Principles of Operation
Air Bag and Safety Belt Pretensioner Supplemental Restraint System (SRS)
Principles of Operation
The Restraints Control Module (RCM) continuously receives/monitors all inputs from the following Supplemental Restraint System (SRS) components.
- LH and RH front impact severity sensors
- Driver and passenger front door side impact sensors
- Driver and passenger second row side impact sensors
- Safety belt buckle switches (driver and passenger)
- Driver seat track position sensor
- Occupant Classification System Module (OCSM)
If the RCM detects sudden vehicle deceleration and/or lateral deceleration based on all the information received from all sensors and switches and determines that deployment is required, the RCM sends voltage and current to deploy the appropriate SRS components.
The RCM contains an internal sensor that detects vehicle rollover angles. If the RCM determines that a rollover condition exists and deployment is required, the RCM deploys the applicable SRS components.
During a frontal or side crash, the RCM can deploy the following SRS components, based on crash severity and sensor input.
- Safety belt retractor pretensioner(s)
- Seat side air bag(s)
- Safety Canopy(R)
- Driver and passenger dual-stage air bag(s) (one or both stages)
The fact that the safety belt pretensioners, Safety Canopy(R) or air bags did not activate for both front seat occupants in a crash does not indicate that something is wrong with the system.
The RCM carries out a self-test of the SRS during each startup. If a SRS fault exists, the air bag warning indicator illuminates and remains illuminated for the rest of the ignition cycle. In addition to the self-test at start up, the RCM continuously monitors all of its SRS components and circuitry for correct operation.
Air Bag Warning Indicator
The air bag warning indicator:
- is located in the Instrument Panel Cluster (IPC) and proves out by lighting for 6 seconds and then turn off.
- flashes and/or illuminates based on the message the IPC receives from the RCM.
- illuminates if the IPC 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 Service and Repair.
Clockspring
The clockspring 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 a dual-stage air bag, upon receiving a flow of current from the RCM, deploying at 1 of 2 different rates depending upon vehicle impact severity and impact sensor input.
High Speed Controller Area Network (HS-CAN)
This vehicle utilizes a communication system called a High Speed Controller Area Network (HS-CAN). The RCM communicates with various modules for required information about the vehicle. Refer to Module Communications Network Testing and Inspection for information about the HS-CAN.
Occupant Classification System (OCS) System
The Occupant Classification System (OCS) system is found only on the front passenger seat. The OCS system classifies the size of the front passenger seat occupant and provides this information to the RCM. The RCM uses this information (and other inputs) in determining the deployment strategy of the passenger air bag and pretensioner. The OCS system is comprised of 4 OCS weight sensor bolts that are mounted to each corner of the seat track and serviced as an assembly with the seat track. The OCSM is mounted to the electrical bracket underneath the seat cushion pan and can be serviced separately from the seat track assembly.
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 Safety Belt System Testing and Inspection. To deactivate or reactivate the passenger Belt-Minder(R) feature, refer to Instrumentation, Message Center, and Warning Chimes Service and Repair or the Owner's Literature.
Belt Tension Sensor (BTS)
The Belt Tension Sensor (BTS):
- is a 3-wire Hall-effect sensor located at the safety belt anchor point and is part of the front passenger safety belt and retractor assembly.
- is used in conjunction with the OCS system.
The BTS is 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 as a child, the RCM automatically deactivates the passenger air bag module and illuminates the Passenger Air Bag Deactivation (PAD) indicator.
Passenger Air Bag Deactivation (PAD) Indicator
The PAD indicator is a visual indicator used to inform the front seat occupants of the front passenger air bag state. The PAD indicator (part of the hazard/traction control switch assembly) is located in the instrument panel visible to each front seat occupant.
The RCM controls the state of the PAD indicator through a direct hard-wired connection, based on information provided by the OCSM. The PAD indicator is lit to indicate the front 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.
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.
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 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 vehicle impact severity and sensor input.
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(R) and cause serious personal injury or death.
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: This vehicle is equipped with the SYNC(R) feature which contains the 911 assist option. Refer to the Owner's Literature.
