Principles of Operation
PRINCIPLES OF OPERATIONA supplemental restraint system (SRS) consists of a dual stage driver and passenger air bag module, safety belt buckle pretensioners, adaptive load limiting retractors and safety canopies (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. 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 instrument cluster (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 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 restraints control module (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 Warning Indicator
The air bag warning indicator:
- is located in the instrument cluster (IC) module.
- and circuitry prove out is a function of the IC module. The instrument cluster 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 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.
- allows for continuous electrical connections between the driver air bag module and the restraints control module (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.
- is installed new as an assembly.
- is mounted in the center of the steering wheel.
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 restraints control module (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.
Electrical System
The electrical system that supports the 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 a communication path from the RCM to the air bag warning indicator, located in the instrument cluster (IC) module, via the high-speed controller area network (HS-CAN).
- provides the electrical communication path from the RCM to the data link connector (DLC) and all other modules on the HS-CAN.
- provides the electrical communication path from the occupant classification system module (OCSM) to the DLC and all other modules on the HS-CAN.
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 sensor (OCS) system
- 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 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 (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 is an electronic device and can not be tested using a multi-meter.
For these vehicles, the supplemental restraint system (SRS) uses 6 satellite sensors in addition to the restraints control module (RCM). The RCM is mounted to the center tunnel beneath the console. All vehicles will have 2 front impact severity sensors located in the front-center area of the vehicle, behind the grille mounted on the lower radiator support. The first row impact 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.
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 occupant classification sensor (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 occupant classification system module (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 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 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 Vehicle System. For information on the Zero Seat Weight Test and/or System Reset, refer to Occupant Classification Sensor (OCS) System Zero Seat Weight Test or Occupant Classification Sensor (OCS) System Reset. Occupant Classification Sensor (OCS) System Zero Seat Weight Test Programming and Relearning
Occupant Classification System Module (OCSM)
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), 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 data link connector (DLC) with the use of a scan tool.
Occupant Classification Sensor (OCS) Rails (Weight Sensors)
Part 1:
Part 2:
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 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 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 table indicates the passenger air bag status and the PAD indicator status based the size of the front outboard passenger occupant.
Passenger Air Bag And 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 installed new as an assembly.
- 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:
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 and cause serious personal injury or death.
- 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.
CAUTION: When installing a new restraints control module (RCM) it is necessary to carry out programmable module installation (PMI). System failure may occur if PMI is not performed. Refer to Information Bus (Module Communications Network).
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(s) and safety belt adaptive load limiting retractor(s) 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.
When the ignition is cycled (turned OFF and then ON), the instrument cluster (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 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 Canopy Module
WARNING: Anytime the safety canopy 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 or side air curtain module deploying incorrectly and increases the risk of serious personal injury or death in a crash.
The safety canopy 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:
- References to seat side air bag modules refer to the seat-mounted and not to the steering wheel or instrument panel mounted air bag components of the supplemental restraint system (SRS).
- 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 module.
The seat side air bag module:
- will deploy upon receiving a flow of current from the restraints control module (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 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 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 generated by the instrument cluster (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 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.
Bit-Mapped DTCs
Many of the continuous memory and on-demand DTCs that can be present in the restraints control module (RCM) provide general fault information and require accessing the associated 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 do).
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.