XK-8 Convertible (X100) V8-4.0L (AJ27) (2001)
Air Bag Control Module: Description and Operation
NOTE: Due to the importance of the RCM being securely fixed to the vehicle body, the ground connection is made via the fixings and is monitored by
the diagnostic system.
The RCM is mounted on the top of the drive shaft tunnel below the center console. It identifies crash severity, the direction of impact and makes
decisions on deployment of air bag modules and front safety belt pretensioners. It also provides firing signals to all air bag modules and front safety belt
The RCM controls air bag deployment decisions by using signals from its internal accelerometer and the following:
Front crash sensor
Front safety belt buckle sensor
Driver seat track position sensor
Front seat passenger weight sensing system
Occupancy sensing module
Internally, the RCM has two areas that determine which elements of the SRS are to be deployed:
Crash severity evaluation - This area evaluates crash severity by using data from the RCM internal accelerometer, the front crash sensor and the
safety belt buckle sensor. Based on this data, the RCM 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, occupancy sensors, front seat passenger weight sensing system and safety belt
buckle sensors are examined before a decision is made about which restraints should finally be deployed. For instance, if the occupancy sensing
and weight sensing system indicate that the passenger front seat is empty, then no restraint deployment will take place on the passenger side, even
if full deployment takes place on the driver side.
Data from the side crash sensors is used by the RCM in conjunction with acceleration data from the RCM internal accelerometer to make a deployment
decision. The RCM processes the acceleration data and subject to an impact being of high enough severity, decides whether the side air bag module
should be deployed. The decision is forwarded to the deployment handler (within the RCM) which responds appropriately. For example, in the case that
the front seat passenger weight sensing system calculates that the seat is empty, or occupied by a small person, the passenger side air bag module will be
On board testing of the air bag module, front safety belt pretensioner firing circuits, warning indicator circuits and module status (the front and side crash
sensors perform basic self-tests) is performed by the RCM together with the storing of fault codes.
The RCM drives the SRS indicator on the instrument pack. If the warning lamp fails, a fault code is recorded and a warning tone is sounded. 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 such deceleration information, firing delay and fault codes for subsequent access via the diagnostic connector.