Advanced Automotive Fault Diagnosis
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LÔØI NOÙI ÑAÀU, ledklok
- Điều hướng trang này:
- 2.5.10 Faultfinding by luck!
- Figure 2.4
| 2.5.9 Flight recorder tests
It is said that the best place to sit in an aeroplane is on the black box flight recorder! Well, apart from the black box usually being painted bright orange so it can be found after a crash, my reason for men- tioning it is to illustrate how the flight recorder principle can be applied to automotive diagnostics. Most hand-held scopes now have flight record facilities. This means that they will save the signal from any probe connection in memory for later play back. The time duration will vary depending on the available memory and the sample speed but this is a very useful feature. As an example, consider an engine with an intermittent misfire that occurs only under load. If a connection is made to the suspected compon- ent (coil HT output for example), and the vehicle is road tested, the waveforms produced can be examined afterwards. Many engine (and other system) ECUs have built in flight recorders in the form of self- diagnostic circuits. If a wire breaks loose causing a misfire but then reconnects the faulty circuit will be ‘remembered’ by the ECU. 2.5.10 Faultfinding by luck! Or is it logic? If four electric windows stopped working at the same time, it would be very unlikely that all four motors had burned out. On the other hand if just one electric window stopped working, Input 1 Output 1 2 2 ECU 3 3 4 5 Figure 2.3 System block diagram Ω 1 Ω 2 Component such as a sensor With wires disconnected Wires disconnected from ECU ECU Figure 2.4 Ohmmeter tests Diagnostic techniques 15 then it may be reasonable to suspect the motor. It is this type of reasoning that is necessary when faultfinding. However, be warned it is theoretic- ally possible for four motors to apparently burn out all at the same time! Using this ‘playing the odds’ technique can save time when tracing a fault in a vehicle system. For example, if both stop lights do not work and every- thing else on the vehicle is OK, I would suspect the switch (stages 1 to 3 of the normal process). At this stage though, the fault could be anywhere – even two or three blown bulbs. None-the-less a quick test at the switch with a voltmeter would prove the point. Now, let’s assume the switch is OK and it produces an output when the brake pedal is pushed down. Testing the length of wire from the front to the back of the vehicle further illustrates how ‘luck’ comes into play. Figure 2.5 represents the main supply wire from the brake switch to the point where the wire ‘divides’ to each individual stop light (the odds say the fault must be in this wire). For the purpose of this illustration we will assume the open circuit is just before point ‘I’. The procedure continues in one of the two following ways. Either: ● guess that the fault is in the first half and test at point F; ● we were wrong! Guess that the fault is in the first half of the second half and test at point I; ● we were right! Check at H and we have the fault … on test number THREE; or: ● test from A to K in a logical sequence of tests; ● we would find the fault … On test number NINE! You may choose which method you prefer! tải về 7.07 Mb. Chia sẻ với bạn bè của bạn:
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