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kap all phase & 6/11/03 11:38 am Page 45 Student Workbook LV42 Electrical and Electronic Systems (3) LV42/SWB Student Workbook for Technical Certificates in Light Vehicle Maintenance and Repair MODULE LV42 ELECTRICAL AND ELECTRONIC SYSTEMS (3) Contents Page Introduction Revision Exercises SRS Airbag and Seatbelt Pre-tensioners: SRS airbag Deployment conditions Overview of operation Types of SRS airbags Mechanical type Airbag sensor Safety mechanism Electronic type Overview of operation Airbag sensors - external to SRS ECU Safing sensor - inside SRS ECU Mercury switch SRS electrical circuitry Spiral cable Fitting a spiral cable Progress check Other Airbags: Operation Progress check 8 9 10 10 12 13 13 14 14 15 16 17 18 19 20 21 22 Page 23 23 24 25 Safety Information: Electronic SRS airbag Mechanical SRS airbag Connectors Airbag activation prevention mechanism Diagnosis Progress check Storage Electronic type disposal Disposal mechanical type 26 26 27 28 30 31 Seat Belt Pre-tensioners: Piston type Spool type Safety device (mechanical type only) 32 32 33 33 Multiplexing: Multiplex communication system Progress check Multiplex faults Open circuits Daisy chaining Short circuits + B short Ground short Bus cut relays Types of multiplex 34 35 38 39 39 39 40 41 41 42 (Cont.) -1Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Page Noise elimination Progress check Multiplex diagnosis Test equipment Typical wiring diagram Progress check 43 44 45 46 47 48 Vehicle Security – Immobiliser: Operation Progress check 49 49 50 In-car Radio: Broadcasting 51 53 Cassette Tape Player: Tape drive mechanism 54 54 Compact Disc Player 55 Page DVD (Digital Versatile Disc) 56 Speakers: Progress check 57 58 Satellite Navigation: Advantages Disadvantages 59 59 59 GPS (Global Positioning System): How it works 60 60 In-vehicle Communication Systems 62 Telematics: Applications Progress check 63 63 64 -2Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue 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 -3Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue 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 Now consider an open circuit fault in connector B (poor terminal contact) Mark on the voltages at all the key points -4Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Now consider an open circuit fault in connector F (poor terminal contact) Mark on the voltages at all the key points Now consider a high resistance fault in connector D (poor terminal contact) Mark on the voltages at all the key points -5Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Now consider a high resistance fault in connector F (poor terminal contact) Mark on the voltages at all the key points Draw three differing short circuit faults on the circuits below: -6Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue 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 -7Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue 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 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 predetermined 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 -8Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Overview of operation All SRS airbags have at least the following components: • airbag sensor(s) • inflator • bag The sensor detects the impact energy and if this exceeds the predetermined threshold it activates the inflator The inflator instantly generates nitrogen gas through the use of a primer, an igniter charge and gas generant pellets The nitrogen gas inflates a nylon bag that provides the cushioning affect The bag has deflation holes (normally two) that ensure that the bag does not create a rebound effect or obscure the view of the driver where applicable 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: ã mechanical ã electronic -9Copyright â Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Progress check Answer the following questions: Where is the transponder chip located? What is the purpose of the transponder key coil? -50Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue In-car Radio An in-car radio consists of the following components: • antenna • tuner • amplifier • speaker(s) The antenna picks up the radio waves and sends them to the tuner section The tuner selects one of the multitudes of radio signals that the antenna is detecting, turns these into sound signals and sends them to the amplifier The amplifier increases the strength of the sound signals to a useable volume The speaker(s) change the electrical sound signals into sounds that the human ear can detect -51Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Radio waves are transmitted by radio broadcasting stations These waves are generated at different frequencies in order for radio tuners to detect specific waves A wave frequency is the speed at which it changes from a high value to a low value (peaks and troughs) and is measured in Hertz A frequency of Hz means that the wave changes from high to low once every second kHz (kilo Hertz) means it changes one thousand times every second and MHz Mega Hertz) one million times every second GHz (Giga Hertz) means it changes one thousand million times every second! Radio waves are grouped into bands of frequency and they are as follows: 30 kHz to 300 kHz LF (Low Frequency) 300 kHz to MHz MF (Medium Frequency) MHz to 30 MHz MF (High Frequency) 30 MHz to 300 MHz VHF (Very High Frequency) 300 MHz to GHz UHF (Ultra High Frequency) The earth’s ionosphere reflects medium and long radio waves but short and ultra short are not reflected This makes medium and long wave radio signals easier to detect at very long distances -52Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Broadcasting Sound signals such as those produced by a microphone cannot be transmitted Before this can happen the broadcasting station must mix them with a very high frequency electrical wave known as a carrier wave This mixed signal is known as a modulated signal There are two main types of broadcast, AM and FM AM or Amplitude Modulation changes the amplitude (height) of the carrier wave and FM or Frequency Modulation changes the frequency of the carrier wave AM lends itself well to long and medium bands and FM to short bands; to this end AM stations can broadcast far greater distances FM broadcasting often produces far better sound quality This is because signals with better frequency characteristics can be broadcast (higher treble, lower bass) -53Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Cassette Tape Player Professional music tape manufacturers carry out recording music to tape The tape itself consists of a plastic strip with a magnetic medium attached When recording, this magnetic tape is passed across the recording head and electromagnetic poles on the head magnetise the tape in different places When this recorded tape is then passed under the head in play mode, the opposite effect takes place The magnetised portions of the tape create an electrical signal within the head, which is then amplified by the amplifier This signal is then further amplified before being relayed to the speakers Tape drive mechanism The tape drive mechanism is simply an electric motor driven pulley system See diagram above -54Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Compact Disc Player A compact disc player uses a laser beam to detect the presence or absence of pits etched into the surface of a plastic disc (the compact disc or CD) These pits are 0.5 µm (micro metres) wide, 0.9 to 3.3µm long and 0.11µm deep The lengths of the pits represent binary value streams These are interpreted by the playback circuit, converted into analogue signals and amplified before being sent to the speakers The pits are ‘read’ in the following way The laser beam is directed at the surface of the disc, if it strikes the surface of the disc where there is no pit, nearly 100% of the light energy will reflect back This reflected light is directed at a photo diode The photo diode will adopt a level of electrical conductivity dictated by the intensity of light that shines on it If the laser beam strikes a pit in the surface of the disc, then only about 30% of the light energy will reflect This reduces the conductivity of the photo diode and the signal received from it alters accordingly -55Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue DVD (Digital Versatile Disc) DVD is very similar to compact disc technology DVD’s are of the same diameter and thickness as CD’s, and they are made using some of the same materials and manufacturing methods Like a CD, the data on a DVD is encoded in the form of small pits in the track of the disc A DVD is composed of several layers of plastic (polycarbonate), in total about 1.2 millimetres thick This process forms a disc that has microscopic pits arranged as a single, continuous and extremely long spiral track of data Once the clear pieces of polycarbonate are formed, a thin reflective layer is added to the disc, covering the pits Aluminium is used behind the inner layers, but a semi-reflective gold layer is used for the outer layers, allowing the laser to focus through the outer and onto the inner layers After all of the layers are made, each one is coated with lacquer, squeezed together and cured This layering creates what is in effect a third dimension to the disc when compared to a standard CD The data track is incredibly small, just 740 nanometres separate one track from the next (a nanometer is a billionth of a metre) The elongated pits that make up the track are each 320 nanometres wide, a minimum of 400 nanometres long and 120 nanometres high The microscopic dimensions of the pits make the spiral track on a DVD extremely long If you could lift the data track off a single layer of a DVD, and stretch it out into a straight line, it would be almost 7.5 miles long! That means that a double-sided, double-layer DVD would have 30 miles (48 km) of data! This increases its capacity approximately seven fold when compared with a standard CD This added capacity makes it an ideal medium for video games and movies, all of which are finding themselves inside the passenger compartment of the modern motor vehicle -56Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Speakers A speaker receives audio signal voltages from the amplifier Current flows through the moving coil of the speaker due to the application of this voltage The current flowing through the moving coil generates an electromagnetic field, which reacts with the magnetic field created by the permanent magnet of the speaker The interaction of these two magnetic fields causes