Auto Electrics

PART EIGHT: Supplementary restraint systems and vehicle safety.

Part eight: Vehicle safety.

Vehicle safety has come a long way since Volvo engineer Nils Bohlin invented the threepoint seatbelt in 1959. Seatbelts, however, are arguably still the single most effective safety feature in a car. While the first patent for an airbag can be traced back to the 1950s, the basic idea of an inflatable cushion to protect the occupant has changed little. Nevertheless, Advanced Driver Assist Systems (ADAS) and features that contribute to active safety have seen significant developments in recent years, including adaptive cruise control, autonomous emergency braking and blind spot monitoring.

In the automotive industry, occupant and pedestrian protection are divided in two categories.

 Passive Safety This encompasses features that come into play after a collision has occurred and are designed to reduce the risk or severity of injury to occupants or pedestrians. This includes airbags, seatbelts, head restraints, seat design, side impact protection, crumple zones, etc.  Active Safety Anything that contributes to avoiding or reducing the risk of a collision. Brakes to stop the vehicle, tyres for grip, steering, headlights for visibility, comfortable cabin design, stability control, forward alert warning, etc.

These are some of the components that make up a supplementary restraint system.

 Airbag Control Module The central controller for the airbag system. The module is constantly monitoring inputs such as crash sensors, road speed, rate of deceleration, yaw rate (rotation around the vertical axis) and roll-over detection. The airbag module processes the data at an extremely fast rate to determine if deployment of airbags or seatbelt pretensioners is necessary. Most airbag modules have two internal processing areas: crash evaluation and deployment handling. Crash evaluation monitors the inputs to determine if an impact is severe enough to require airbag deployment. The deployment handler monitors seat position, seatbelt buckles and, if necessary, triggers the pyrotechnic igniters in airbags and seatbelts. Airbag modules also manage fault monitoring, system diagnosis, warning lamps, and crash data storage. Modules contain a capacitor that is capable of storing enough energy to deploy the airbags if the power supply is disrupted in a crash. The module also houses a ‘safing sensor’ – this is an inertial measurement unit (IMU) to verify any crash signals received from the external crash sensors. Modules are mounted centrally in the vehicle along the tunnel; correct fitting is essential for the system to function correctly.

MEMS are also used in medical devices, smartphones and fitness trackers.

 Crash Sensors Cars are typically fitted with six or eight crash sensors located strategically around the car to detect impact. Micro electrical mechanical systems (MEMS) or IMUS detect rapid deceleration and feed this back to the airbag module; they’re usually fitted to the front crossmember or chassis leg. Pressure sensors are used in door cavities to detect side impact – when a door is compressed due to a side impact, the sensor detects a spike in pressure and relays this back to the airbag control module. It is important when working on car doors with side impact sensors that all seals, blanking plugs or grommets are refitted or replaced as necessary or you could inhibit the function of the side impact sensors.

 Airbags Most new cars are fitted with driver, passenger, side, curtain and knee airbags. Airbags have a pyrotechnic igniter to light a propellant (sodium azide) that rapidly produces an inert gas, mostly nitrogen, to inflate the airbag. The bag is a folded textile cushion designed to inflate and deflate rapidly. In some cases, the time between the impact and the airbags inflating can be as quick as 150 milliseconds.

Hybrid airbags use a combination of compressed gas and propellant to fill the bag. Single-stage airbags have one igniter, while two-stage airbags are deployed in steps typically 50-100 milliseconds apart, depending on the severity of the impact. Side and curtain airbags protect the torso and head in a side impact. As the space between the occupant and the side of the vehicle is so narrow, they must inflate extremely quickly – in some vehicles this can as short as 15 milliseconds.

Pedestrian airbags are fitted to the bonnet area and deploy to protect against head injury. Pyrotechnic actuators deploy bonnet hinges to reduce impact severity. Sensors mounted behind the front bumper detect pedestrian impact. Hybrid airbag.

 Airbag components are quite reliable and the cause of system faults is often related to wiring. Issues can arise when wiring under a seat gets stretched or trapped with movement of the seat. When working with SRS components, extreme care must always be taken. Most manufacturers recommend disconnecting the 12V battery and allowing time for capacitors to discharge before removal or disconnection. Fitting secondhand or used parts is never recommended. Any work should be carried out by a suitably qualified technician.

 Pyrotechnic seatbelts and

airbags are fitted with shorting bars that bridge the two pins when disconnected from the wiring harness. This is a safety feature to protect against accidental deployment when disconnected or removed from the vehicle.

 Seatbelts Most are fitted with pyrotechnic pretensioners controlled by the airbag module. In the event of a crash, the pretensioner is fired, which takes up any slack in the belt and helps to correctly position the occupant in the seat. As the vehicle slows further, a load limiter allows the occupant to move forward into the inflated airbag. Seatbelt webbing should be checked for any frayed edges and belts should retract when slack.

 Electrical connectors for SRS components feature locking tabs that must be engaged when fitting. The locking tab lifts the shorting bar, enabling deployment. Connectors can break easily if not handled with care. Gold-plated contacts are often used on airbags due to their low resistance.

 Some manufacturers can supply kits for SRS wiring repairs. Yellow wire is often recommended, as it allows technicians to quickly and easily identify where repairs have been carried out. Repairs must be carried out following manufacturer guidelines and using recommended repair methods. Not all brands permit repairs to the airbag wiring harness.

 Warning labels are fitted to sun visors to warn against fitting rearfacing child seats to the passenger front seat. In some models, the airbag can be switched off or disabled, but it’s always best to check your owner’s manual and consult your main dealer for advice when fitting a child seat. Most vehicles use the ISOFIX system for attaching child seats.

 Airbags are clearly marked where fitted. If fitting seat covers to a vehicle with seat airbags, always check if the covers are approved. Approved seat covers are designed with weak spots or perforations to allow for airbag inflation. Non-approved seat covers can be a hazard.