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Student Workbook

LV42 Electrical and Electronic

Systems (3)

kap all phase 2 & 3 6/11/03 11:38 am Page 45

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV42 ELECTRICAL AND ELECTRONIC

Airbag sensors - external to SRS 14 Safety device (mechanical type only) 33

ECU

Page Page

Introduction

Most vehicle manufacturers fit as standard many advanced features During this course of study we will look in detail at the following systems:

• SRS airbag and seatbelt pre-tensioner

• in-car entertainment systems

• satellite navigation systems

• multiplex communication

• diagnosis of such systems

Revision Exercises

When studying such systems it is easy to overwhelmed by the apparent

complexity It is important to maintain a focus on the basics, as they will

always apply

Complete the following exercises on the next few pages to confirm your

understanding of fundamental electrics

1 The diagram below shows a simple earth switched lamp circuit

Consider this circuit as serviceable and in the condition pictured

(switch closed or on)

Mark on the voltages at all the key points

2 Now consider an open circuit fault in connector B (poor terminal

contact)

Mark on the voltages at all the key points

3 Now consider an open circuit fault in connector F (poor terminal

contact)

Mark on the voltages at all the key points

4 Now consider a high resistance fault in connector D (poor terminal

contact)

Mark on the voltages at all the key points

5 Now consider a high resistance fault in connector F (poor terminal

contact)

Mark on the voltages at all the key points

6 Draw three differing short circuit faults on the circuits below:

SRS Airbag and Seatbelt Pre-tensioners

All vehicle safety systems fall into one of two categories, passive or active

A passive safety system is one that acts to limit injury during an accident

An active safety system is one that acts to prevent the accident from

happening in the first place

SRS (Supplementary Restraint System) airbags are a good example of a

passive safety system Airbags are now positioned to protect the driver and passengers in the event of impact from virtually any direction, with the

exception of rear end shunts (where the head restraint is of primary

importance) Driver’s airbags are housed within the steering wheel pad,

passenger airbags within the dashboard, curtain shield airbags in the roof

runner trim and side airbags in the seat back Some manufacturers are now fitting airbags to protect the driver’s knees and also airbags that deploy across the front screen Rollover protection airbags can now be found under roof

linings on high specification vehicles

Seat belt pre-tensioners work in conjunction with frontal protection airbags Upon deployment of the front airbags, explosive charges remove any slack

from the seat belt preventing the occupant from adopting a position

dangerously close to the deploying airbag The importance of wearing

seatbelts in a vehicle equipped with airbags cannot be over stressed Without this restraining mechanism, airbags can indeed be lethal Hence the term

SRS – Supplementary Restraint System

SRS airbag

The diagrams above show the underlying principle of the airbag During

frontal impact the airbag deploys automatically providing a cushioning effect between the occupant and the vehicle In an accident, events take place very quickly and the SRS airbag system must be capable of responding to this A vehicle travelling at 30 mph in collision with a non-deformable barrier will

come to a complete standstill in under 100ms (100 thousandths of a second

or 1 tenth) From impact detection to full deployment can take as little as

20ms (20 thousandths of a second)

Without the protection of the airbag, the impact experienced by the occupant

at this speed (30 mph) is the equivalent of a fall from a third floor window

Deployment conditions

An airbag deploys with considerable force (it has to in order to be effective) For this reason it is vital that deployment only occurs in the event that the

accident requires it If an airbag were to deploy during minor collisions (such

as ‘kerbing’ the vehicle or knocking a parking post at low speed) the

deployment could result in serious injury

To this end, the SRS airbag system will not deploy the airbags until a

pre-determined threshold is reached This threshold is the equivalent impact

energy released if a vehicle was to collide with a non-deformable barrier at a speed of 12 to 14 mph

Nitrogen gas is not harmful (the air that you breathe is 78% nitrogen) The

explosive charge that generates the nitrogen gas does generate high

temperatures that can result in superficial burns The nylon bag itself can

cause minor abrasions to the face During deployment, ‘smoke’ is generated Most of this ‘smoke’ is talcum powder, which is used as a lubricant, but some compounds of sodium are also generated These compounds are mostly

harmless but can cause minor skin irritation Face and hands should be

washed as soon as practicable after deployment is experienced

Deployment is very loud! During controlled deployment (discussed later) ear defenders must be used People involved in accidents where deployment has occurred often state that the noise of the impact obscures the noise of

deployment, which gives you some idea of the levels of noise that an accident generates!

