Description

Air Bag Systems

NOTE:
A = Hardwired; D = High speed CAN (controller area network) bus

















System Operation
In a collision, the sudden deceleration or acceleration is measured by the impact sensors and the accelerometers in the restraints control module. The restraints control module evaluates the readings to determine the impact point on the vehicle and whether the deceleration/acceleration readings exceed the limits for firing any of the air bags, pretensioners, and battery disconnect unit. During a collision, the restraints control module only fires the air bags and pretensioners if the safing function confirms that the data from the impact sensor(s) indicates an impact limit has been exceeded.
The restraints control module incorporates the following impact thresholds to cater for different accident scenarios:
- Front impact, pretensioners
- Front impact, driver and passenger air bags stage 1, belt unfastened
- Front impact, driver and passenger air bags stage 1, belt fastened
- Front impact, driver and passenger air bags stage 2, belt unfastened
- Front impact, driver and passenger air bags stage 2, belt fastened
- Rear impact
- Driver side impact
- Passenger side impact.
The front impact thresholds increase in severity from pretensioners, through to driver and passenger air bag stage 2, belt fastened.

Firing Strategies
The safety belt pretensioners are fired when the pretensioner impact limit is exceeded. The RCM (restraints control module) only fires the pretensioners if the related safety belt is fastened.
The driver and passenger air bags are only fired in a frontal impact. If an impact exceeds a stage 1 limit, but is less than the corresponding stage 2 limit, only one inflator in each air bag is fired (stage 2 is still fired for disposal after a delay of 100ms). If an impact exceeds the stage 2 limit, the two inflators in each air bag are fired simultaneously.
The passenger air bag is disabled unless the front passenger seat is occupied by a large person (NAS only), or the passenger air bag deactivation switch is on (all except NAS & AUS).
The stage 2 inflator of the driver air bag is disabled if the driver seat is forward of the switching point of the seat position sensor.
If there is a fault with a safety belt buckle sensor, the RCM (restraints control module) assumes the related safety belt is fastened for the pretensioner firing strategy and unfastened for the driver and passenger air bag firing strategies. If there is a fault with the occupant classification sensor, the RCM (restraints control module) disables the passenger air bag. If there is a fault with the passenger air bag deactivation switch, the RCM (restraints control module) disables the passenger air bag.
If a side impact limit is exceeded, the RCM (restraints control module) fires the side air bag and the side head air bag on that side of the vehicle. If the side impact limit on the front passenger side of the vehicle is exceeded, the RCM (restraints control module) also evaluates the input from the occupant classification sensor, and fires the side air bag only if the front passenger seat is occupied by a large person (NAS only).
If multiple impacts occur during a crash event, after responding to the primary impact the RCM (restraints control module) will output the appropriate fire signals in response to any further impacts if unfired units are available.









The battery disconnect unit is fired:
- At driver and passenger air bag belt fastened threshold in a frontal impact
- At the driver and passenger side impact threshold in a side impact
- At the rear impact threshold in a rear impact.

Crash Signal
When the RCM (restraints control module) outputs any of the fire signals it also outputs a crash signal to the RJB (rear junction box) and the ECM (engine control module) on the High speed CAN (controller area network). The crash signal is also hardwired to the ECM (engine control module) and the RJB (rear junction box). The instrument cluster picks up the crash signal from the High speed CAN (controller area network) and gateways it to the LCM (lighting control module). On receipt of the crash signal, the RJB (rear junction box) goes into a crash mode and the ECM (engine control module) cuts the power supply
to the fuel pump relay. In the crash mode, the RJB (rear junction box):
- Activates all of the unlock signals of the vehicle locking system, even if the vehicle is already unlocked.
- Ignores all locking/superlocking inputs until it receives an unlock input, when it returns the locking system to normal operation.
- Activates the interior lamps. The interior lamps remain on permanently until they are manually switched off at the lamp unit, or the RJB (rear junction box) crash mode is switched off and they return to normal operation.
- Disables the rear window child lock input until the crash mode is switched off.
- Sends a crash message to the LCM (lighting control module), to activate the hazard flashers. The hazard flashers remain on until cancelled by the hazard warning switch or the crash mode is switched off.
The RJB (rear junction box) crash mode is switched off by a valid locking and unlocking cycle of the locking system.

