Air Bag Systems: Description and Operation

Air Bag Supplemental Restraint System (SRS)

COMPONENT LOCATION - SHEET 1 OF 3









COMPONENT LOCATION - SHEET 2 OF 3









COMPONENT LOCATION - SHEET 3 OF 3









INTRODUCTION

WARNING: All pyrotechnic devices are dangerous. Before performing any procedures on any pyrotechnic device, read all information contained within the Standard Workshop Practices section of this product. For additional information, refer to Standard Workshop Practices Standard Workshop Practices
The SRS (supplemental restraint system) provides additional protection for occupants in certain vehicle accident conditions.
The SRS (supplemental restraint system) comprises the following sub-systems:
- Driver and passenger front air bag system.
- Roll over protection system (convertible only).
- Driver and passenger side air bag system.
- Safety belt pretension system.
- Passenger occupancy detection system
- Passenger occupancy classification system
The SRS (supplemental restraint system) features selective activation of the air bags and safety belt pre-tensioners, and two stage driver and passenger air bags. The RCM (restraints control module) monitors internal and external sensors and activates the required safety belt pre-tensioners, air bags and roll-over protection hoops (convertible only) if the sensors detect an impact or roll-over above preset limits.
The Roll-over Protection System comprises two spring loaded hoops located behind the rear seat. The hoops incorporate a pin to break the rear screen glass as the hoop is released. The hoops are released by a pyrotechnic device when the RCM (restraints control module) detects a roll-over condition.

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 are to be deployed by using two internal areas:

Crash severity evaluation
This area 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.

Deployment handler
The status of the seat track position sensor(s) and safety belt buckle sensors are examined before a decision is made about which restraints should finally be deployed.
Data from the side crash sensors is used by the RCM 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 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 IDS (Integrated Diagnostic System) over the CAN (controller area network) bus. If a fault that could cause a false fire signal is detected, the 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 AIR BAG 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. It has two electrical connectors that are color coded and mechanically keyed to the respective connector on the inflator.

PASSENGER AIR BAG MODULE









The passenger air bag 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.

SIDE AIR BAG MODULE









The side air bag module is mounted in the outboard bolster of each front seat and uses compressed gas to inflate. It provides protection for the thorax (the part of the trunk between the neck and the abdomen) and the head. In an air bag deployment situation, it deploys through the stitch seam in the side bolster. To ensure the air bag always emerges at the same point, a chute is attached to the inside of the trim cover and wrapped around the air bag module.

IMPACT SENSORS





Two front impact sensors are attached to the body behind the radiator grille. The RCM (restraints control module) processes the crash data sent by the crash sensor against stored data, and deploys the front air bags and pretensioners.
Two side impact sensors are mounted at the base of the B-pillars to facilitate lateral impact sensing. In the event of a side impact, the RCM (restraints control module) processes the crash data against the stored data. The RCM (restraints control module) will deploy the side air bag module on the side the deployment request was initiated.

SEAT POSITION SENSOR(S)









A seat position sensor is installed on the driver seat and, up to VIN (vehicle identification number) B03409, some front passenger seats. The seat position sensor allows the RCM (restraints control module) to detect when the seat is forward of a given point on the seat track. The seat position sensor consists of a Hall effect sensor attached to the seat frame and a target plate on the seat base. While the ignition is on, the RCM (restraints control module) supplies the sensor with a power supply of 12V nominal, and monitors the return voltage. When the seat frame moves forwards, the sensor moves over the target plate, which changes the reluctance of the sensor. The change of voltage 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 target plate.
When a 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 or passenger air bag, as applicable. When the 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
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.
For additional information, refer to Safety Belt System Description and Operation

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.
Each pretensioner has a tube containing an inflator and a piston. The inflator is connected to the RCM (restraints control module). The piston is attached to a steel cable, the opposite end of which is attached to the safety belt buckle.
On receipt of a fire signal from the RCM (restraints control module), the inflator generates nitrogen gas that rapidly expands to drive the piston along the tube, pulling the cable and drawing the safety belt buckle downwards.

AIR BAG WARNING INDICATOR





The air bag warning indicator consists of a amber LED (light emitting diode) behind a graphic in the tachometer of 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) illuminates the air bag warning indicator if a fault is detected, and for approximately 6 seconds during the indicator 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 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.