Types of SRS airbags

Airbag systems are categorised into two main types:

• mechanical

• electronic

Mechanical type

The mechanical type system is completely self-contained within the airbag

assembly itself and is not electrical in anyway

During impact (the accident) the weight will roll forward when the

predetermined impact threshold is reached This movement tilts the trigger shaft, which releases the firing pin When the firing pin strikes the primer it

ignites; this generates the gas required for inflation

Safety mechanism

The mechanical type airbag is particularly dangerous to the technician The nature of its deployment mechanism is such that if the airbag was to be

dropped when removed it could deploy To reduce the likelihood of this

happening, manufacturers design a safety device into the mechanism The airbag assembly cannot actually be removed from the car unless this safety device is engaged; it is not physically possible to do so

There are two types of safety mechanism but their action is similar The one pictured to the right is the pull type In order to engage the safety device (and remove the airbag assembly) the rod must be withdrawn fully

Note: The rod cannot be removed completely from the airbag assembly; it is captive The action of withdrawing the rod locks the weight in place This

makes the assembly safe

The second type of safety mechanism is shown above The only difference is that this rod is withdrawn by rotating it, normally with a tamperproof Torx bit Although these safety mechanisms provide a high degree of protection it

should be noted that the greatest of care should be taken with mechanical

airbags They are explosive devices and must be treated with respect A rifle loaded and cocked with the safety catch applied is far from risk free and a

mechanical airbag is very similar in its design and capability

Note: Under no circumstances apply force to a mechanical airbag assembly i.e with a hammer The airbag is highly likely to deploy, especially if it is fitted

to the car and the safety mechanism is not engaged

Crash sensor ON

Range of operation

AND Safing Sensor ON

Crash sensor ON

Range of operation

AND 11 Ignition Ignition

The airbag sensors (crash sensors or impact sensors) are designed to provide

an electrical signal to the SRS ECU (sometimes referred to as a centre airbag sensor assembly) The receipt of such a signal from an airbag sensor (there are normally two of them, one mounted on each of the inner wings under the

The SRS ECU will not deploy the airbag(s) until it receives a confirmation

signal from the safing sensor This sensor is mounted inside the SRS ECU itself, hence this is sometimes referred to as the centre airbag sensor

assembly Deployment will only take place if at least one external sensor and the safing sensor confirm impact energy is sufficient There always has to be

a signal from the safing sensor Once impact is confirmed, the SRS ECU will allow current to flow through the primer by grounding its supply circuit This confirmation approach prevents unnecessary deployment through a fault

which could be disastrous

Airbag sensors – external to SRS ECU

Airbag sensors are of an eccentric mass design This eccentric mass rotates forwards upon collision bringing together two electrical contacts When

contact is made, current flows to the SRS ECU providing an impact signal

Safing sensor – inside SRS ECU

The safing sensor is of a different design Mounted inside the SRS ECU, this sensor is a strain gauge type A strain gauge experiences a change in

resistance in accordance to flexing experienced This flexing of the assembly

is caused by the impact energy and is converted into an electrical signal

through the use of an integrated circuit (IC)

Mercury switch

Occasionally, the safing sensor is a mercury switch type although these are rare In this type, mercury rolls forward to make an electrical contact when the impact threshold is exceeded

Note: Upon deployment through impact, eccentric mass airbag sensors must

be replaced This is because the large amount of current flowing through

them during deployment renders the contacts unserviceable

Note: If the safing sensor inside the SRS ECU is of the mercury switch type, the ECU must be replaced in the event of deployment for the same reason

SRS electrical circuitry

The diagram above shows a typical SRS airbag circuit configuration It can be seen that the SRS ECU receives a power supply via three fuses This is to reduce the amount of current that flows through the terminals on the ECU