Restraints Control Module





The RCM (restraints control module)is installed on the top of the transmission tunnel, in line with the B pillars, and controls operation of the SRS (supplemental restraint system). The main functions of the RCM (restraints control module) include:
- Crash detection and recording
- Air bag and pre-tensioner firing
- Self test and system monitoring, with status indication via the air bag warning lamp and non volatile storage of fault information.
The RCM (restraints control module) determines which elements of the SRS (supplemental restraint system) are to be deployed by using two internal areas:
- Crash severity evaluation
- Deployment handler.
Crash severity evaluation evaluates crash severity by using data from the RCM (restraints control module) internal accelerometer, the front crash sensor and the safety belt buckle sensor. Based on this data, the RCM (restraints control module) decides which level of air bag module deployment is required and forwards the information to the second area, the deployment handler.
The deployment handler evaluates the status of the seat track position sensor and safety belt buckle sensors before a decision is made about which restraints should finally be deployed.
Data from the side crash sensors is used by the RCM (restraints control module) in conjunction with acceleration data from the RCM (restraints control module) internal accelerometer to make a deployment decision. The RCM (restraints control module) processes the acceleration data and subject to an impact being of high enough severity, decides whether the side air bag module should be deployed.
On board testing of the air bag modules, front safety belt pretensioner firing circuits, warning indicator circuits and module status (the crash and side impact sensors perform basic self-tests) is performed by the RCM (restraints control module) together with the storing of fault codes.
The RCM (restraints control module) drives the SRS (supplemental restraint system) indicator on the instrument pack via a CAN (controller area network) signal. If the warning lamp fails, a fault code is recorded and a warning tone is sounded in place of the lamp if a further fault occurs. It also provides a temporary back-up power supply to operate the air bag modules in the event that in crash conditions, the battery supply is lost. In the event of a crash, it records certain data which can be accessed via the diagnostic connector.
A safing sensor in the RCM (restraints control module) provides confirmation of an impact to verify if air bag and pretensioner activation is necessary. A roll-over sensor monitors the lateral attitude of the vehicle. Various firing strategies are employed by the RCM (restraints control module) to ensure that during an accident only the appropriate air bags and pretensioners are fired. The firing strategy used also depends on the inputs from the safety belt switches and the occupant monitoring system.
An energy reserve in the RCM (restraints control module) ensures there is always a minimum of 150 milliseconds of stored energy available if the power supply from the ignition switch is disrupted during a crash. The stored energy is sufficient to produce firing signals for the driver air bag, the passenger air bag and the safety belt pretensioners.
When the ignition is switched on the RCM (restraints control module) performs a self test and then performs cyclical monitoring of the system. If a fault is detected the RCM (restraints control module) stores a related fault code and illuminates the air bag warning indicator. The faults can be retrieved by the recommended Jaguar diagnostic tool over the CAN (controller area network) bus. If a fault that could cause a false fire signal is detected, the RCM (restraints control module) disables the respective firing circuit, and keeps it disabled during a crash event.

Clock Spring





The clockspring is installed on the steering column to provide the electrical interface between the fixed wiring harness of the steering column and the components that rotate with the steering wheel, i.e. the driver air bag, the horn and the steering wheel switch packs.
The clockspring consists of a plastic cassette which incorporates an outer cover fixed to the steering column and an inner rotor which turns with the steering wheel. Four securing lugs attach the cover to the multifunction switch on the steering column. The rotor is keyed to the steering wheel by a drive peg. A lug on the underside of the rotor operates the self-canceling feature of the turn signal indicator switch. A ribbon lead, threaded on rollers in the rotor, links two connectors on the cover to two connectors on the rotor. Link leads for the driver air bag are installed in one of the connectors on the rotor.
To prevent damage to the ribbon lead, both the steering and the clockspring must be centralized when removing and installing the clockspring or the steering wheel. The clockspring is centralized when the drive peg is at six o'clock and 50 - 100% of a yellow wheel is visible in the viewing window.
Replacement clocksprings are fitted with a stopper, which locks the cover to the rotor, in the central position. The stopper must be broken off when the replacement clockspring is installed.

Drivers Airbag Module





The driver air bag module is controlled by the RCM (restraints control module) which chooses between single or dual stage deployment, depending on the occupant position and the crash severity. To reduce the risk of an air bag module induced injury to a driver that is positioned close to the steering wheel, the air bag module deploys radially. It has a non-azide propellant that reduces particulates and effluents. It consists of a two stage inflator with separate chambers for the two inflation stages, each being independently activated by the RCM (restraints control module). It has two electrical connectors that are color coded and mechanically keyed to the respective connector on the inflator.

