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:
- Front impact severity sensor
- Driver and passenger front door side impact sensors
- Driver and passenger C-pillar side impact sensors
- Safety belt buckle switches (driver and passenger)
- Driver seat track position sensor
If the RCM detects sudden vehicle deceleration and/or lateral deceleration based on information received from 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 may deploy the following SRS components, based on crash severity and sensor input.
- Safety belt buckle pretensioner(s)
- Safety Canopy(R)
- Seat side air bag(s)
- Driver and passenger dual-stage air bags (one or both stages)
The fact that the safety belt buckle pretensioner(s), safety belt retractor(s) or air bags did not activate for both front seat occupants in a crash does not mean that something is wrong with the SRS system.
The RCM carries out a self-test of the complete SRS during each startup. During normal operation, the Instrument Cluster (IC) illuminates the air bag warning indicator continuously for 6 seconds. If the SRS is free of faults, the air bag warning indicator turns off and remains off. 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 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, the front air bag module must 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.
Air Bag Warning Indicator
The air bag warning indicator:
- is located in the IC and proves out by lighting for 6 seconds and then turns off.
- flashes and/or illuminates based on the message the IC receives from the RCM.
- illuminates if the IC does not receive a message from the RCM.
Clockspring
The clockspring is mounted on the steering column, behind the steering wheel and allow for continuous electrical connections between the driver air bag module and the RCM while the steering wheel turns.
Driver Air Bag Module
The driver air bag module is a dual-stage air bag. Upon receiving a flow of current from the RCM, it deploys at 1 of 2 different rates depending upon vehicle impact severity and sensor input.
Event Notification Signal
The purpose of the deployment event notification feature is to provide other vehicle subsystems with information pertaining to restraint system deployment or fuel cutoff status. Signal communication between the RCM and fuel pump control module allows the PCM to initiate fuel cut-off to disable the fuel system in the event of a crash.
The RCM does not monitor the event notification signal circuit faults and does not set a DTC if a fault occurs.
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. For information about the HS-CAN, refer to Module Communications Network Description and Operation.
Impact Sensors
For this vehicle, the SRS uses 5 satellite sensors in addition to the RCM. The RCM is mounted to the center tunnel. All vehicles have one front impact severity sensor located in the front-center area of the vehicle, behind the bumper mounted on the lower radiator support. The first row impact sensors (pressure-based sensors) are located in each of the front doors behind the trim panel, the second row sensors are located on each C-pillar. The side impact sensors are not interchangeable between the first and second row. Mounting orientation is critical for correct operation of all impact sensors.
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. It cannot be interchanged between Expedition and Navigator vehicles.
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.
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 Module Configuration Programmable Module Installation (PMI).
NOTE: This vehicle may be equipped with the SYNC(R) feature which contains the 911 assist option. Refer to the Owner's Literature.
NOTE: Carrying out Programmable Module Installation (PMI) does 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 a new PCM is installed, the air bag warning indicator will illuminate when the ignition switch is ON. The warning indicator will remain on until PCM configuration is restored. Clear the RCM DTCs.
The RCM carries out the following functions:
- Deploys the air bag(s)/Safety Canopy(R) in the event of a deployable crash
- Activates the safety belt buckle pretensioners to remove slack from the safety belt in the event of a crash
- Activates the driver and passenger retractor to control the tension of the driver and passenger safety belts in the event of a crash
- Monitors the SRS for faults
- Sends a message to the IC to illuminate the air bag warning indicator if a fault is detected
The RCM monitors the SRS for possible faults. If a fault is detected, the RCM requests the IC to illuminate the air bag warning indicator. When the ignition is turned OFF and then ON, the IC proves out the air bag warning indicator by lighting for 6 seconds. If no faults are detected by the RCM, the IC turns the air bag warning indicator off and it remains off. If the RCM detects a fault, it sends a message to the IC 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 IC automatically activates an audible chime.
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.
Safety Belt Buckle Pretensioners
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 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 belts are buckled or unbuckled.
The RCM also communicates the driver safety belt buckle switch status to the IC, 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 [1][2]Description and Operation.
Safety Canopy(R) Module
WARNING: Anytime the Safety Canopy(R) 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(R) or side air curtain module deploying incorrectly and increases the risk of serious personal injury or death in a crash.
The Safety Canopy(R) module:
- deploys upon receiving a flow of current from the RCM, initiated by the RCM.
- is mounted above the headliner.
- attaches from the A-pillar frame to the D-pillar frame.
Seat Side Air Bag Module
The side air bag module:
- deploys upon receiving a flow of current from the RCM, initiated by the RCM.
- is mounted in the seat backrest.
- is used in conjunction with a Safety Canopy(R) module.
Seat Track Position Sensor
The seat track position sensor indicates to the RCM the position of the driver seat along the seat track . 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 this information (and other 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 located in and controlled by the IC. If a fault is detected with the air bag warning indicator, a DTC is stored in the IC memory. Upon receiving the message from the RCM that a SRS fault has been detected, the IC sounds the secondary air bag warning chime in a pattern of 5 sets of 5 beeps.