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 front impact severity sensor
- Driver front door side impact sensor
- Passenger front door side impact sensor
- Driver second row side impact sensor
- Passenger second row side impact sensor
- 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 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 may deploy the following SRS components, based on crash severity and sensor input.
- Safety belt buckle pretensioner(s)
- Seat side air bag(s)
- Safety Canopy(R)
- Driver/passenger dual-stage air bag (one or both stages)
The fact that the safety belt pretensioners or air bags did not activate for both front seat occupants in a collision does not mean that something is wrong with the system.
The RCM performs a self-test of the SRS during each startup. If a SRS fault exists, the air bag warning indicator will illuminate and remain 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 and controlled by the Instrument Cluster (IC) module. The IC module will prove out the air bag warning indicator by lighting 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 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:
- 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 a dual-stage air bag, deploying at 1 of 2 different rates depending upon vehicle impact severity and impact sensor input.
- cannot be interchanged between Expedition and Navigator vehicles.
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 Information Bus for additional information about the HS-CAN.
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.
For these vehicles, the SRS uses 5 satellite sensors in addition to the RCM. The RCM is mounted to the center tunnel beneath the console. All vehicles will have one front impact severity sensor located in the front-center area of the vehicle, behind the grille mounted on the lower radiator support. The first row impact sensors (pressure) 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 installed new as an assembly.
- is a dual-stage air bag, deploying at 1 of 2 different rates, depending upon vehicle impact severity and impact sensor input.
- is mounted in the passenger side of the instrument panel.
- 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 Information Bus.
NOTE: This vehicle may be equipped with the SYNC(R) V2 feature which contains the 911 assist option. Refer to the Owner's Literature.
NOTE: Carrying out Programmable Module Installation (PMI) will 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. Programmable Module Installation (PMI)
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. Refer to Computers and Control Systems.
The RCM carries out the following functions:
- Deploys the air bag(s) in the event of a deployable crash
- Deploys the canopy(s) in the event of a rollover condition is detected
- Activates the safety belt buckle pretensioners to remove slack from the safety belt
- Monitors the SRS for faults
- Sends a message to the IC module to illuminates 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 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 IC module to turn the air bag warning indicator on and it will remain 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 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 is removed from the RCM.
A new RCM installed in a vehicle will flash Lamp Fault Code (LFC) 54 until Programmable Module Installation (PMI) has been carried out on the RCM. If a LFC is present after completing PMI, a fault is present in the SRS and on-demand DTCs must be retrieved and diagnosed.