Passenger Airbag Module





The passenger airbag module is controlled by the RCM (restraints control module) which chooses between single or dual stage deployment, depending on the occupant status and the crash severity. It consists of a two stage inflator with two air bag electrical connectors to accommodate the two stage inflation.
The heated gas inflator consists of a high-pressure mix of clean air and hydrogen gas, triggered by two separate ignition squibs. It produces a controlled generation of clean gas to rapidly fill the air bag. It is classified as a stored flammable gas (not as an explosive) and as such, has less restrictive storage and transportation requirements. It produces a very clean burn and almost no particulates and is almost free of any toxins, making disposal or recycling much easier.

Head Airbag





Side air curtains protect against head injuries in a side impact, while also helping to ensure unrestrained occupants are not ejected through open or broken windows during a rollover event.
A single inflator mounted behind the rear seat unfurls the curtain and fills five separate chambers to provide head protection cushions for occupants in side-impact and roll-over events. Steel ramps guarantee the curtain does not snag on the interior trim as it unfurls, while tethers at the front and rear of the curtain ensure the curtain is held taut.
The side air curtain deflation characteristics are deliberately slow to ensure it remains inflated throughout the duration of a vehicle rollover event.

Side Airbag Module





A side air bag is attached to the outside of each front seat backrest frame, under the backrest cover.
The side air bags are handed, and each consist of a molded plastic case which contains the folded air bag and the inflator. A cable connects the igniter of the inflator to a connector in the main seat harness connector block located under the front edge of the seat cushion.
When the air bag deploys it forces the front edge of the molded plastic case apart and splits open the backrest cover.
The side air bags use compressed argon as the inflation medium. The inflated volume of each side air bag is 12 liters (0.42 ft3).

Impact Sensors





Impact sensors are installed in the front and both sides of the vehicle. The use of multiple impact sensors provides shorter air bag trigger times, through faster detection of lateral and longitudinal acceleration, and improves detection accuracy.
There are two front impact sensors, attached to the headlamp surround panel below the headlamps.
There are four side impact sensors located in the passenger compartment, as follows:
- One attached to the base of each B/C pillar
- One attached to the base of each D pillar.
Each impact sensor incorporates an accelerometer and a microchip powered by a feed from the RCM (restraints control module). The power feed also provides the interface connection through which the side impact sensor communicates with the RCM (restraints control module) using serial data messages. Acceleration is evaluated by the microchip and transmitted to the restraints control module, which then makes the decision on whether or not to activate the air bags and pretensioners.
When the ignition is switched on the RCM (restraints control module) supplies power to the impact sensors, which perform a self test. After satisfactory self tests the impact sensors continually output 'digital acceleration' messages to the restraints control module. If a fault is detected the relevant impact sensor sends a fault message, instead of the digital acceleration message, to the restraints control module. The RCM (restraints control module) then stores a related fault code and illuminates the air bag warning indicator. Faults can be retrieved by the Jaguar approved diagnostic system from the RCM (restraints control module) via the high speed CAN (controller area network) bus connection.

Seat Position Sensor
The seat position sensor allows the RCM (restraints control module) to detect when the driver seat is forward of a given point on the seat track. The seat position sensor consists of a Hall effect sensor attached to the driver seat frame. While the ignition is on, the RCM (restraints control module) supplies the sensor with power, and monitors the return current. When the seat frame moves forwards, the sensor moves over the edge of the seat track, which changes the reluctance of the sensor. The change of current is detected by the RCM (restraints control module) and used as a switching point. The switching point is when the center of the sensor is 3 ± 4 mm from the leading edge of the seat track.
When the driver seat is forward of the switching point, the RCM (restraints control module) increases the time delay between firing the two stages of the inflator in the driver air bag. When the driver seat is rearward of the switching point, the RCM (restraints control module) uses the normal time delay between firing the two stages.

Safety Belt Sensor
A safety belt switch is installed in the buckle of each front safety belt to provide the RCM (restraints control module) with a status signal of the related safety belt(s). When the safety belt is unfastened the switch outputs a low current to the RCM (restraints control module). When the safety belt is fastened the switch outputs a high current to the RCM (restraints control module).

Pretensioners





The pretensioners are used to tighten the front safety belts during a collision to ensure the occupants are securely held in their seats. A pretensioner is integrated into each front safety belt buckle and attached to a bracket on the inboard side of the seat.
Each pretensioner has a tube containing propellant and a piston. The piston is attached to a steel cable, the opposite end of which is attached to the safety belt buckle. A squib in the base of the tube provides an ignition source when triggered by a fire signal from the RCM (restraints control module).
On receipt of a fire signal from the RCM (restraints control module), the squib ignites the propellant. The propellant produces nitrogen gas that rapidly expands to drive the piston along the tube, pulling the cable and drawing the buckle downwards.

