Principles of Operation
PRINCIPLES OF OPERATIONA supplemental restraint system (SRS) consists of a dual stage driver and passenger air bag module, safety belt buckle pretensioners, side air bag modules and side air curtains (containing an inflator/squib or gas generator and an air bag), impact sensors, a restraints control module (RCM), a clockspring, an air bag warning indicator, occupant classification sensor (OCS) system and a passenger air bag deactivation (PAD) indicator. These components are all interconnected by a wiring harness and powered by the vehicle's 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. The RCM performs an internal, external circuitry and component "self-test" during each startup. If a SRS fault exists, the RCM sends a message to instrument cluster (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 SRS 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)/side air curtains and/or activates the safety belt pretensioners/adaptive load limiting retractor(s) to control the tension of the safety belt(s).
In certain side collisions, the air curtain and seat-mounted side air bag on the side affected by the collision will be inflated, except that the OCS system will deactivate the passenger seat-mounted side air bag if it detects an empty unbuckled passenger seat or an unbuckled child or small person in the passenger seat. The side air curtain was designed to inflate between the side window area and occupant to further enhance the head protection provided to occupants in side impact collisions. The seat-mounted side air bag was designed to inflate between the door panel and occupant to further enhance the protection provided occupants in side impact collisions.
The RCM communicates through the data link connector (DLC) the current and historical DTCs on the high-speed controller area network (HS-CAN). The RCM also communicates over the HS-CAN to the instrument cluster module, occupant classification system module (OCSM) and PCM.
Air Bag Warning Indicator
The air bag warning indicator:
- is located in the instrument cluster (IC) module.
- lamp and circuitry prove out is a function of the instrument cluster module. The instrument cluster 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 instrument cluster module receives from the restraints control module (RCM).
- will illuminate if the instrument cluster 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.
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 restraints control module (RCM) when the steering wheel is turned.
Driver Air Bag Module
The driver air bag module:
- is installed as an assembly.
- is a dual-stage air bag, deploying at 1 of 2 different rates depending upon impact severity.
- is mounted in the center of the steering wheel.
Electrical System
The electrical system that supports the supplemental restraint system (SRS):
- is powered from the battery through the ignition circuit.
- provides the electrical path from the restraints control module (RCM) to the SRS components.
- provides a communication path from the RCM to the air bag warning indicator, located in the instrument cluster (IC) module, via the high-speed control area network (HS-CAN).
- provides the electrical communication path from the RCM to the DLC and all other modules on the HS-CAN.
High-Speed Controller Area Network (HS-CAN)
This vehicle utilizes a communication system called a high-speed controller area network (HS-CAN). The HS-CAN consists of a twisted pair of wires connected to some of the following:
- ABS module
- Instrument cluster (IC) module
- PCM
- Transmission control module (TCM)
- Restraints control module (RCM)
- Occupant classification system module (OCSM)
- Data link connector (DLC)
The HS-CAN circuits use a bias voltage of approximately 2.5 volts. The HS-CAN also uses 2 terminating resistors, one contained within the PCM, the other in the instrument cluster (IC) module. The terminating resistors are not serviced separately. The terminating resistors have a value of 120 ohms each, for a normal operating system total of 60 ohms. The HS-CAN may operate with only one terminating resistor and may communicate with only one circuit functioning. Refer to Information Bus (Module Communications Network).
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.
The impact sensors provide data to the restraints control module (RCM) for use in calculating impact severity. This is accomplished using various electrical sensors located throughout the vehicle.
For these vehicles, the supplemental restraint system (SRS) employs 6 impact sensors. One of the sensors is integral to the RCM and is not separately serviceable. The RCM is mounted on the center tunnel between the front seats. There is one front impact severity sensor located in the front-center of the vehicle, behind the grille and 4 side impact sensors. The first row side impact sensors are located in each of the front doors behind the trim panel, the second row side impact sensors are located on or near the base of the C-pillar. Mounting orientation is critical for correct operation of all impact sensors.
The side impact sensors are not interchangeable between the first and second row.