The SRS ECU applies a voltage to each of the front airbag sensors and also controls the operation of the SRS warning lamp on the combination meter

Upon impact, both or one of the front airbag sensors will make the circuit (it can be seen that they are simply switches) applying voltage to the SRS ECU

If the ECU receives a confirming signal from the safing sensor mounted inside the ECU, it will deploy the airbag by sending a controlled current through the inflator which is integral with the bag

Spiral cable

The driver’s airbag is mounted in the centre of the steering wheel Because the steering wheel rotates (on some cars as much as three full revolutions

from lock to lock) a special electrical contact is required to facilitate a

connection to the airbag assembly Before the advent of airbags, a similar

problem was faced with connecting the horn push when mounted in the

middle of the steering wheel This problem was overcome through the use of slip rings and brushes This type of contact would not be sufficiently reliable for such a vital system as an airbag so an alternative solution was sought

The spiral cable is a component that allows rotation to take place without

straining or breaking the cable It is in effect a spool of cable that has

sufficient length to accommodate the rotation of the steering wheel

Fitting a spiral cable

The spiral cable must be centralised before fitting If this is not done, the

cable will have plenty of movement in one direction but insufficient in the

other, and it will break when the steering is put on full lock in that direction

New spiral cables are supplied locked in the central position with a small

plastic peg Ensure that the steering is dead ahead, fit the cable and only

then remove the peg Spiral cables that have been removed and are to be

refitted can be ‘timed’ through the use of a coloured tooth inside the assembly and a match mark (usually an arrow)

Progress check 1

Answer the following questions:

1 Which sensor must always turn on in order for deployment to take

place?

2 What is the purpose of the spiral cable?

Operation

The operation of side and curtain shield airbags is very similar to that of driver and passenger airbags The key difference is the provision of additional crash sensors designed to detect impact from the side These sensors work in an identical way to those already discussed but they are normally far quicker in their ability to react (there is less crumple zone to the side of the vehicle)

These sensors are normally mounted at the bottom of the vehicle’s B posts There maybe some differences in the way in which the inflator mechanism

operates with some airbag assemblies

A slightly different approach taken with a curtain shield airbag is shown above This type of airbag is referred to as a microgas type Rather than relying on heat and gas generating pellets, the airbag has a compressed gas canister The igniter fires a ‘nail’ into the canister, piercing it and releasing the gas into the bag

Progress check 2

Answer the following questions:

1 What type of impact is a curtain shield airbag designed to protect the

occupants from?

2 What gas is used to inflate the airbag assembly?

3 Where are the side impact sensors normally located?

Safety Information

Electronic SRS airbag

Before carrying out any work on electronic SRS airbag systems (with the

exception of fault code extraction) it is essential that the vehicle battery be

disconnected Disconnecting the vehicle battery prevents any current flowing

in the system and therefore prevents deployment

IMPORTANT

ONCE THE VEHICLE BATTERY HAS BEEN DISCONNECTED, YOU MUST

WAIT AT LEAST 90 SECONDS BEFORE CARRYING OUT ANY WORK ON THE SYSTEM

This is because the SRS ECU contains large capacitors that are capable of retaining sufficient charge to deploy the airbags during this time These are there to enable deployment to take place in the event that the vehicle’s battery

is destroyed during the collision

Never use memory keepers when working on SRS airbag systems

Always follow manufacturers’ specific procedures when working on these

Never use an ohmmeter to measure the resistance of an airbag or

pretensioner assembly Ohmmeters create current flow in the circuits they

measure and this will deploy them (very dangerous)

Always replace the SRS ECU if a vehicle collision has resulted in deployment Treat SRS ECU’s as toxic waste when disposing of them if they are known to contain the mercury switch type safing sensor

Never disconnect/reconnect the SRS ECU if it is not mounted to the vehicles body; static could cause deployment

