Vehicle Electronic Systems and Fault Diagnosis

Allan Bonnick

• Publisher: Routledge
• Year: 2014
• ISBN:
  9781136076527
• Edition: 1
• Topic: Technologie et ingénierie
• Subtopic: Ingénierie civile

The average car now contains much more electronic circuitry than would have been the case, even five years ago. This leaves many technicians struggling to keep up with current developments in the repair and maintenance of these electronic systems. Often, texts covering vehicle electronics dwell on unnecessary maths and general electronics principles. This practical guide discusses electronics ony within the context of the vehicle system under consideration and thus keeps theory to a minimum. Using numerous diagrams, photographs and step by step instructions, this book gives a clear description of vehicle electronic systems and fault diagnosos and than continues on to the testing and repair of these systems. Regular reviews and summaries help consolidate learning and make this book ideal for workshop and classroom use.

1
Structure of the vehicle system

Before getting involved with electronics, it is helpful to remember that much of a motor vehicle remains fundamentally unchanged. For example, engines have pistons and poppet valves, brakes have drums or discs, suspension systems have springs, of some sort, and gearboxes have spur gears or epi-cyclic gear trains - as did the model T Ford.

Traditional and electronic skills required

Much of the work that technicians perform is therefore traditional, i.e. checking brake pads and discs for wear, routine servicing such as oil and filter changes, etc. Failure of a petrol engine to start on a wet morning is just as likely to be due to damp leads as ever it was. A persistent misfire may well be due to a spark plug failure, or loss of compression arising from a burnt valve, or some other mechanical failure. Just because a vehicle is fitted with a number of electronic devices does not mean that one should immediately suspect them, or assume that because they are there nothing can be done to remedy a problem.

Figure 1.1 shows part of a list of service tasks for a fairly modern vehicle; it also shows the ‘traditional’ nature of the work that has to be done. If a technician does this work properly, i.e. safely, methodically and thoroughly, with proper checks being performed at appropriate stages, then he/she already possesses some of the important attributes needed for effective work on electronically controlled systems.

It is also important to remember that there are well over 20 million vehicles on the road in Britain, and that the average life of a vehicle is estimated at 10 years. Older vehicles generally require more repairs than newer ones, which means that there will probably be quite a bit of repair work to be done in the future. Much of this repair work is likely to be on systems that have an electronics element and it is important for vehicle repairers to be able to deal with this aspect, as well as the ‘traditional’ aspects. This means that a mechanic needs to have good all-round ‘traditional’ skills backed up by the skills necessary to cope with the ‘electronic’ aspect. Throughout this text, the intention is to show that the ‘average’ well-motivated conventional mechanic is quite capable of acquiring the ability to deal with the electronic element of vehicle repair and maintenance.

Fig. 1.1 A section of a job sheet

It is tempting to believe that the problem of electronics can be ‘got round’ by employing an electronics specialist. This might just about be feasible in a very large workshop, but for the majority of garages it is unlikely to be commercially viable. So it falls to the vehicle technician to extend her/his range of skills to encompass electronics and thus be competent to tackle the full range of service and repair operations required by motorists and vehicle operators. Knowledge of electronics alone, in isolation from the other skills that a vehicle technician needs in order to perform routine work, is not sufficient because very often the failure of an electronically controlled system is not due to the electronics but to some aspect of the conventional vehicle technology, e.g. anti-lock brakes playing up because of a wheel-bearing failure.

It is important to keep this in mind because the competent vehicle technician needs a platform of traditional skills to perform the bulk of his work. In order to cope with modern vehicle systems, however, these traditional skills need enlarging to encompass the electronics that are to be found on current and future vehicles.

Electronic systems contain elements in common and, in the first instance, I think it useful to review some of the systems currently in use on vehicles as this will serve to show what these ‘elements in common’ are.

Petrol injection systems

When dealing with vehicle technology it is common practice to start with the engine; so for this review of electronic systems I shall start with a petrol injection system. The petrol injection system shown in Figure 1.2 is of the multi-point type: that is to say, there is a separate injector for each cylinder of the engine. Petrol injection is widely used, in conjunction with an exhaust catalyst, to provide effective control of exhaust emissions.

At this stage I want to draw your attention to some specific parts of Figure 1.2. Note the inlet valve, above the piston. Just above the inlet valve, and to the right, is a fuel injector. This injector is operated by electro-magnetism in accordance with electrical pulses which are transmitted by the ECU (electronic control unit). The black line with the arrow on it is the cable (wire) that carries the electric current. The injector could be described as an actuator. To the left of the piston is the water jacket and there you will see the coolant temperature sensor. Here the black line, with the arrow pointing to the electronic control unit (ECU), carries the electrical signal that represents temperature to the ECU. The air-flow meter is another sensor.

The ECU is central to the operation of the system because it is here that the computing capacity is held, which enables the system to function.

This brief examination of the fuel injection system shows that it has four basic elements:

• actuators (injectors)

• sensors

• cables (connecting the elements together)

• electronic control unit

Fig. 1.2 An engine management system

Anti-lock brakes

Now let us take a brief look at another commonly used system – anti-lock brakes (ABS). ABS is used to provide enhanced braking in difficult driving conditions. Figure 1.3 shows a three-dimensional diagram of a vehicle equipped with ABS.

Here again, I want you just to pick out the main components: we have wheel-speed sensors, the hydraulic pump and accumulator (actuator), and the ABS computer (ECU), and all of these components are connected together by cable. Thus, this system has the same four basic elements as the fuel injection system, i.e.:

• actuator

• sensors

• ECU (computer)

• interconnecting cables

Fig. 1.3 Anti-lock braking (Toyota)

It seems that the majority of vehicle electronic systems have a similar structure: it is useful to know this because each of these four elements must work properly otherwise the system itself will not work, and this property, i.e. the four basic elements of the system, is a useful concept in fault diagnosis. These systems are often represented by a diagram similar to Figure 1.4.

Fig. 1.4 The basic elements of an electronic system

The system diagram shown in Figure 1.4 greatly simplifies the topic. The actual circuits on the vehicle contain a great deal more detail than is shown here. However, the systems are fundamentally similar and it is useful to be able to hold on to this basic simplicity when one is studying an actual circuit diagram, because it helps to remind you what you are looking for.

Before moving on to consider more detail about systems, it is useful to give some thought to the conventional methods used to repair vehicle electronic systems.

Electronic system repair

Individual components such as the controller (ECU), some sensors and actuators are not designed to be repaired in garages. In most garages the function of the technician is to determine which component of an electronic system is defective and to replace that component correctly. The actual method for testing a system is dependent on t...