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, seat side air bags and safety canopies (containing an inflator/squib or gas generator and an air bag), crash sensors, a restraints control module (RCM), a clockspring and an air bag warning indicator. These components are all interconnected by a wiring harness and powered by the vehicle's electrical system. 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 an SRS fault exists, the air bag warning indicator will illuminate and remain illuminated for the rest of the key cycle. In addition to the self-test at start up, the RCM continuously monitors all of its external and internal circuitry for faults.

In addition to the self-test at start up, the RCM continuously monitors all of its external and internal circuitry for faults. 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 and PCM.

In a frontal collision, the front impact severity sensor located in the front of the vehicle detects sudden vehicle deceleration and sends an electrical signal to the RCM. The RCM uses the information from the front impact severity sensor in the deployment determination. If the RCM determines that air bag(s), safety belt pretensioner(s) and/or safety canopy 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 to control the tension of the safety belt(s).

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 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 RCM when the steering wheel is turned.

Driver Air Bag Module
The driver air bag module:
- is installed new 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.
- cannot be interchanged between Expedition and Navigator vehicles.

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 the electrical path from the RCM to the air bag warning indicator.
- provides the electrical path from the RCM to the DLC.
- provides the electrical path from the RCM to the instrument cluster module.

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 module
- PCM
- Transmission control module (TCM)
- Four wheel drive (4WD) control module (if equipped)
- restraints control module (RCM)
- Air suspension module (vehicle dynamics module [VDM]) (if equipped)
- 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 to some of the control modules with only one circuit functioning. For information regarding HS-CAN, 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.

For these vehicles, the supplemental restraint system (SRS) uses 5 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 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.

Loops/Squib
All deployable devices contain an initiating device called a squib. The squib is part of the deployment loop. Air bag/safety canopy 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.

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 impact severity.
- 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 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 the installation of a new 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 the restraints control module (RCM) DTCs. Refer to Computers and Control Systems.
- When installing a new RCM, always make sure the correct RCM is being installed. RCMs are not interchangeable between vehicles or vehicle lines. If an incorrect RCM is installed, erroneous DTCs and incorrect diagnosis will result.

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 supplemental restraint system (SRS) for faults.
- sends a message to the instrument cluster module to illuminates the air bag warning indicator if a fault is detected.
- communicates through the DLC the current or historical DTCs.

The RCM monitors the SRS for possible faults. If a fault is detected, the RCM will request the instrument cluster to illuminate the air bag warning indicator. When the ignition is turned OFF and then ON, the instrument cluster will prove out the air bag warning indicator by lighting for 6 seconds. If no faults are detected by the RCM the instrument cluster 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 instrument cluster to turn the air bag warning indicator on and it will remain on for the rest of the key cycle. 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 is removed from the RCM.

A new RCM installed in a vehicle will flash 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.

Safety Belt Buckle Switches
As part of the supplemental restraint system (SRS), the driver and front outboard 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 connected or disconnected. The RCM uses this information in determining the deployment rate of the dual-stage driver and passenger air bag modules.

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

Safety Canopy Module
Vehicles are equipped with safety canopies for protection during side impacts or rollover. Vehicles with safety canopies require a specific headliner. When installing a new headliner on a vehicle equipped with safety canopies, make sure a headliner for safety canopies is being used. The word AIRBAG will appear on the headliner where it meets each B-pillar trim panel.

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.
- is standard equipment.
- cannot be interchanged from side to side.
- is used in conjunction with seat side air bag module.

Seat Track Position Sensor
The seat track position sensor is mounted to a bracket attached to the outboard side of the driver seat track. The seat track position sensor informs the restraints control module (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 controlled by the instrument cluster module. If the instrument cluster module has detected a fault with the air bag warning indicator a DTC will be stored in memory of the instrument cluster module. Upon receiving the message from the RCM that a supplemental restraint system (SRS) fault has been detected, the instrument cluster module 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 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.
- is standard equipment.