75 An approach to fault finding

Having dealt with tools and technology in previous chapters and with skills and method in the sections above, it is appropriate to turn to more specific detail. Whilst the self-diagnosis capacity of the on-board computer (ECM) and its code storing capacity is a powerful aid to fault diagnosis and repair, it is by no means a complete solution. By taking an example of an engine management system, such as that shown in Fig. 7.4, it may be seen that there are many factors that relate to the operation of the engine.

Fig. 7.4 A modern engine management system (Lexus)

At the 'heart' of the diagram (Fig. 7.4) and the engine system itself, is the engine. In order for this engine to operate it must have:

3. compression in the cylinder;

4. a cooling system to prevent overheating;

5. a lubrication system to deal with friction and assist with cooling;

6. the mechanism of the engine, i.e. pistons and piston rings, exhaust and inlet valves, connecting rods and bearings, crankshaft, bearings and a flywheel. These are required to convert the energy of combustion into mechanical energy at the flywheel.

Whilst failures do occur in areas (4), (5) and (6), on modern vehicles, they happen far less frequently than they once did. Most failures in automotive computer controlled systems are likely to be related to the areas (1), (2) and (3). In other words, faults are more likely to be found in some areas than others. In fact the term used is probability. When we are looking for causes of faults we can safely say that some possible causes are more likely to be the reasons for the failure than others. These likely causes have a high probability of being the reason for failure.

To take a simple case: some time ago, a certain vehicle had its ignition distributor situated behind the radiator and facing directly into oncoming rain. In very wet weather it was quite common for these vehicles to suffer breakdowns. The cause was often found to be that water had affected the distributor. This became common knowledge and one had a good chance of getting the diagnosis right first time by going direct to the most probable cause, wetness at the distributor. This type of knowledge (based on experience) is based on empirical evidence, i.e. we know that it is so because that is what happens.

Evidence gained by experience is gathered by motoring organizations, such as the AA and the RAC, and also by technicians who are called upon to recover and repair vehicles. This evidence often points to common causes of failure, such as discharged batteries, lack of fuel, poor maintenance etc.

Whilst urging technicians to remain aware of the value of knowledge gained by experience, we now turn to the instruments and tools aspects of computer controlled systems diagnosis.

Firstly, remember the six steps; one of these is 'carry out a thorough visual inspection'. This is a step that is often overlooked until one has got well into the fault finding procedure. Carrying out this relatively simple step can often avoid much unnecessary work. By taking an example of a system failure and by working through a diagnostic procedure it is possible to establish a 'methodology' for fault diagnosis. The example chosen is a vehicle that is not 'pulling' well, the engine check lamp shows a malfunction and the workshop documentation provides the following information which is also shown on the flow chart in Fig. 7.5.

Flow chart showing a diagnostic technique

Flow chart showing a diagnostic technique


Engine speed sensor signal out of limits.


Low engine power.

Fault code:

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