Principles of Operation

PRINCIPLES OF OPERATION

A supplemental restraint system (SRS) consists of a driver and passenger dual stage air bag module, safety belt buckle pretensioners, safety belt retractor pretensioners, safety canopies (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 on the high-speed controller area network (HS-CAN). The RCM also communicates over the HS-CAN to the instrument cluster (IC) module, PCM and the occupant classification system module (OCSM).

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), safety belt buckle pretensioner(s), safety belt retractor pretensioner(s) and/or safety canopy 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)/safety canopies and/or activates the safety belt 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.
- lamp 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, it is mandatory that the front air bag module be remotely deployed using the appropriate air bag disposal procedure.
- For information on the 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.

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.

Electrical System
The electrical system that supports the air bag supplemental restraint system (SRS):
- is powered from the battery through the ignition circuit.
- provides the electrical path from the 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 controller area network (HS-CAN).
- provides the electrical communication path from the RCM to the data link connector (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 the following:
- ABS module
- instrument cluster (IC) module
- PCM
- Transmission control module (TCM)
- Restraints control module (RCM)
- Occupant classification sensor (OCS) system
- Data link connector (DLC)

The HS-CAN circuits use a bias voltage of approximately 2.5 volts, one is a positive 2.5 volts while the other is a negative 2.5 volts. The HS-CAN also uses 2 terminating resistors, one contained within the PCM, the other in the 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 will operate with only one terminating resistor and has the capability to communicate some messages to some of the control modules with only one circuit functioning, but at a reduced rate of performance. Refer to Information Bus (Module Communications Network).

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

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 installed or an OCS service kit is installed. A scan tool is used to trigger the active command to carry out the OCS system reset.

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 disconnect of the electrical connector from the OCSM.

An OCS system service kit has the OCSM electrical connector glued to the module. 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 between the seat cushion foam and pan, 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 restraints control module (RCM), via a 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. 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 via the data link connector (DLC) with the use of a scan tool.

The OCS system also interprets a variable voltage signal provided by the safety belt tension sensor to identify the presence of a cinched child safety seat in the front outboard passenger seat installed according to manufacturer instructions. The OCSM then communicates with 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.

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 components (seat cushion foam pad, bladder with pressure sensor, seat wiring harness and OCSM) are calibrated to each other and are serviced as an assembly. OCS system components are not to be installed 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 state. The PAD indicator is a stand-alone indicator 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 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 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
The passenger 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 passenger side of the instrument panel.

Restraints Control Module (RCM)

WARNING:
- Do not handle, move or change the original horizontal mounting position of the restraints control module (RCM) while the RCM is connected and the ignition switch is ON. Failure to follow this instruction may result in the accidental deployment of the safety canopy and cause serious personal injury or death.
- 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: When installing a new restraints control module (RCM), it is necessary to carry out programmable module installation (PMI). System failure may occur if PMI is not performed.

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 carries out the following functions:
- Deploys the air bag(s)/safety canopies 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
- Activates the driver and passenger retractor pretensioner(s) to control the tension of the driver and passenger safety belts in the event of a deployable collision
- Monitors the supplemental restraint system (SRS) for faults
- Sends a message to the instrument cluster (IC) module to illuminate the air bag warning indicator if a fault is detected
- Communicates through the DLC the current or historical DTCs

The RCM monitors the SRS for possible faults. If a fault is detected, the RCM will request the IC module to illuminate the air bag warning indicator. When the ignition is turned OFF and then ON, the IC module will prove out the air bag warning indicator by lighting for 6 seconds. If no faults are detected by the RCM the IC module will turn the air bag warning indicator off and it will remain off. If the RCM detects a fault it will send a message to the instrument cluster to turn the air bag warning indicator on and it will remain on for the rest of the key cycle. 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 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 RCM whether the safety belts are connected or disconnected. The RCM uses this information in determining the deployment rate of the dual-stage driver and passenger air bag modules.

Safety Belt Buckle Pretensioners
As part of the supplemental restraint system (SRS), the driver and front passenger safety belt buckles are equipped with pretensioners. The safety belt buckle pretensioners are pyrotechnic devices integrated to the driver and passenger safety belt buckle assemblies. 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 crash event force exceeding a programmed limit.

Safety Belt Retractor Pretensioner
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 are pyrotechnic devices integrated to the driver and passenger safety belt retractor assemblies. The safety belt retractor pretensioners work in conjunction with the safety belt buckle pretensioners and control the tension of the driver and passenger safety belts in the event of a deployable collision. The RCM monitors the readiness of the safety belt retractor pretensioners. The RCM uses this information and all other information provided by the restraints system to determine what action is to be taken.

Safety Canopy Module

WARNING: Anytime the safety canopy or side air curtain module has deployed, a new headliner and new A-, B-, C- and D-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 safety canopy module:
- is installed as an assembly.
- is mounted above the headliner.
- attaches from the A-pillar frame to the C-pillar frame.

NOTE:
- There are different safety canopy modules for this vehicle line. The safety canopy modules are NOT interchangeable. If installing a new safety canopy module, the correct safety canopy module MUST be installed for the vehicle application.
- There are different safety canopy modules for vehicles equipped with a moon roof and vehicles without a moon roof. The safety canopy modules are NOT interchangeable. If installing a new safety canopy module, the correct safety canopy module MUST be installed for the vehicle application.

Vehicles With Moon Roof:

Vehicles Without Moon Roof:

Seat Track Position Sensor
The seat track position sensor is mounted to a bracket attached to 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 rate of the dual-stage driver 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 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 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.

Sensors

WARNING: If a vehicle has been in a crash, inspect the restraint control module (RCM) and the impact sensors (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 airbags have deployed. failure to follow these instructions may result in serious personal injury or death in a crash.

For this vehicle line, the supplemental restraint system (SRS) uses 5 satellite sensors in addition to the restraint control module (RCM). The RCM is mounted to the center tunnel beneath the console. All vehicles have one front impact severity sensor located in the front-center area of the vehicle, behind the grill mounted on the lower radiator support. The first row impact sensors are mounted behind the trim panel near the floor mounted on the B-pillar, the second row sensors are located on each C-pillar. The front and side impact sensors are not interchangeable. Mounting orientation is critical for correct operation of all impact sensors.


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 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 safety canopy 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 installed as an assembly.
- is mounted in the driver or passenger seat backrest.
- is used in conjunction with a safety canopy module.