Principles of Operation
Air Bag and Safety Belt Pretensioner Supplemental Restraint System (SRS)
Principles of Operation
During a frontal or side crash, the Restraints Control Module (RCM) may deploy the safety belt retractor pretensioners, side air bag(s) and/or none, one or both stages of the driver/passenger dual-stage air bag based on crash severity and conditions. The fact that the pretensioners or air bag(s) did not deploy for both front seat occupants in a collision does not indicate a fault with the Supplemental Restraint System (SRS).
In a frontal collision, the front impact severity sensor detects sudden vehicle deceleration and sends an electrical signal to the RCM. The RCM uses the information from the front impact severity sensor, safety belt buckle switches and the Occupant Classification System Module (OCSM) to determine deployment. If deployment is required, the RCM sends voltage and current to the air bag module(s) and safety belt retractor pretensioner(s).
In a side collision, the side impact sensor(s) detects sudden vehicle lateral deceleration and sends an electrical signal to the RCM. The RCM uses the information from the side impact sensor, safety belt buckle switches and the OCSM to determine deployment. If deployment is required, the RCM sends voltage and current to the side air bag module(s) and safety belt retractor pretensioner(s).
The RCM performs a complete system "self-test" during each startup. If an SRS fault exists, the air bag warning indicator will illuminate and remain illuminated for the rest of the ignition cycle. In addition to the self-test at start up, the RCM continuously monitors all SRS components and circuitry for correct operation. The RCM communicates current and historical DTCs through the Data Link Connector (DLC) using a scan tool. The RCM also communicates to the OCSM on a dedicated High Speed Controller Area Network (HS-CAN) communication system.
Air Bag Warning Indicator
The air bag warning indicator:
- is located in the Instrument Cluster (IC) module.
- and circuitry prove out is a function of the RCM. The RCM 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 SRS fault.
- will illuminate if the RCM is disconnected.
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 between the steering mounted controls and various other components.
Driver Air Bag Module
The driver air bag module:
- is installed new 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.
- will deploy upon receiving a flow of current from the RCM.
Impact Sensors
The impact sensors provide data to the RCM for use in calculating impact severity. This is accomplished using various electrical sensors located throughout the vehicle. A front impact severity sensor is located in the front-center of the vehicle, behind the radiator grille. In addition, there are side impact sensors located near the base of each B-pillar.
Loops/Squibs
All deployable devices contain an initiating device called a squib. The squib is part of the deployment loop. Air bag/safety canopies 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 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 OCSM which is also mounted to the seat frame. Pressure is applied to the OCS system bladder when the weight of any occupant or object in the front passenger seat is sensed. 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 (Empty, Below Threshold, Indeterminate and/or Above Threshold), the OCSM will inform the RCM of the necessary information via a dedicated HS-CAN communication system. 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 Passenger Air Bag Deactivation (PAD) indicator.
When an OCS system fault is present, the OCSM communicates the fault information to the RCM. Upon receiving the fault information from the OCSM, the RCM will illuminate the air bag warning indicator or the air bag warning chime will be activated (if an air bag warning indicator fault is present). The SRS then defaults the passenger air bag module to ON (activated) regardless of the size of the occupant in the front outboard passenger seat.
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 OCSM 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 OCSM also interprets a variable voltage signal provided by the Belt Tension Sensor (BTS) to identify the presence of a properly fastened child safety seat in the front outboard passenger seat. The OCSM then communicates with the RCM, automatically deactivating the passenger air bag module. Refer to BTS.
Passenger Air Bag Deactivation (PAD) Indicator
NOTICE: The PAD indicator is part of the hazard/traction control switch assembly and cannot be serviced separately.
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 located 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 OCSM. The PAD indicator is lit to indicate the passenger air bag module is deactivated (OFF). An exception 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 is in the RUN position, the PAD indicator prove-out period is initiated by the RCM. The RCM briefly activates the PAD indicator to verify operation.
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.
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
The passenger air bag module:
- is a dual-stage air bag, deploying at 1 of 2 different rates depending upon impact severity and safety belt usage.
- will deploy upon receiving a flow of current from the 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.
NOTICE: Prior to the installation of a new 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 once installed. Refer to Information Bus.
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.
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 is mounted on the center tunnel under the instrument panel. The RCM:
- deploys the air bags in the event of a deployable impact.
- activates the safety belt retractor pretensioners to remove slack from the safety belt webbing.
- monitors the SRS for faults.
