Principles of Operation
PRINCIPLES OF OPERATIONA supplemental restraint system (SRS) consists of a driver and passenger dual stage air bag module, retractor pretensioners and seat side air bag modules (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 air bag warning indicator will illuminate and remain illuminated for the rest of the key cycle.
In addition to the self-test at start up, the RCM continuously monitors all of its external and internal circuitry for faults. The RCM communicates through the data link connector (DLC) the current and historical DTCs. The RCM also communicates to the occupant classification system module (OCSM) on a dedicated high-speed controller area network (HS-CAN) communication system.
In a frontal collision, the front impact severity sensor located in the front of the vehicle detects sudden vehicle deceleration and sends an electrical signal to the RCM. The RCM uses the information from the front impact severity sensor and the OCSM in deployment determination. If the RCM determines that air bag(s), retractor pretensioner(s) and/or side air bag 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 retractor pretensioners to control the tension of the safety belt(s).
Air Bag Warning Indicator
The air bag warning indicator:
- is located in the instrument cluster (IC) module.
- indicator and circuitry prove out is a function of the restraints control module (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 supplemental restraint system (SRS) fault.
- will illuminate if the RCM is disconnected.
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 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 restraints control module (RCM).
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 the electrical path from the RCM to the air bag warning indicator, located in the instrument cluster (IC) module.
- provides the electrical communication path from the RCM to the data link connector (DLC) and all other modules.
- provides the electrical communication path from the occupant classification system module (OCSM) to the RCM on a dedicated high-speed controller area network (HS-CAN) communication system.
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. A front impact severity sensor is located in the front-center of the vehicle, behind the 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
WARNING: Make sure the front passenger seat repair is complete, the seat and all attached components (head restraint, seat side shield, etc.) are correctly assembled, and the seat is correctly installed to the vehicle before carrying out the System Reset. Failure to follow these instructions may result in incorrect operation of the occupant classification sensor (OCS) system and increases the risk of serious personal injury or death in a crash.
CAUTION: It is necessary to carry out the occupant classification sensor (OCS) system reset when a front passenger seat cushion is disassembled, a new trim cover is installed or an OCS system service kit is installed. A scan tool is used to carry out the OCS system reset command. Failure to follow this instruction may result system failure and the incorrect operation of the OCS system.
NOTE: To identify between a production occupant classification sensor (OCS) system and a OCS system service kit, inspect the occupant classification system module (OCSM) electrical connector.
A production OCS system allows the disconnection of the OCSM electrical connector.
A service OCS system service kit has the electrical connector glued to the OCSM, it cannot and should not be disconnected or altered.
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 system. Based on preprogrammed set points, the OCSM will inform the restraints control module (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 passenger air bag deactivation (PAD) indicator.
When an OCS system fault is present, the air bag warning indicator lamp will illuminate or the air bag warning chime will be activated (if an air bag warning indicator fault is present). The supplemental restraint system (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 safety belt tension sensor 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 Safety Belt Tension Sensor.
Passenger Air Bag Deactivation (PAD) Indicator
CAUTION: The passenger air bag deactivation (PAD) indicator is part of the hazard/traction control switch assembly and cannot be separately serviced.
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 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 system module (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 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 occupants correct functional operation of the PAD indicator.
When an OCS system fault is present the supplemental restraint system (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 indicate the passenger air bag status and the PAD indicator status based the size of the front outboard 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 and safety belt usage.
- 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.
NOTE: When installing a new restraints control module (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 supplemental restraint system (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 LFC detected.
- communicates through the data link connector (DLC) the current or historical DTCs.
- signals the instrument cluster (IC) module to activate a chime if the air bag warning indicator is not available 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 switch is in the RUN position, the RCM will illuminate the restraints indicator lamp 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 key cycle. The RCM will communicate current and history 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 the IC module to 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.
Safety Belt Buckle Switch
As part of the supplemental restraint system (SRS), the driver and passenger safety belt buckles are equipped with a Hall-effect switch. The safety belt buckle switches indicate to the restraints control module (RCM) whether the safety belt buckles are buckled or unbuckled.
The RCM uses this information in determining the deployment rate of the dual-stage driver and passenger air bag modules.
The safety belt buckle switches are also used for the driver safety belt warning system and the driver and passenger Belt Minder. Refer to Seat Belt Systems.
Safety Belt Pretensioners
As part of the supplemental restraint system (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 restraints control module (RCM) when the module detects an impact event force exceeding a programmed limit. Refer to Seat Belt Systems.
Safety Belt Tension Sensor
The safety belt tension sensor:
- is part of the front outboard passenger safety belt and retractor assembly.
- is located at the safety belt anchor point.
- 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 outboard passenger seat. The safety belt tension sensor 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 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 to be a child, the RCM will then automatically deactivate the passenger air bag module.
Seat Side Air Bag Module
NOTE: References to side air bag modules refer to the seat-mounted and not to the steering wheel or instrument panel-mounted air bag components of the supplemental restraint system (SRS).
The seat 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/passenger seat back.
Seat Track Position Sensor
The seat track position sensor is a Hall-effect sensor located on the driver seat track. The seat track position sensor informs the restraints control module (RCM) of the driver seat position. The RCM uses this information in determining the deployment rate of the dual-stage driver air bag module.
Secondary Air Bag Warning (Chime)
The secondary air bag warning chime, is an audible chime generated by the instrument cluster (IC) module. If the restraints control module (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, the RCM will sound the secondary air bag warning chime in a pattern of 5 sets of 5 beeps.
Bit-Mapped DTCs
Many of the continuous memory and on-demand DTCs that can be present in the restraints control module (RCM) provide general fault information and require accessing the bit-mapped parameter identification (PIDs) (fault PIDs) to identify the specific concern. DTCs that use fault PIDs are conceptually different from conventional DTCs.
Conventional DTCs identify a specific concern for a given component and point to a particular diagnostic path. In the diagnostic path, PIDs are sometimes used to determine the root cause.
DTCs that use fault PIDs do not identify the specific concern. The DTC identifies the component(s) or type of components in which the concern exists. The next level, fault PIDs, identifies the specific device and fault condition. Fault PIDs are available for both on-demand (active) and continuous memory (historic) DTCs. Those associated PIDs are an extension of the information provided by the DTC and are identified by the same DTC number. Using both DTC and the fault PID is necessary to define the specific fault present (in the same manner as normal DTCs).
A scan tool must be used to view DTCs and their fault PIDs. Once a scan tool has retrieved a DTC, use the scan tool to view the fault PIDs. Viewing the fault PIDs must be carried out to identify the specific concern that is present. When the viewing of fault PIDs has been carried out, the scan tool can display the PIDs associated with that DTC, including the status or state that exists (on-demand DTC) or existed (continuous memory DTC). Refer to the manufacturer's instructions for the scan tool being used on how to view fault PIDs.