate the chime.
The chime is a series of 5 sets of 5-tone bursts. If the chime is heard and/or the air bag warning indicator is illuminated, the SRS and the air bag warning indicator require repair. When the ignition is cycled (turned off and then on), the air bag warning indicator will remain lit for 6 seconds and then go out. If a SRS fault exists, the air bag warning indicator will then flash the 2-digit LFC. The air bag warning indicator will flash the LFC 3 times, then it will remain illuminated for the rest of the key cycle. The RCM will also communicate the current and historical DTCs through the DLC, using the scan tool.
LFCs are prioritized. If 2 or more faults occur at the same time, the fault having the highest priority will be displayed first. After that fault has been corrected, the next highest priority fault will be displayed.
The RCM includes a backup power supply. This feature provides sufficient backup power to deploy the SRS components in the event that the ignition circuit is damaged in a collision before air bag deployment is activated. The backup power supply will deplete its stored energy approximately one minute after power and/or ground has been removed from the RCM.