Airbag Warning Indicator





The air bag warning indicator consists of a red LED (light emitting diode) behind a SRS (supplemental restraint system) graphic in the instrument cluster.
Operation of the air bag warning indicator is controlled by a high speed CAN (controller area network) bus message from the RCM (restraints control module) to the instrument cluster. The RCM (restraints control module) sends the signal to illuminate the air bag warning indicator if a fault is detected, and for approximately 6 seconds during the bulb check at the beginning of each ignition cycle.

Occupant Monitoring
There are two types of occupant monitoring:
- In all markets except NAS & Australia, vehicles have an occupant detection sensor
- In NAS markets, vehicles have an occupant classification system
For markets which have an occupant detection sensor, this has no interface with the restraints system and only provides the belt reminder function.
For markets that have an occupant classification system, this provides the RCM (restraints control module) with the occupancy status of the front passenger seat. The restraints control module uses this and the seat buckle status in the evaluation of the firing strategy for the passenger front air bag, side air bag, and pretensioner.

Safety Belt Sensors
The buckle of each front safety belt incorporates a Hall effect sensor that provides a safety belt status signal to the RCM (restraints control module). The RCM (restraints control module) broadcasts the status of the two front safety belts on the high speed CAN (controller area network) bus for use by the instrument cluster. In the event of a front impact the RCM (restraints control module) will deploy the pretensioners provided the safety belt buckles are fastened. The safety belt buckle pretensioners have a lower deployment threshold than that required by the air bags. Hence it is possible during a minor collision, which exceeds the deployment threshold, that only the safety belt buckle pretensioners will deploy.

Passenger Airbag Deactivation Indicator

Passenger Airbag Deactivation Warning Lamp





The passenger air bag deactivation indicator is installed on the center switch pack of the instrument panel. When appropriate, the indicator illuminates to advise front seat occupants that the passenger air bag is disabled. Operation of the indicator is controlled by the RCM (restraints control module). The RCM (restraints control module) illuminates the indicator when:
- There is a fault with the passenger air bag firing circuit(s).
- The passenger air bag is deactivated with the passenger air bag deactivation switch (where fitted).
- Required by passenger seat occupant monitoring (see below).

Passenger Airbag Deactivation Switch (All Except NAS)
The passenger air bag deactivation switch provides a method of manually disabling the passenger air bag. The switch is installed in the front passenger end of the instrument panel and operated by the ignition key.
When the passenger air bag deactivation switch is operated, it changes a ground connection between two pins in the connectors of the RCM (restraints control module). When the passenger air bag deactivation switch is selected to OFF, the RCM (restraints control module) disables the passenger air bag and, if the front passenger seat is occupied, illuminates the passenger air bag deactivation indicator.

Occupant Detection System





There are two types of occupant monitoring:
- In all markets except NAS & Australia, vehicles have an occupant detection sensor
- In NAS markets, vehicles have an occupant classification system
For markets which have an occupant detection sensor, this has no interface with the restraints system and only provides the belt reminder function.
For markets that have an occupant classification system, this provides the RCM (restraints control module) with the occupancy status of the front passenger seat. The RCM (restraints control module) uses this and the seat buckle status in the evaluation of the firing strategy for the passenger front air bag, side air bag, and pretensioner.

Occupant Classification System





The occupant classification system can determine if the front passenger seat is unoccupied, occupied by a small person, or occupied by a large person. The occupant classification system consists of:
- A pressure pad, installed under the cushion of the front passenger seat, which is connected to a pressure sensor
- A safety belt tension sensor, integrated into the anchor point of the front passenger safety belt
- An occupant classification module, installed under the front passenger seat.
The pressure pad is a silicone filled bladder. Any load on the pressure pad is detected by the pressure sensor.
The safety belt tension sensor is a strain gauge that measures the load applied by the safety belt anchor to the anchor bolt. The sensor is located in the lower safety belt anchor point.

SAFETY BELT TENSION SENSOR





The occupant classification module supplies a reference voltage to the pressure sensor and the safety belt tension sensor and, from the returned signals, measures the loads acting on the pressure pad and the safety belt tension sensor. The load measurement from the safety belt tension sensor is used to produce a correction factor for the load measurement from the pressure pad. The tightness of the safety belt affects the load acting on the pressure pad, so without the correction factor the occupant classification module cannot derive an accurate occupancy status.
The occupant classification module translates the load readings into a seat occupancy status and transmits the result to the RCM (restraints control module), on a dedicated high speed CAN (controller area network) bus link. The occupant classification module incorporates two load limits for the seat cushion: When the load exceeds the lower limit, but is less than the upper limit, the occupant is classified as small; when the upper limit is exceeded, the occupant is classified as large.