RESTRAINT SYSTEMS (page 2 of 3)

RESTRAINT SYSTEMS

DESCRIPTION AND OPERATION

75-21

Front and Rear Impact Firing Strategy (NAS Only)

The battery disconnect unit is fired:

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At driver and passenger airbag limit 2 in a frontal impact

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At the battery disconnect unit limit in a side impact

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At the rear impact limit in a rear impact.

If one of the side impact limits is exceeded the DCU fires the side airbag and the head airbag(s) on that side of the 
vehicle. If the side impact limit on the front passenger side of the vehicle is exceeded, the DCU also evaluates the 
input from the seat occupancy module, and fires only the head airbag(s) if the front passenger seat is unoccupied.

In a roll-over, the airbags, pretensioners and battery disconnect unit are only fired if a related impact limit is exceeded. 
If multiple impacts occur during a crash event, after responding to the primary impact the DCU will output fire signals 
in response to secondary and further impacts if unfired units are available.

Crash Signal
When the DCU outputs any of the fire signals it also outputs a crash signal to the Body Control Unit (BCU) and the 
ECM. On receipt of the crash signal, the BCU goes into a crash mode and the ECM cuts the power supply to the fuel 
pump relay. In the crash mode, the BCU:

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Activates all of the unlock signals of the vehicle locking system, even if the vehicle is already unlocked

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Ignores all locking/superlocking inputs until it receives an unlock input, when it returns the locking system to 
normal operation

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Activates the interior lamps. The interior lamps remain on permanently until they are manually switched off at the 
lamp unit, or the BCU crash mode is switched off and they return to normal operation

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Disables the rear window child lock input until the crash mode is switched off

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Sends a crash message to the Light Check Module (LCM), via the K bus, instrument pack and I bus, to activate 
the hazard warning lamps. The hazard warning lamps remain on until cancelled by the hazard warning switch or 
the crash mode is switched off.

The BCU crash mode is switched off by a valid locking and unlocking cycle of the locking system.

Seat Belt Status

Passenger 

Seat Status

Strategy

Driver

Passenger

Applicable 

Pretensioner

Driver Airbag

Passenger 

Airbag

Fastened

–

–

Fired at 

pretensioner or 

rear impact limit

Fired at limit 2 or 

above

–

Unfastened

–

–

Not fired

Fired at limit 1 or 

above

–

–

Fastened

Occupied

Fired at 

pretensioner or 

rear impact limit

–

Fired at limit 2 or 

above

  

Unoccupied

Fired 

at 

pretensioner or 

rear impact limit

–

Not fired

–

Unfastened

Occupied

Not fired

–

Fired at limit 1 or 

above

  

Unoccupied

Not 

fired

–

Not fired