LR3/Disco 3
SRS OPERATION
General
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 and high
speed CAN bus messages from the ABS (anti-lock brake system) module and the steering angle 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 side impact.
RH 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 seat 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 large person, i.e. someone over a given weight (NAS only).
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
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). The time delay between firing the two stages of the inflator in the driver
air bag is increased if the driver seat is forward of the seat position sensor switching point.
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 detection system, or if there is a fault with the passenger air bag deactivation switch, the RCM
(restraints control module) increase the time delay between firing the two stages of the inflator in the passenger air bag.
If a side impact limit is exceeded, the RCM (restraints control module) fires the side air bag and the side air curtain(s) 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 system, and fires the side air bag only if the front
passenger seat is occupied by a large person (NAS only).
The side air curtain(s) on both sides of the vehicle are fired if the roll-over limit is exceeded.
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.
20
Right front impact sensor return
Input
21
Left front impact sensor power and data
Input/Output
22
Left front impact sensor ground
Input
23 and 24
Not used
-