The RCM carries out the following functions:
- Deploys the air bag(s)/Safety Canopy(R) in the event of a deployable crash
- 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 crash
- Monitors the SRS for faults
- Sends a message to the IPC to 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 a fault is detected, the RCM requests the IPC to illuminate the air bag warning indicator. When the ignition is turned OFF and then ON, the IPC proves out the air bag warning indicator by lighting for 6 seconds. If no faults are detected by the RCM, the IPC turns the air bag warning indicator off and it remains off. If the RCM detects a fault, it sends a message to the IPC to turn the air bag warning indicator on and it remains on for the rest of the ignition cycle. If the RCM requests illumination of the air bag warning indicator and the air bag warning indicator does not function, the IPC automatically activates 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 depletes its stored energy approximately one minute after power and/or ground is removed from the RCM.
Post Crash Alert Function
The post crash alert is a function controlled by the Body Control Module (BCM). If the RCM determines that an impact of a pre-determined severity (the air bags may or may not be deployed) has occurred, the RCM sends a message to the BCM over the High Speed Controller Area Network (HS-CAN) and the post crash alert function is activated. The post-crash alert function activates the hazard lamp function and cycles the horn on 3 times, then off for 4 seconds, and repeats this cycle until the battery is discharged or the function is turned off. The post crash alert function can be turned off by:
- pressing the hazard flasher lamp switch (which may need to be pressed more than once).
- pressing the Remote Keyless Entry (RKE) transmitter UNLOCK button.
- pressing the RKE transmitter PANIC button.
Safety Belt Buckle Switches
The safety belt buckle switches indicate to the RCM whether the safety belts are buckled or unbuckled.
The RCM also communicates the driver safety belt buckle switch status to the IPC, which monitors the information to control the safety belt warning indicator. For information on the safety belt warning indicator, refer to Instrumentation, Message Center, and Warning Chimes Testing and Inspection.
Safety Belt Retractor Pretensioner
Safety belt retractor pretensioners control the tension of the driver and passenger safety belts in the event of a deployable collision.
Safety Canopy(R) Module
WARNING: Anytime the Safety Canopy(R) 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(R) or side air curtain module deploying incorrectly and increases the risk of serious personal injury or death in a crash.
NOTE: The Safety Canopy(R) modules are NOT interchangeable. If installing a new Safety Canopy(R) module, the correct Safety Canopy(R) module MUST be installed for the vehicle application.
The Safety Canopy(R) module:
- is mounted above the headliner.
- attaches from the A-pillar frame to the B-pillar or C-pillar frame.
Seat Track Position Sensor
The seat track position sensor indicates the position of the driver seat along the seat track to the RCM. The sensor detects the presence of a shunt bracket on the track, indicating that the seat has moved forward beyond a certain point of adjustment. The RCM uses seat track position sensor information to determine 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 located in and controlled by the IPC. If a fault is detected with the air bag warning indicator, a DTC is stored in memory of the IPC. Upon receiving the message from the RCM that a SRS fault has been detected, the IPC sounds the secondary air bag warning chime in a pattern of 5 sets of 5 beeps.
Impact Sensor
For this vehicle line, the SRS uses up to 6 satellite sensors in addition to the RCM. The RCM is mounted to the center tunnel beneath the console (if equipped) or front center seat. All vehicles have 2 front impact severity sensors located in the front of the vehicle, mounted on the radiator support. The first row impact sensors (side restraints sensor 1) are mounted in each of the front doors behind the trim panel, the second row impact sensors (side restraints sensor 2, Crew Cab and SuperCab) are located on each C-pillar. The driver/passenger front door side impact sensor are pressure sensors and are not interchangeable with other sensors on the vehicle. Mounting orientation is critical for correct operation of all impact sensors.
Seat Side Air Bag Module
The side air bag module:
- deploys upon receiving a flow of current from the RCM, initiated by the side restraints sensor and internal RCM circuitry.
- is mounted in the seat backrest.
- is used in conjunction with a Safety Canopy(R) module.