Loops/Squibs
All deployable devices contain an initiating device called a squib. The squib is part of the deployment loop. Air bag/safety canopies/side air curtain 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 Sensor (OCS) System
The occupant classification sensor (OCS) system is found only on the front passenger seat. The front passenger OCS system is comprised of the following: 2 OCS rails (weight sensors) that are mounted to the underside of each side seat track; an occupant classification system module (OCSM) mounted to the electrical bracket underneath the seat cushion pan. The weight of any occupant or object on the front passenger seat is electronically communicated to the OCSM.
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.
When the front passenger seat is removed for service, the Zero Seat Weight Test must be carried out after the installation of the front passenger seat. When an OCS system component is installed new, the System Reset must be carried out after the installation of the front passenger seat. The Zero Seat Weight Test and/or System Reset must be carried out only as instructed to do so in the Vehicle System. For information on the Zero Seat Weight Test and/or System Reset, refer to Occupant Classification Sensor (OCS) System Zero Seat Weight Test or Occupant Classification Sensor (OCS) System Reset. Occupant Classification Sensor (OCS) System Zero Seat Weight Test Programming and Relearning
Occupant Classification System Module (OCSM)
Based on programmed limits, the occupant classification system module (OCSM) will inform the restraints control module (RCM), via a high-speed controller area network (HS-CAN), of the weight of any occupant or object on the front passenger seat. The RCM uses this information in determining if the passenger air bag module or passenger seat side air bag module is to be deployed in the event of a deployable collision.
The OCSM monitors the occupant classification sensor (OCS) system for faults and communicates on-demand and continuous DTCs via the data link connector (DLC) with the use of a scan tool.
Occupant Classification Sensor (OCS) Rails (Weight Sensors)
The components that make up the OCS system (occupant classification system module [OCSM], inboard OCS rail and outboard OCS rail) are serviced separately.
Passenger Air Bag Deactivation (PAD) Indicator
The passenger air bag deactivation (PAD) indicator is a visual indicator used to inform the front seat occupants of the passenger air bag deactivation state. The PAD indicator is located in the integrated control panel (ICP) in a position visible to each front seat occupant.
The restraints control module (RCM) controls the state of the PAD indicator through a direct hardwire connection, based on information provided by the occupant classification sensor (OCS) system. The PAD indicator is lit to indicate the passenger air bag module is deactivated (off). Some exceptions to this are made in certain scenarios such as 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 cases, the PAD indicator is unlit when the passenger air bag module is activated (on).
When the ignition switch is in the RUN 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 seat occupants proper functional operation of the PAD indicator.
When an OCS fault is present, the supplemental restraint system (SRS) defaults the passenger air bag module to on (activated) regardless of the size of the occupant in the front passenger seat. The PAD indicator will be unlit. For information on the OCS, refer to Occupant Classification Sensor (OCS) System.
The following table indicates the passenger air bag status and the PAD indicator status based the size of the front passenger occupant.
Passenger Air Bag and PAD Indicator Status:
Passenger Air Bag Module
NOTE: References to the passenger air bag module must not be confused with the side (seat-mounted) air bag components of the supplemental restraint system (SRS).
The passenger air bag module:
- is a dual-stage air bag, deploying at 1 of 2 different rates depending upon impact severity, safety belt usage and passenger seat occupant classification.
- will deploy upon receiving a flow of current from the restraints control module (RCM).
- is installed as an assembly.
- is mounted in the right 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.
CAUTION: Prior to removal of the restraints control module (RCM) module, 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 (Module Communications Network).
NOTE:
- When a new PCM is installed, the air bag warning indicator will illuminate when the ignition switch is ON. The warning indicator will remain on until PCM configuration is restored. Clear restraints control module (RCM) DTCs.
- 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:
- deploys the air bag(s)/side air curtain(s) in the event of a deployable collision.
- activates the safety belt retractor pretensioner to control tension on the safety belt in the event of a deployable collision.
- monitors the supplemental restraint system (SRS) for faults.
- sends a message to the instrument cluster module to flash/illuminate the air bag warning indicator if a fault is detected.
- communicates through the data link connector (DLC) the current or historical DTCs.
The RCM monitors the SRS for possible faults. If an SRS fault exists, the RCM sends a message to instrument cluster (IC) module to illuminate the air bag warning indicator.
