Principles of Operation
Air Bag and Safety Belt Pretensioner Supplemental Restraint System (SRS)
Principles of Operation
A Supplemental Restraint System (SRS) consists of the following:
- Dual stage driver and passenger air bag module
- Safety belt buckle pretensioners
- Adaptive load limiting retractors
- Safety canopies
- Impact sensors (driver\center front crash sensor, passenger front crash sensor first and second row driver and passenger side impact sensors)
- Clockspring
- An air bag warning indicator (located in the Instrument Cluster (IC))
- Restraints Control Module (RCM)
- Occupant Classification Sensor (OCS) system
- Passenger Air Bag Deactivation (PAD) indicator
In addition to dual stage deployment (advanced restraint system) driver and front passenger air bag modules, this vehicle is equipped with a passenger air bag adaptive tether as well as driver and passenger air bag canister vents.
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 RCM sends a message to IC module to illuminate the air bag warning indicator. In addition to the self-test at start up, the RCM continuously monitors all of its external and internal circuitry for faults.
In a frontal collision, the impact sensors located in the front of the vehicle detect the sudden deceleration and send an electrical signal to the RCM. The RCM uses the information from the impact sensors and the Occupant Classification System Module (OCSM) in the deployment determination. If the RCM determines that SRS 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/adaptive load limiting retractor(s) to control the tension of the safety belt(s).
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 IC module, OCSM and PCM.
Adaptive Load Limiting Safety Belt Retractor
This vehicle is equipped with driver and front passenger adaptive load limiting retractors. Adaptive load limiting retractors are pyrotechnic devices integrated to the driver and front passenger safety belt retractor assemblies. Adaptive load limiting retractors work in conjunction with the safety belt buckle pretensioners and control the tension of the driver and front passenger seat belts in the event of a deployable collision. The RCM monitors the readiness of the adaptive load limiting retractors. The RCM uses this information and all other information provided by the restraints system to determine what action is to be taken.
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.
Air Bag Warning Indicator
The air bag warning indicator:
- is located in the IC module.
- 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.
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:
- contains dual stage deployment (advanced restraint system).
- is equipped with canister vent.
In addition to dual stage deployment (advanced restraint system) driver air bag module, this vehicle is equipped with a driver air bag canister vent. Canister venting is a deployable device integrated to the driver air bag module. Canister venting controls the inflation rate of the driver air bag and the escape rate of gases from the vent.
Deployable Steering Column
This vehicle is equipped with a deployable steering column. The deployable steering column includes a deployable device that, once deployed, reduces the amount of force necessary to collapse the steering column during a crash event. The deployable device is activated by the RCM depending on when the module detects a crash event force exceeding a programmed limit and the position of the driver seat. After deployment, a new steering column must be installed.
High Speed Controller Area Network (HS-CAN)
This vehicle utilizes a communication system called a HS-CAN. The HS-CAN consists of a twisted pair of wires connected to the following:
- ABS module
- Accessory Protocol Interface Module (APIM)
- IC module
- OCS system
- PCM
- Transmission Control Module (TCM)
- RCM
- Four-Wheel Drive (4WD) module
- 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 total 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.
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 6 satellite sensors in addition to the RCM. The RCM is mounted to the center tunnel beneath the console. All vehicles will have 2 front impact severity sensors (driver/center front crash sensor and passenger front crash 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 sensors) are located in each of the front doors behind the trim panel. The second row impact sensors are located on each C-pillar. Mounting orientation and torque is critical for correct operation of all impact and rollover sensors.
Deployment Loop
The deployment loop is made up of the RCM, deployable device (each containing a squib), air bag(s), safety belt pretensioners (safety canopies/side air curtains, deployable steering columns, load limiting retractor, adaptive tether, adaptive vent, if equipped) and associated circuits.
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
The OCS system is found only on the front passenger seat. The front passenger OCS system is comprised of 2 OCS rails (weight sensors) that are mounted to each side of the seat track and an OCSM which is mounted to the electrical bracket underneath the seat cushion pan. The components that make up the OCS system are serviced separately. The weight of any occupant or object on the front passenger seat is electronically communicated to the OCSM.