SAFETY BELT SWITCHES
A safety belt switch is installed in the buckle of each front safety belt to provide the RCM (restraints control module) with status signals of the related safety belts. When a 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).

OCCUPANT DETECTION SYSTEM









The occupant detection system can only determine if the front passenger seat is occupied or unoccupied. The occupant detection system consists of a captive sensor mat installed between the foam padding and the cover of the front passenger seat cushion and a control module.
The mat's output is fed to the control module which gives either empty or occupied state signal to the instrument cluster. The instrument cluster illuminates the safety belt minder lamp when it detects that the seat is occupied and the safety belt is not engaged.

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.
The occupant classification system has 4 possible states which are detailed in the following table.





ROLL OVER PROTECTION SYSTEM
The roll over protection system fitted to the convertible vehicle comprises two roll over protection hoops located behind the rear seat. The roll over protection system is controlled by the RCM (restraints control module). The RCM (restraints control module) has an internal roll over sensor which triggers the pyrotechnic latches that hold the roll over protection hoops in place.

Roll Over Protection Hoops

NOTE:
A= Deployed, B= Normal state









The roll over protection system hoops are located behind the rear seat underneath the tonneau cover. The tonneau cover has two hatches set into it to allow the hoops to break through once triggered. The hoops are spring loaded and held in place by a pyrotechnic latch. The latch is fired in response to a trigger from the RCM (restraints control module). Once fired the latch opens and the hoops are released. As the hoops move upwards a ratchet in the body of the hoops locks into place and prevents the hoops from being forced back down by the weight of the vehicle. Once deployed the hoops cannot be re-set and should be replaced with new components.
The hoops have a small pin mounted on top which breaks the rear screen if the convertible top is up. This allows clear passage for the roll over protection hoops to operate.

CONTROL DIAGRAM

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









PRINCIPLES OF OPERATION
In a collision, the sudden deceleration or acceleration is measured by the safing sensor in the RCM (restraints control module) and by the impact sensors. The RCM (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 or pretensioners. During a collision, the RCM (restraints control module) only fires the air bags and pretensioners if the safing sensor confirms that the data from the remote sensor(s) indicates an impact limit has been exceeded. The RCM (restraints control module) also monitors the vehicle for a roll-over accident using the internal roll-over sensor.
The RCM (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 2, belt unfastened.
- Front impact, driver and passenger air bags stage 1, belt fastened.
- Front impact, driver and passenger air bags stage 2, belt fastened.
- Rear impact.
- LH (left-hand) side impact.
- RH (right-hand) side impact.
- Roll-over.
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 either the pretensioner impact limit or the roll-over limit is exceeded. The RCM (restraints control module) only fires the pretensioners if the related safety belt is fastened. For the front passenger pretensioner to fire, the seat must also be occupied by a person.
The driver and passenger air bags are only fired in a frontal impact that exceeds the stage 1 threshold. Both stages of the inflator in the driver and passenger air bags are fired. At impacts between the stage 1 and 2 thresholds, the delay between the firing of the two stages varies with the severity of the impact; the more severe the impact the shorter the delay. At stage 2 impact thresholds and above, the two stages of the inflator are fired almost simultaneously. The time delay between firing the two stages of the inflator is increased if the related seat is forward of the seat position sensor switching point.
If a side impact limit is exceeded, the RCM (restraints control module) fires the side air bag on that side of the vehicle.
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.

Crash Signal
When the RCM (restraints control module) outputs any of the fire signals, it also outputs a hard wired crash signal to the ECM (engine control module) and changes the high speed CAN (controller area network) bus output message from 'no crash' to 'crash condition'. The high speed CAN (controller area network) bus message is used by the CJB (central junction box).
On receipt of the crash signals:
- The ECM (engine control module) disables the fuel pump.
- The auxiliary junction box enters the crash mode and:
- Activates all of the unlock signals of the vehicle locking system, even if the vehicle is already unlocked, repeatedly for 3 seconds, in case a lock button is pressed during the crash.
- Ignores all locking and superlocking inputs until the crash mode is cancelled, when it returns the locking system to normal operation.
- Activates all of the courtesy lamps, except for the approach lamps. The activated courtesy lamps remain on until crash mode is cancelled, when they return to normal operation.
- Activates the hazard warning lamps. The hazard warning lamps remain on until crash mode is cancelled.
The crash mode is cancelled by cycling the ignition mode.