When the ignition is cycled (turned OFF and then ON), the instrument cluster will prove out the air bag warning indicator by lighting for 6 seconds and then turn off. If a current SRS fault exists, the RCM requests illumination of the air bag warning indicator and will remain illuminated for the rest of the key cycle. The RCM will also communicate the on-demand (current) and continuous (historical) DTCs through the DLC, to the scan tool. If the RCM requests illumination of the air bag warning indicator and the air bag warning indicator does not function, the IC module will automatically activate an audible chime. The chime is a series of 5 sets of 5 tone bursts. If the chime is heard, the SRS and the air bag warning indicator require repair.
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.
Safety Belt Pretensioners
As part of the supplemental restraint system (SRS), the driver and front passenger safety belt retractors are equipped with pretensioners. The safety belt retractor pretensioners remove excess slack from the safety belt webbing. The pretensioners are activated by the restraints control module (RCM) when the module detects a collision event force exceeding a programmed limit.
Safety Belt Buckle Switches
As part of the supplemental restraint system (SRS), the driver and front passenger safety belt buckles are equipped with safety belt buckle switches. The safety belt buckle switches indicate to the restraints control module (RCM) whether the safety belts are buckled or unbuckled. The RCM uses this information in determining the deployment of the dual-stage driver and passenger air bag modules.
Safety Belt Tension Sensor
The safety belt tension sensor:
- is part of the front passenger safety belt retractor assembly.
- is located at the safety belt anchor point on the passenger seat.
- is used in conjunction with the occupant classification sensor (OCS) system.
- is a 3-wire Hall-effect sensor that is part of the front passenger safety belt and retractor assembly.
The safety belt tension sensor is used by the OCS system to identify the presence of a child safety seat on the front passenger seat. The safety belt tension sensor senses the tension on the safety belt assembly then provides an output to the occupant classification system module (OCSM), indicating that the safety belt assembly is cinched. After sensing the weight applied to the seat by the occupant and using the safety belt tension sensor input, the OCSM determines how the occupant should be classified and communicates this information to the restraints control module (RCM). If the occupant is classified as a child, the RCM will then automatically deactivate the passenger air bag module and illuminate the passenger air bag deactivation (PAD) indicator.
Seat Track Position Sensors
The seat track position sensors are mounted to a bracket attached to the driver and passenger seat track. The seat track position sensor informs the restraints control module (RCM) of the driver/passenger seat position. The RCM uses this information in determining the deployment of the dual-stage driver/passenger air bag module.
Secondary Air Bag Warning (Chime)
The secondary air bag warning chime, is an audible chime controlled by the instrument cluster (IC) module. If the IC module has detected a fault with the air bag warning indicator, a DTC will be stored in the memory of the IC module. Upon receiving the message from the restraints control module (RCM) that a supplemental restraint system (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.
Side Air Bag Module
NOTE: References to side air bag modules refer to the seat-mounted, not to the steering wheel or instrument panel mounted air bag components of the supplemental restraint system (SRS).
A side air bag module provides protection of the thorax area (between the neck and abdomen) of the body, working in conjunction with the head protection provided by a side air curtain module.
The side air bag module:
- will deploy upon receiving a flow of current from the restraints control module (RCM), initiated by the side impact sensor and internal RCM circuitry.
- is installed as an assembly.
- is mounted in the driver or passenger seat backrest.
- is used in conjunction with a side air curtain module.
Side Air Curtain Module
WARNING: Anytime the safety canopy or side air curtain module has deployed, a new headliner and new A-, B- and C-pillar upper trim panels and attaching hardware must be installed. Remove any other damaged components and hardware and install new components and hardware as needed. Failure to follow these instructions may result in the safety canopy or side air curtain module deploying incorrectly and increases the risk of serious personal injury or death in a crash.
The side air curtain module:
- will deploy upon receiving a flow of current from the restraints control module (RCM) initiated by the side impact sensors and internal RCM circuitry when a side impact is detected.
- is installed as an assembly.
- is mounted above the headliner.
- attaches from the A-pillar frame to the C-pillar frame.
- cannot be interchanged from side to side.