the coil to move or vibrate (hence its name) and this movement is relayed to the speaker cone Any movement of the speaker cone creates sounds waves in the air (pressure variations) -57Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Progress check Answer the following questions: What does FM stand for? Why does a DVD have a larger capacity than a CD? What percentage reflection does a CD pit produce? -58Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Satellite Navigation Satellite navigation systems are now commonplace on vehicles, most manufacturers at least offering it as an option but often as standard equipment Advantages • guides user to unfamiliar destinations • avoids the use of maps when driving • accurate and timely instructions Disadvantages • limited use for the majority of journeys • reduced perceived value • systems not aware of current traffic problems (unless specifically equipped) -59Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue GPS (Global Positioning System) The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of satellites placed into orbit by the U.S Department of Defence GPS was originally intended for military applications, but in the 1980s, the U.S Government made the system available for civilian use GPS works in any weather conditions, anywhere in the world, 24 hours a day The Global Positioning System (GPS) comprises three parts: • 24 satellites that orbit the Earth • ground control stations, which monitor the satellites • GPS receiver mounted on the vehicle The satellites are synchronised to emit encoded navigational information (exact positioning and exact time) Any vehicle equipped with a GPS receiver will be able to intercept these transmissions Using a simple mathematical formula derived from triangulation, the receiver is able to calculate its own longitude, latitude, velocity and even altitude Accuracy with these systems for civilian applications is typically 100 metres How it works GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth GPS receivers take this information and use triangulation to calculate the user's exact location -60Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received The time difference tells the GPS receiver how far away the satellite is With distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude) Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance and distance to destination -61Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue In-vehicle Communication Systems The pace of the working world has increased beyond all recognition over the last few decades People are finding themselves spending far more time in their cars during the working day than ever before The importance of being able to communicate whilst in the car is paramount CB (Citizens Band) radio has been available across the globe for many years now Its principle of operation is similar to that of the radio as discussed earlier in this workbook; the primary difference being that the set is both a transmitter and a receiver and works within a very narrow band of frequency dictated by the Government In-car telephones are now also proving popular The inherent dangers (and illegality) of using a mobile phone with no hands-free kit will only increase their popularity -62Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Telematics The French word télématique was coined in the 1970s to denote the combination of télécommunications and informatique 'computing' Telematics is best defined as the discipline which has emerged from the coming together of the electronics, communications and information technologies The subject matter of telematics extends over a broad area: from fundamentals of electronics, telecommunications and information engineering, to the application of these technologies in areas such as the design, implementation, use and evaluation of services at a distance via networks In a motor vehicle context, telematics is the ability of the vehicle to communicate with information technology systems external to that vehicle Applications Vehicle telematics at the time of publication is in its infancy A few examples of potential applications are as follows: • fault detection at distance and parts ordering (AA and RAC) • internet browsing • electronic traffic avoidance (Traffic Master linked to satellite navigation) • automatic billing on toll roads • automatic speed regulation • logistics communications • vehicle tracking Who knows what the future could hold? -63Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Progress check Answer the following questions: What does GPS stand for? How many satellites (minimum) are required to ascertain longitude, latitude and height? What is telematics? -64Copyright © Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue ... 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue Safety Information Electronic SRS airbag Before carrying out any work on electronic SRS airbag systems (with the exception... Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue SRS Airbag and Seatbelt Pre-tensioners All vehicle safety systems fall into one of two categories, passive... Airbag systems are categorised into two main types: ã mechanical ã electronic -9Copyright â Automotive Skills Limited 2003 All Rights Reserved LV42: Electrical and Electronic Systems (3) Issue