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Home >> Mercury >> 2008 >> Sable FWD V6-3.5L >> Repair and Diagnosis >> Restraints and Safety Systems >> Air Bag Systems >> Testing and Inspection >> Initial Inspection and Diagnostic Overview >> Principles of Operation

Principles of Operation

PRINCIPLES OF OPERATION

A 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. These components are all interconnected by a wiring harness and powered by the vehicle's battery. 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 module to flash a coded sequence which is called a LFC on the air bag warning indicator. Each LFC is flashed 3 times after which the air bag warning indictor will remain lit for the remainder of the key-on cycle. 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 OCS system 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 instrument cluster module, occupant classification system module (OCSM) and PCM.

Air Bag Warning Indicator
The air bag warning indicator:
- is located in the instrument cluster module.
- lamp and circuitry prove out is a function of the instrument cluster module. The instrument cluster 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 instrument cluster module receives from the restraints control module (RCM).
- will illuminate if the instrument cluster 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:
- is installed as an assembly.
- is a dual-stage air bag, deploying at 1 of 2 different rates depending upon impact severity.
- is mounted in the center of the steering wheel.

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 module, via the high-speed controller area network (HS-CAN).
- provides the electrical communication path from the RCM 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 control area network (HS-CAN). The HS-CAN consists of a twisted pair of wires connected to some of the following:
- Instrument cluster module
- PCM
- Restraints control module (RCM)
- Occupant classification system module (OCSM)
- 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 instrument cluster module. The terminating resistors are not serviced separately. The terminating resistors have a value of 120 ohms each, for a normal operating system total of 60 ohms. The HS-CAN may operate with only one terminating resistor and may communicate with only one circuit functioning (depending on the circuit fault), at a reduced rate of performance. 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.

The impact sensors provide data to the restraints control module (RCM) for use in calculating impact severity. This is accomplished using various electrical sensors located throughout the vehicle.

For these vehicles, the SRS employs up to 7 impact sensors. One of the sensors is integral to the RCM and is not separately serviceable. The RCM is mounted on the center tunnel between the front seats. There are 2 front impact severity sensors located in the front-center of the vehicle, behind the grille. In addition, there are 4 side impact sensors. The first row side impact sensors are located in each of the front doors behind the trim panel, the second row side sensors are located on or near the base of the C-pillar. Mounting orientation is critical for correct operation of all impact sensors. The side impact sensors are not interchangeable between the first and second row.

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

CAUTION:
- It is necessary to carry out the occupant classification sensor (OCS) system reset when a front passenger seat cushion is disassembled, a new trim is cover installed or an OCS system service kit is installed. A scan tool is used to trigger the active command to carry out the OCS system reset. Failure to follow these instructions may result in incorrect operation of the OCS system and may cause system failure.
- Make sure the seat is completely assembled to prevent system failure before carrying out the occupant classification sensor (OCS) system reset active command.

NOTE: To identify between a production occupant classification sensor (OCS) system and an OCS system service kit, inspect the occupant classification system module (OCSM) electrical connector.

A production OCS allows the disconnection of the OCSM electrical connector. An OCS service kit has the electrical connector glued to the OCSM, it cannot and should not be disconnected or altered. An OCS service kit also has an in-line connector between the OCSM and the seat wiring harness.

The OCS is found only on the front passenger seat. The front passenger seat OCS is comprised of a silicone gel-filled bladder mounted in the seat cushion, a pressure sensor that is mounted to the seat frame and a OCSM which is also mounted to the seat frame. Pressure is applied to the OCS bladder when weight of any occupant or object in the front passenger seat is present. The pressure is then transferred through a tube, is sensed by the OCS pressure sensor, then electronically communicated to the OCSM. Based on preprogrammed set points, the OCSM will inform the restraints control module (RCM), via a high-speed controller area network (HS-CAN), of the necessary information. The RCM uses this information in determining if the passenger air bag module is to be deployed in the event of a deployable collision. The RCM may also use this information to illuminate/not illuminate the passenger air bag deactivation (PAD) indicator. For information regarding PAD indicator operation, refer to Passenger Air Bag Deactivation Indicator.

When an OCS system fault is present, the supplemental restraint system (SRS) defaults the passenger air bag module to ON (activated) regardless of the size of the occupant in the front passenger seat.

The OCS system components (seat cushion foam pad, bladder with pressure sensor and OCSM) are calibrated to each other and are serviced as an assembly. OCS system components are not to be installed separately.

The OCSM also interprets a variable voltage signal provided by the safety belt tension sensor to identify the presence of a properly fastened child safety seat in the front passenger seat. The OCSM then communicates with the RCM, automatically deactivating the passenger air bag module. Refer to Safety Belt Tension Sensor.

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.

Occupant Classification System Module (OCSM)
Based on programmed limits, the OCSM will inform the restraints control module (RCM), via a high-speed controller area network (HS-CAN), of the necessary information. 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.

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 located in the integrated control panel (ICP) 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 system module (OCSM). The PAD indicator is lit to indicate the passenger air bag module is deactivated (off). Some exceptions to this are made in certain scenarios such as 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 cases, the PAD indicator is unlit when the passenger air bag module is activated (on).

