Restraint systems (Operation): Description and operation

OPERATION

CAUTION: The diagnostic control unit is a non-serviceable component and no attempt should be made to repair or modify the unit.

The diagnostic control unit (DCU) which is mounted within the passenger compartment performs two functions:

1. It continually monitors the airbag system for faults;
2. It provides a fire signal to the airbag(s) and pretensioners in the event of a crash.

The SRS warning lamp illuminates to inform the driver of any airbag system faults.

The diagnostic control unit comprises of the following circuits which control the airbag system:

1. The crash sensor determines the severity of any impact and can discriminate between rough road conditions and an actual crash.
2. The safing sensor is wired in series with the crash sensor. The fire signal from the crash sensor therefore, passes through the safing sensor which confirms a crash is taking place.
3. The regulator circuit maintains stability of the system in the event of battery voltage drop.
4. The back-up power supply provides power to the system in the event of the battery being damaged or disconnected during the impact.

When triggered, the fire signal from the diagnostic control unit passes via the airbag harness to the airbag modules. Grains of Nitrocellulose and Nitroglycerine inside the airbag module, ignite and combine in a chemical reaction to form a large amount of Nitrogen gas leading to inflation of the airbag in approximately 30 milli-seconds. A similar gas explosion forces pistons along the length of the pretensioner modules which pull the seat buckles downwards, removing any slack in the belt mechanism.

As the occupant moves forward into the airbag it immediately deflates to provide progressive deceleration and reduce the risk of injuries.

Sequence of operation:

1. The main sensor and the safing sensor are activated.
2. Power is supplied to the airbag igniter(s) and pretensioners by the battery or the back-up circuit.
3. Airbag and pretensioners deploy.

It takes about 0.1 seconds from the beginning of the airbag deployment until it is completely deflated.

System check

The SRS warning lamp, located in the instrument pack, illuminates when the electrical circuits are switched on whilst the system performs a self diagnosis test. If the system finds no fault during self diagnosis the light will extinguish after approximately 5 seconds and remain extinguished.

In the event of a fault in the system, the warning light will either:

1. Illuminate continuously;
2. Illuminate for 0.5 second, extinguish, then illuminate continuously;
3. Fail to illuminate during the self diagnosis test.

Fault diagnosis

Diagnosis is accomplished using TestBook connected to the diagnostic socket which is located on a bracket on the right hand side behind the centre console.