- illuminates the air bag warning indicator if a fault is detected.
- illuminates the PAD indicator when conditions warrant illumination.
- flashes the air bag warning indicator to indicate the Lamp Fault Code (LFC) detected.
- communicates through the DLC the current or historical DTCs.
- will active the chime, located in the IC module, if an air bag warning indicator fault is present and another SRS fault exists.
- records deployable and non-deployable impact data.
The RCM monitors the SRS for possible faults. If a fault is detected while the ignition is in the RUN position, the RCM will illuminate the air bag warning indicator located in the IC module.
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 an SRS fault exists, the air bag warning indicator will then flash the 2-digit LFC associated with that fault. The air bag warning indicator will flash the LFC 5 times, then remain illuminated for the rest of the ignition cycle. The RCM will communicate current and historical DTCs through the DLC using a scan tool. If the air bag warning indicator does not function, and the RCM detects a fault condition, the RCM will signal 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.
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 power to deploy the front air bags, seat side air bags and the safety belt retractor pretensioners in the event that the ignition circuit is damaged in a collision before the air bags are deployed. The backup power supply will deplete its stored energy reserve in approximately one minute after the power and/or ground is removed from the RCM.
Restraint System Diagnostic Tool Warning
WARNING: Remove restraint system diagnostic tools from the vehicle prior to road testing. If tools are not removed, the supplemental restraint system (SRS) device may not deploy in a crash. Failure to follow this instruction may result in serious personal injury or death in a crash and possibly violate vehicle safety standards.
Safety Belt Buckle Switch
As part of the SRS, the driver and passenger safety belt buckles are equipped with safety belt buckle switches. The safety belt buckles are comprised of integrated circuits called Hall-effect switches. The safety belt buckle switches indicate to the RCM whether the safety belt buckles are buckled or unbuckled.
The RCM supplies current to the safety belt buckle switch. Current flows through the switch, buckled or unbuckled, to ground. The RCM will sense the difference in this current draw, 6 mA (unbuckled) or 15 mA (buckled), and use this information in determining the deployment rate of the dual-stage driver and passenger air bag modules. If the RCM detects current out of this range, it will set a DTC.
The safety belt buckle switches are also used for the driver safety belt warning system and the driver and passenger Belt-Minder(R). Refer to Seat Belt Systems.
Safety Belt Pretensioners
As part of the SRS the safety belt retractors are equipped with pretensioners. The pretensioners remove excess slack from the safety belt webbing. The pretensioners are activated by the RCM when the module detects an impact event force exceeding a programmed limit. Refer to Seat Belt Systems.
Belt Tension Sensor (BTS)
The BTS
- is part of the front passenger safety belt and retractor assembly.
- is located at the safety belt anchor.
- is used in conjunction with the OCS system.
- is a 3-wire Hall-effect sensor that is part of the front passenger safety belt retractor.
The BTS is a variable output sensor used by the OCS system to identify the presence of a child safety seat on the front passenger seat. The BTS senses the tension on the safety belt assembly then provides an output to the OCSM, indicating that the safety belt assembly is cinched. After sensing the weight applied to the seat by the occupant and using the BTS input, the OCSM determines how the occupant should be classified and communicates this information to the RCM. If the occupant is classified to be a child, the RCM will then automatically deactivate the passenger air bag module.
Seat Side Air Bag Module
The seat 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 installed as an assembly.
- is mounted in the driver/passenger seat back.
Seat Track Position Sensor
As part of the SRS, the driver seat is equipped with a seat track position sensor. The seat track position sensor is comprised of integrated circuits called Hall-effect switch. The seat track position sensor indicates the position of the driver seat, forward or rearward, to the RCM. The RCM uses this information in determining the deployment rate of the dual-stage driver air bag module.
The RCM supplies current to the seat track position sensor. Current flows through the switch, both in the forward or rearward seat positions, to ground. The RCM will sense the difference in this current draw, 6 mA (forward) or 15 mA (rearward), and use this information in determining the deployment rate of the dual-stage driver air bag module. If the RCM detects current out of range, it will set a DTC.
Secondary Air Bag Warning (Chime)
The secondary air bag warning chime is an audible chime generated by the IC module and controlled by the RCM. If the RCM has detected a fault with the air bag warning indicator, a DTC will be stored in memory of the RCM. When a Supplemental Restraint System (SRS) fault has been detected and an air bag warning indicator fault is present, the RCM will sound the secondary air bag warning chime in a pattern of 5 sets of 5 beeps.