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 Restraint 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 the front passenger seat is removed for service, the Zero Seat Weight Test must be carried out after the installation of the front passenger seat. When a new OCS system component is installed, the System Reset must be carried out after the installation of the front passenger seat. The Zero Seat Weight Test and/or System Reset must be carried out only as instructed in the diagnostic/repair information. For information on the Zero Seat Weight Test and/or System Reset, refer to Occupant Classification Sensor (OCS) System Zero Seat Weight Test Component Tests and General Diagnostics or Occupant Classification Sensor (OCS) System Reset Programming and Relearning.
Occupant Classification System Module (OCSM)
Based on programmed limits, the OCSM will inform the RCM, via a HS-CAN, the weight of any occupant or object on the front passenger seat. The RCM uses this information in determining if the passenger air bag module or passenger seat side 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 DLC with the use of a scan tool.
Occupant Classification Sensor (OCS) Rails (Weight Sensors)
WARNING: Do not disassemble the occupant classification sensor (OCS) rail or tighten or loosen any of the nuts and bolts installed to the OCS rail body. Only the 8 bolts that attach the 2 OCS rails to the seat track may be removed and installed. Failure to follow these instructions may result in incorrect operation of the OCS system and increase the risk of serious personal injury or death in a crash.
NOTE: The illustration below is indicating the fasteners of the OCS rails (weight sensors) that must not be tightened or loosened. The OCS rails are installed individually as an assembly and require no adjustment.
Passenger Air Bag Deactivation (PAD) Indicator
The 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 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 exception to this is when the front passenger seat is determined to be empty and the passenger safety belt buckle is unbuckled indicating 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 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 correct operation.
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 the size of the front outboard passenger occupant.
Passenger Air Bag Deactivation (PAD) Indicator Status
Passenger Air Bag Module
The passenger air bag module:
- contains dual stage deployment (advanced restraint system).
- is equipped with canister vent.
- is equipped with adaptive tether.
- is mounted in the passenger side of the instrument panel.
In addition to the dual stage deployment (advanced restraint system) front passenger air bag module, this vehicle is equipped with a passenger air bag adaptive tether, as well as a passenger air bag canister vent. Canister venting is a deployable device integrated to the passenger air bag module. Canister venting controls the inflation rate of the passenger air bag and the escape rate of gases from the vent.
Adaptive tether is a deployable device integrated to the passenger air bag module. Adaptive tether restricts the rearward movement of the passenger air bag.
Passenger Air Bag Module
NOTE: Illustration shown as if looking through the glove compartment opening, rotated slightly for clarity.
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)
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(s) and safety belt adaptive load limiting retractor(s) to control tension on the safety belt.
- monitors the SRS for faults.
- requests the illumination of the air bag indicator if a fault is detected.
- communicates the on-demand or continuous DTCs through the DLC.
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 IC.
When the ignition is cycled (turned OFF and then ON), the IC module will prove out the air bag indicator by lighting the indicator 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 ignition 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
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 RCM when the module detects a crash event force exceeding a programmed limit.
Safety Belt Buckle Switches
The front safety belt buckles are equipped with 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 supplies current to the safety belt buckle switch. Current flows through the switch, buckled or unbuckled, to ground. The RCM will sense the difference in this current draw, 6 mA (unbuckled) or 15 mA (buckled), and use this information in determining the deployment rate of the dual-stage driver and passenger air bag modules. If the RCM detects current out of this range, it will set a DTC.
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 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 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:
- is installed as an assembly.
- is mounted above the headliner.
- attaches from the A-pillar frame to the D-pillar frame on Explorer/Mountaineer and from the A-pillar frame to the C-pillar frame on Explorer Sport Trac.
Seat Side Air Bag Module
NOTE: When servicing a seat equipped with a seat side air bag module, refer to Seats.
A seat 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(R) module.
The seat side air bag module:
- will deploy upon receiving a flow of current from the RCM initiated by the side impact sensor(s) and internal RCM circuitry.
- is installed as an assembly.
- is mounted in the driver/passenger seat back.
- is used in conjunction with a Safety Canopy(R) module.
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 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 generated by the IC module. If the IC module has detected a fault with the air bag warning indicator, a DTC will be stored in the memory of the IC module. Upon receiving the message from the RCM that a 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.