Mechanical SRS airbag

When any work is to be carried out on such a vehicle that could result in

strong mechanical shocks, always engage the safety mechanisms on the

airbags and pre-tensioners

Do not bring magnets close to mechanical airbag and pre-tensioner

assemblies

When a vehicle has been involved in a collision, even if the airbags or

pre-tensioners have not deployed, always inspect the airbags and pre-pre-tensioners

Never use SRS airbag components from a donor vehicle; always use new

parts

Never attempt to repair SRS airbag assemblies (including sensors, airbags, pre-tensioners, ECU and loom); always replace the faulty part

Never expose airbag components to heat

At any sign of damage of any description to SRS components, replace them with new ones

When disposing of airbag or pre-tensioner assemblies, always deploy them first This includes scrapping a vehicle with such assemblies in situ

Never place removed airbags deployment side down; if they deploy they will take off like a rocket

Never use grease or cleaning agents on SRS components

Always follow manufacturers’ specific instructions when diagnosing SRS

airbag faults

Connectors

SRS airbag connectors (and loom conduits) are always coloured yellow to aid identification

Because of their importance, SRS airbag connectors are always of a most

secure design Normal ‘push/ pull’ connectors would be insufficient due to the risk of inadvertent disconnection Shown above is a typical SRS connector

To release the connector the lock must be disengaged and the hinged gate lifted up Only then can the connector be disconnected

Airbag activation prevention mechanism

It has already been stated that there is a very real risk of deployment if an

ohmmeter is used on the system in a way that creates current flow through

the airbag or pretensioner assembly To reduce the likelihood of this

occurrence, a mechanism is in place that shorts together the connector

terminals on the assembly upon disconnection This will prevent a potential difference from being created across the primer and therefore prevent

accidental deployment However, this mechanism is a last chance device and

no substitute for knowledge!

Diagnosis

Diagnosis of SRS airbag circuits must never be carried out without reference

to specific manufacturers’ instructions This is because the use of an

ohmmeter to diagnose wiring faults (within the loom only) is often required,

and this presents a very real risk of accidental deployment if procedures are not followed to the letter

Progress check 3

Answer the following questions:

1 What is the purpose of the airbag activation prevention mechanism?

2 What colour are SRS airbag connectors?

3 Why must an SRS ECU never be unplugged unless it is fitted to the

vehicle body?

Whether the articles should be kept in a licensed store or at registered

premises depends on the number of inflators/tensioners that the workshop

proposes to keep in stock

For most workshops a secure steel cabinet is the most practical method of

storing inflators/tensioners Provided that the cabinet is used solely for the

storage of inflators/tensioners and is located in a cool dry area well away from flammable gases/liquids and sources of ignition, no further precautions need

be taken to comply with the law However, it would be helpful for the fire

service to be aware of the contents of the cabinet as they may have to enter the building in the event of a fire This objective can be easily achieved by

simply displaying the orange hazard warning diamond 1.4s on the door of the cabinet

Registration of the premises is simply a matter of completing an application form (FSG5118) available from local Fire Safety Offices A small fee is

payable and the registration must be renewed annually

It may well be the practice of some motor manufacturers to preclude their

dealerships from having to keep a stock of inflators/tensioners by supplying the units from a central distribution depot on an "as needed" basis Where

this policy applies and the replacement unit is immediately fitted to the vehicle,

it may not be necessary to register the premises Any units kept in stock for future use will be deemed, as being kept for the purpose of sale and

Electronic type disposal

SRS airbags and seatbelt pre-tensioners must never be disposed of without first deploying them A typical deployment tool to aid the deployment of an

electronic type airbag is shown above It is no more than a switched circuit that enables a standard vehicle battery to be connected to the airbag or pre-tensioner assembly and current applied

It is best to use this tool to deploy the airbag or pretensioner whilst the

assembly is still fitted to the vehicle If this is not possible, the following

procedure should be observed The airbag should be firmly secured to an old wheel and tyre assembly, deployment side uppermost This wheel should

then have at least three tyres (no wheels) stacked on top of it This will keep what amounts to a controlled explosion contained

Disposal mechanical type

Disposal of the mechanical type requires the suspending of the assembly

inside the tyres and dropping it in order to bring about deployment