Air Bag Systems: Description and Operation
Fig. 1:
WARNING: THE VEHICLE MAY NOT COMPLY WITH LEGISLATIVE SAFETY STANDARDS IF ANY 'SRS' COMPONENT DOES NOT CONFORM TO THE ORIGINAL MANUFACTURERS SPECIFICATION FOR THAT MARKET.
THE NORTH AMERICAN / CANADIAN STEERING WHEEL AND PASSENGER AIRBAG MODULES (IF EQUIPPED) ARE NOT INTERCHANGEABLE WITH THOSE OF OTHER MARKETS.
All US market XJS models (As of 1990, VIN 165791) are fitted with a driver air bag as a supplementary restraint system.
The air bag module (Fig. 1) is located under the centre trim pad of the steering wheel and comprises the air bag, inflator / sensor assembly and inflator / sensor housing. Each component is described below. The module is protected by the trimmed cover which splits horizontally as the air bag inflates. The module is non-serviceable and must not be dismantled, transferred to or recycled for use on another vehicle.
For the 1994 Model Year, all vehicles are fitted as standard with both driver and passenger side 'Supplementary Restraint System' (SRS), a system introduced at 1993.5 model year. North American and Canadian vehicles will now be fitted with a tear loop seat belt buckle and knee bolster on the driver side to complement those units currently fitted on the passenger side.
The passenger airbag is fitted in the area normally occupied by the glove box and when activated, exits through veneer faced deployment doors on the fascia.
Note: To ensure operator safety during removal and handling of airbags, a mechanism is built into each module to allow it to be armed and disarmed.
The airbag modules CANNOT be removed from either the steering wheel or fascia unless that module has been disarmed.
Driver Inflator / Sensor Assembly
Fig. 1:
The inflator/sensor assembly consists of the components which are illustrated in Fig. 1 and described in the following paragraphs:
Two spherical 'Sensing Masses' (housed in cylinders) which move forward and rotate a pair of D-Shafts when the vehicle suffers a frontal impact of sufficient severity. Thus causing significant movement of the driver relative to the steering wheel.
Two pivoted D-shafts each retaining a firing pin.
Two firing pins which ignite a primer when released by the D-shafts.
A primer which reacts with a propellant to produce nitrogen gas which inflates the air bag. Two bias springs which return each D-shaft to its seated position if an impact is of insufficient strength to fully trigger the sensors.
A safety shaft and safety levers which prevent the D-shafts from rotating and releasing the firing pins after the Disarming Procedure has been carried out. This allows service / maintenance work to be safely carried out.
Upper / Lower Steering Column Assembly
The upper steering column (and brackets) transmit the crash pulse to the steering wheel/air bag module sensors. The steering column collapses in the event of a crash and absorbs impact energy.
Dash Liner
The dash liner replaces that fitted to non-air bag vehicles. Although similar in appearance to the old liner, the new one is of a heavier construction, to work as part of the occupant restraint system.
Body Components
In the event of a frontal impact, the vehicle body will deform, absorbing the energy of the impact. The crash pulse is transmitted through the vehicle body and steering column mounting brackets to the steering column and then to the steering wheel hub, the air bag module and the sensor.
Arming / Disarming Mechanism (Driver Airbag)
Should maintenance of the steering-related components be required involving the steering wheel / air bag module. A safety device is built into the inflator / sensor assembly to allow safe maintenance of steering wheel components which involves the removal of the steering wheel / air bag module. Arming and disarming is controlled by an arming screw housed in the steering wheel hub.
Armed condition:
When screwed clockwise against its stop, i.e. fully home, the arming screw (4 Fig. 1) forces a chamfered safety shaft (2 Fig. 1) between the two safety levers (1 Fig. 1). The two levers, fixed at one end, are forced away from the 'D' shaft (3 Fig. 1), allowing the D shaft to be moved by the sensing mass (5 Fig. 1).
Disarmed condition:
When the arming screw (4 Fig. 2) is turned anti-clockwise to its stop (12 turns, approximately 15mm total movement), spring pressure (6 Fig. 2) forces the chamfered safety shaft (2 Fig. 2) away from the levers (1 Fig. 2) and tension returns the levers to rest. When in this state, the levers lie across the D-shaft (3 Fig. 2) and block any movement of the sensing mass (5 Fig. 2) so disarming the system.