Discovery 2. Manual — part 640

RESTRAINT SYSTEMS

DESCRIPTION AND OPERATION

75-1

RESTRAINT SYSTEMS

DESCRIPTION AND OPERATION

SRS component layout

RHD shown, LHD similar

1 DCU
2 SRS warning lamp
3 Driver airbag module
4 Driver seat belt pretensioner
5 Passenger airbag module
6 Passenger seat belt pretensioner

RESTRAINT SYSTEMS

75-2

DESCRIPTION AND OPERATION

SRS block diagram

1 DCU
2 SRS warning lamp
3 Driver airbag module
4 Driver seat belt pretensioner
5 Passenger airbag module
6 Passenger seat belt pretensioner

RESTRAINT SYSTEMS

DESCRIPTION AND OPERATION

75-3

Description - SRS

General
The purpose of the Supplementary Restraint System (SRS) is to reduce the impact of the driver and passenger
against the vehicle's interior in the event of a serious accident.

The SRS system is a stand-alone system. There are no remotely mounted crash sensors. The only external input to
the DCU is power. The system only becomes live when the ignition is switched on.

The SRS consists of the following components:

l

Diagnostic and control unit (DCU).

l

SRS warning lamp.

l

Rotary coupler.

l

Driver airbag module.

l

Driver seat belt pretensioner.

l

Passenger airbag module.

l

Passenger seat belt pretensioner.

The passenger airbag is optional in some markets. All other components are standard fit in all markets. The SRS
functions the same in all markets.

RESTRAINT SYSTEMS

75-4

DESCRIPTION AND OPERATION

Driver airbag module

The driver airbag module is located in the steering wheel and is controlled by the DCU. A bag within the module
inflates to protect the driver in the event of a collision severe enough to deploy the system.

The driver airbag module connects to the steering column wiring harness via a rotary coupler.

Within the driver airbag module is a squib which, when given the correct current by the DCU, inflates the airbag by
initiating a chemical reaction. In deployment, the airbag inflates within a fraction of a second and then deflates at a
controlled rate as the weight of the driver presses against the bag. Vents in the bag control airbag deflation.

The DCU regularly applies a test current to the driver airbag squib to confirm continuity of the ignition circuitry.

The DCU is capable of detecting a short to ground, a short to battery and an open circuit for the driver airbag module
squib. It will also identify if the configuration of the DCU differs from that actually fitted to the vehicle.

The DCU also stores the following additional information on each fault:

l

A count indicating the number of times the fault has occurred.

l

A start time and an end time for the fault to indicate duration of the fault.