Principles of Operation

PRINCIPLES OF OPERATION

A supplemental restraint system (SRS) consists of a driver and passenger dual stage air bag module and safety belt buckle pretensioners (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. 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 pretensioner(s) 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 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 instrument cluster module. The IC 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 restraints control module (RCM).
- will illuminate if the IC module does not receive a message from the RCM.

Clockspring
The clockspring:
- is mounted on the steering column, behind the steering wheel.
- continuously transfers electrical signals from the driver air bag module to the restraints control module (RCM).

Driver Air Bag Module
The driver air bag module:
- contains dual stage deployment (advanced restraint system).
- is installed new as an assembly.
- is mounted in the center of the steering wheel.

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 restraints control module (RCM) to the SRS components.
- provides the electrical path from the RCM to the data link connector (DLC).
- provides the electrical path from the occupant classification system module (OCSM) to the DLC.
- provides the electrical path (via HS-CAN) from the RCM to the instrument cluster (IC) module.

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
- Occupant classification system module (OCSM)
- PCM
- Transmission control module (TCM)
- Restraints control module (RCM)
- Four wheel drive (4WD) module
- 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 instrument cluster 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 some messages to some of the control modules 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.

NOTE: Do not probe any impact sensor. The impact sensor can not be tested using a multi-meter.

For these vehicles, the supplemental restraint system (SRS) uses 2 satellite sensors in addition to the RCM. The RCM is mounted to the center tunnel beneath the instrument panel. All vehicles will have 2 front impact severity sensors located in the front-center area of the vehicle, behind the grille mounted on the radiator support. Mounting orientation and torque is critical for correct operation of all impact sensors and the RCM.

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

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 system 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 occupant classification system module [OCSM]) are calibrated to each other and are serviced as an assembly.

Occupant Classification Sensor (OCS) System Module
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 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 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.

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 a stand-alone lamp installed into the vehicle instrument panel 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 disabled. An exemption to this is when the front passenger seat is determined to be empty and the passenger safety belt buckle is unbuckled, 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 RCM defaults the passenger air bag module to the last valid state received from the OCS system until the ignition switch is turned OFF. If the OCS system fault is still present when the ignition switch is turned ON, the RCM defaults the passenger air bag module to enabled regardless of the size of occupant in the front passenger seat. The PAD indicator will be unlit. For information on the OCS system, refer to Occupant Classification Sensor System.

The following table indicates the passenger air bag status and the PAD indicator status based on the size of the front outboard passenger occupant.

Passenger Air Bag and PAD Indicator Status:

Passenger Air Bag Module
The passenger air bag module:
- contains dual stage deployment (advanced restraint system).
- is installed new 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.

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) in the event of a deployable crash
- Activates the safety belt buckle pretensioner to control tension on the safety belt
- Monitors the supplemental restraint system (SRS) for faults
- Requests the illumination of the air bag indicator if a fault is detected
- Communicates through the data link connector (DLC) the on-demand or continuous DTCs

The RCM monitors the SRS for possible faults. If a fault is detected while the ignition switch is in the ON position, the RCM will request illumination of the air bag indicator located in the instrument cluster (IC) module.

When the ignition is cycled (turned OFF and then ON), the air bag indicator will prove out by lighting for 6 seconds and then off. If a SRS fault exists, the air bag indicator will illuminate and 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 indicator and the air bag 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 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 the battery ground cable is disconnected.

Safety Belt Buckle Pretensioners
As part of the supplemental restraint system (SRS), the safety belt buckles are equipped with pretensioners. The safety belt buckle pretensioners remove excess slack from the safety belt webbing. The pretensioners are activated by the restraints control module (RCM) when the module detects a crash event force exceeding a programmed limit.

Safety Belt Buckle Switches
As part of the supplemental restraint system (SRS), the front safety belt buckles are equipped with safety belt buckle switches. The safety belt buckle switches are comprised of integrated circuits called Hall-effect sensors. The safety belt buckle switches (Hall-effect sensors) are located in the driver and front passenger safety belt buckles. 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 rate of the dual-stage driver and passenger air bag modules. The RCM also communicates the driver safety belt buckle switch status to the instrument cluster (IC) module, which monitors the information to control the safety belt warning indicator. For information on the safety belt warning indicator, refer to Instrument Panel, Gauges and Warning Indicators.

Safety Belt Tension Sensor
The safety belt tension sensor is used by the occupant classification sensor (OCS) system to identify the presence of a child safety seat on the front outboard passenger seat, when the child safety seat is installed according to manufacturer's instructions. 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 to be a child, the RCM will then automatically deactivate the passenger air bag module.

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 OCS system.
- is a 3-wire Hall-effect sensor that is part of the front passenger safety belt and retractor assembly.

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. Based on programmed limits, the seat track position sensor will inform 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 the instrument cluster module has detected a fault with the air bag warning indicator, a DTC will be stored in the memory of the instrument cluster 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.

Occupant Classification System Module (OCSM)
System Reset active command sets the zero set point or rezeros the OCS system. Refer to Occupant Classification Sensor.

Bit-Mapped DTCs
Many of the continuous memory and on-demand DTCs that can be present in the restraints control module (RCM)/occupant classification system module (OCSM) provide general fault information and require accessing the bit-mapped 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.