Discovery 2. Manual - part 641

 

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Discovery 2. Manual - part 641

 

 

RESTRAINT SYSTEMS

DESCRIPTION AND OPERATION

75-5

Passenger airbag module

The passenger airbag module is located in the instrument panel above the glove box and is controlled by the DCU.  
A bag within the module inflates to protect the passenger in the event of a collision severe enough to deploy the 
system.  

Within the passenger 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 passenger presses against the bag.  Vents in the bag control airbag deflation.

The DCU regularly applies a test current to the passenger 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 passenger 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.

RESTRAINT SYSTEMS

75-6

DESCRIPTION AND OPERATION

Seat belt pretensioners

The seat belt pretensioners are located within the front seat belt anchors and are controlled by the DCU.  They are 
pyrotechnic devices that, when deployed, tighten the front seat belts.  This works in conjunction with the airbag 
module(s) to protect front seat occupants in the event of an accident.

The DCU regularly applies a test current to the seat belt pretensioner squibs 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 squibs.  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.

RESTRAINT SYSTEMS

DESCRIPTION AND OPERATION

75-7

SRS warning lamp

The SRS warning lamp located in the instrument cluster indicates the following:

l

The status of the system readiness check on start-up.

l

A fault within the SRS system.

The input to the SRS warning lamp from the DCU is 12 volts during the readiness test, 0 volts if no fault present and 
12 volts if a fault is detected.

The DCU is capable of detecting an open circuit or a short circuit for the SRS warning lamp. 

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

l

A count indicating the number of times the fault occurred.

l

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

RESTRAINT SYSTEMS

75-8

DESCRIPTION AND OPERATION

Diagnostic and Control Unit (DCU)

The SRS DCU is an electronic single point crash sensor. It is capable of sensing if a crash has taken place. If all the 
relevant parameters are met, then the DCU deploys the airbag(s) and seat belt pretensioners.

The DCU is bolted to the transmission tunnel beneath the centre console in the area of the handbrake.

The DCU performs the following functions:

l

Internal self test, system monitoring and fault detection.

l

SRS warning lamp operation.

l

System deployment.

Self test, system monitoring and fault detection
The DCU performs a self-test during start-up and then continually during operation of the system. The following 
components are tested:

l

The decelerometer.

l

The electromechanical safing sensor.

l

The microprocessor.

l

The continuity of squib ignition circuits.

l

The SRS warning lamp.

If the DCU detects a fault, it stores the fault code in memory.  Diagnose the DCU with TestBook.  The DCU cannot 
be serviced.  It must be replaced if it is faulty.

SRS warning lamp operation
The DCU controls the SRS warning lamp contained in the instrument cluster. 

System deployment
The DCU determines when the system deploys based on inputs from both the decelerometer and the 
electromechanical safing sensor. Both are located within the DCU. Neither are serviceable.

The electromechanical safing sensor uses a magnet and ball bearing system to provide a mechanical confirmation of 
vehicle deceleration.  The ball bearing is held in place by the magnet.  The strength of the magnet is calibrated such 
that the force required for the ball bearing to move away from the magnet will only occur if the vehicle decelerates 
suddenly, as in an accident. 

The SRS deploys only if both the decelerometer reading and the safing sensor operation occur at the same time.

 

 

 

 

 

 

 

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