Advanced Automotive Fault Diagnosis
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LÔØI NOÙI ÑAÀU, ledklok
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- 2.3.2 Examples
particular, logical way. I would like to stress the need for a logical process again – with a quotation! ‘Logic is the beginning of wisdom not the end .’ (Spock to Valeris, Star Trek II) 2.3 Diagnostics on paper 2.3.1 Introduction This section is again a way of changing the way you approach problems on a vehicle. The key message is that if you stop and think before ‘pulling the car to pieces’, it will often save a great deal of time. In other words, some of the diagnostic work can be done ‘on paper’ before we start on the vehicle. To illustrate this, the next section lists symptoms for three separate faults on a car and for each of these symptoms, three possible faults. All the faults are possible but in each case choose the ‘most likely’ option. 2.3.2 Examples Symptoms Possible faults A The brake/stop lights are 1. Two bulbs and 12 LEDs reported not operating. On blown checking it is confirmed 2. Auxiliary systems relay that neither of the two open circuit bulbs or the row of 3. Brake light switch not high-mounted LEDs is closing operating as the pedal is pressed. All other systems work correctly B An engine fitted with full 1. Fuel pump output management system tends to pressure low stall when running slowly. 2. Idle control valve sticking It runs well under all other 3. Engine speed sensor wire conditions and the reported loose symptom is found to be intermittent C The off side dip beam 1. Two bulbs blown headlight not operating. This 2. Main lighting fusible link is confirmed on examination blown and also noted is that the 3. Short circuit between off off side tail light does not side tail and dip beam work lights The most likely fault for example A, is number 3. It is possible that all the lights have blown but unlikely. It could not be the auxiliary relay because this would affect other systems. For example B, the best answer would be num- ber 2. It is possible that the pump pressure is low but this would be more likely to affect operation under other conditions. A loose wire on the engine speed sensor could cause the engine to stall but it would almost certainly cause misfire under other conditions. The symptoms in C would suggest answer 1. The short circuit suggested as answer 3 would be more likely to cause lights and others to stay on rather that not work, equally the chance of a short between these two circuits is remote if not impos- sible. If the lighting fusible link were blown then none of the lights would operate. The technique suggested here relates to stages 1 to 3 of the ‘the six stages of fault diagnosis’ process. By applying a little thought before even Diagnostic techniques 9 taking a screwdriver to the car, a lot of time can be saved. If the problems suggested in the previ- ous table were real we would at least now be able to start looking in the right area for the fault. tải về 7.07 Mb. Chia sẻ với bạn bè của bạn:
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