When the ignition switch is in the RUN 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 seat occupants proper functional operation of the PAD indicator.

When an OCS system fault is present, the supplemental restraint system (SRS) defaults the passenger air bag module to on (activated) regardless of the size of the occupant in the front passenger seat. The PAD indicator will be unlit. For information on the OCS system, refer to Occupant Classification Sensor (OCS) System.

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 PAD Indicator Status:






Passenger Air Bag Module

NOTE: References to the passenger air bag module must not be confused with the side (seat-mounted) air bag components of the supplemental restraint system (SRS).

The passenger air bag module:
- is a dual-stage air bag, deploying at 1 of 2 different rates depending upon impact severity, safety belt usage and passenger seat occupant classification.
- will deploy upon receiving a flow of current from the restraints control module (RCM).
- is installed as an assembly.
- is mounted in the right side of the instrument panel.

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: Prior to removal of the restraints control module (RCM) module, it is necessary to upload module configuration information to the scan tool. This information needs to be downloaded into the new RCM module once installed. Refer to Information Bus (Module Communications Network).

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 restraints control module (RCM) DTCs.
- When installing a new 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)/safety canopies in the event of a deployable collision.
- activates the safety belt buckle pretensioner and safety belt adaptive load limiting retractor to control tension on the safety belt in the event of a deployable collision.
- monitors the SRS for faults.
- sends a message to the instrument cluster module to flash/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 an SRS fault exists, the RCM sends a message to instrument cluster module to flash a coded sequence which is called a LFC on the air bag warning indicator.

When the ignition is cycled (turned OFF and then ON), the instrument cluster will prove out the air bag warning indicator by lighting for 6 seconds and then turn off. If a current SRS fault exists, the RCM requests illumination of the air bag warning indicator and will 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 warning indicator and the air bag warning indicator does not function, the instrument cluster 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 has been removed from the RCM.

Safety Belt Buckle Pretensioners
As part of the supplemental restraint system (SRS), the driver and front passenger 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 collision event force exceeding a programmed limit.

Safety Belt Buckle Switches
As part of the supplemental restraint system (SRS), the driver and front passenger safety belt buckles are equipped with safety belt buckle switches. 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 of the dual-stage driver and passenger air bag modules.

Safety Belt (Adaptive Load Limiting) 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 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 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.

Safety Belt Tension Sensor
The safety belt tension sensor:
- is part of the front passenger safety belt retractor assembly.
- is located at the safety belt anchor point on the passenger seat.
- is used in conjunction with the occupant classification sensor (OCS) system.
- is a 3-wire Hall-effect sensor that is part of the front passenger safety belt and retractor assembly.

The safety belt tension sensor is used by the OCS system to identify the presence of a child safety seat on the front passenger seat. The safety belt tension sensor senses the tension on the safety belt assembly then provides an output to the occupant classification system module (OCSM), indicating that the safety belt assembly is cinched. After sensing the weight applied to the seat by the occupant and using the safety belt tension sensor input, the OCSM determines how the occupant should be classified and communicates this information to the restraints control module (RCM). If the occupant is classified as a child, the RCM will then automatically deactivate the passenger air bag module and illuminate the passenger air bag deactivation (PAD) indicator.

Safety Canopy Module

WARNING: Anytime the safety canopy 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 or side air curtain module deploying incorrectly and increases the risk of serious personal injury or death in a crash.

The safety canopy module:
- will deploy upon receiving a flow of current from the restraints control module (RCM) initiated by the side impact sensors and internal RCM circuitry when a side impact or vehicle rollover condition is detected.
- is installed as an assembly.
- is mounted above the headliner.
- attaches from the A-pillar frame to the D-pillar frame (Taurus X).
- attaches from the A-pillar frame to the C-pillar frame (Taurus and Sable).
- cannot be interchanged from side to side.

Seat Track Position Sensors
The seat track position sensors are mounted to a bracket attached to the driver and passenger seat track. The seat track position sensors informs the restraints control module (RCM) of the driver/passenger seat position. The RCM uses this information in determining the deployment of the dual-stage driver/passenger air bag module.

Secondary Air Bag Warning (Chime)
The secondary air bag warning chime, is an audible chime controlled by the instrument cluster module (ICM). If the ICM has detected a fault with the air bag warning indicator, a DTC will be stored in the memory of the ICM. Upon receiving the message from the RCM that a supplemental restraint system (SRS) fault has been detected, the ICM will sound the secondary air bag warning chime in a pattern of 5 sets of 5 beeps.

Side Air Bag Module

NOTE:
- References to side air bag modules refer to the seat-mounted, 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 side air bag module, refer to Seats.

A 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 side air bag module:
- will deploy upon receiving a flow of current from the restraints control module (RCM), initiated by the side impact sensor and internal RCM circuitry.
- is installed as an assembly.
- is mounted in the driver or passenger seat backrest.
- is used in conjunction with a safety canopy module.

Bit-Mapped DTCs
Many of the continuous memory and on-demand DTCs that can be present in the restraints control module (RCM)/occupant classification system module (OCSM) provide general fault information and require accessing the 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).

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.
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