Advanced Automotive Fault Diagnosis
tải về 7.07 Mb. Chế độ xem pdf
|
LÔØI NOÙI ÑAÀU, ledklok
- Điều hướng trang này:
- 2.5.7 Black box technique
| 2.5.6 On and off load tests
On load means that a circuit is drawing a current. Off load means it is not! One example where this may be an issue is when testing a starter circuit. Battery voltage may be 12 V off load but only 9 V when on load. A second example is the supply voltage to the positive terminal of an ignition coil via a high resistance connection (corroded switch terminal for example). With the ignition on and the vehicle not running, the reading will almost certainly be battery voltage because the ignition ECU switches off the primary circuit and no volt drop will show up. However, if the circuit were switched on (with a fused jumper lead if necessary) a lower reading would result showing up the fault. 2.5.7 Black box technique The technique that will be covered here is known as ‘black box faultfinding’. This is an excellent technique and can be applied to many vehicle systems from engine management and ABS to cruise control and instrumentation. As most systems now revolve around an ECU, the ECU is considered to be a ‘black box’, in other words we know what it should do but how it does it is irrelevant! Any colour, so long as it’s black [Henry Ford (1920s)]. I doubt that he was referring to ECUs though … Figure 2.3 shows a block diagram that could be used to represent any number of automobile electrical or electronic systems. In reality the arrows from the ‘inputs’ to the ECU and from the ECU to the ‘outputs’ are wires. Treating the ECU as a ‘black box’ allows us to ignore its com- plexity. The theory is that if all the sensors and associated wiring to the ‘black box’ are OK, all the output actuators and their wiring are OK and the supply/earth connections are OK, then the fault must be the ‘black box’. Most ECUs are very reliable, however, and it is far more likely that the fault will be found in the inputs or outputs. V 1 V S V 2 V 3 V 4 tải về 7.07 Mb. Chia sẻ với bạn bè của bạn:
|