Advanced Automotive Fault Diagnosis
2.5.4 Volt drop testing
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- 2.5.5 Testing for short circuits to earth
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2.5.4 Volt drop testing Volt drop is a term used to describe the difference between two points in a circuit. In this way we can talk about a voltage drop across a battery (normally about 12.6 V) or the voltage drop across a closed switch (ideally 0 V but may be 0.1 or 0.2 V). The first secret to volt drop testing is to remem- ber a basic rule about a series electrical circuit: The sum of all volt drops around a circuit always adds up to the supply. The second secret is to ensure that the circuit is switched on and operating – or at least the cir- cuit should be ‘trying to operate’! In Figure 2.2 this means that V 1 ⫹ V 2 ⫹ V 3 ⫽ V s . When electrical testing, therefore, and if the battery voltage measured as say 12 V, a reading of less than 12 V at V 2 would indicate a volt drop between the terminals of V 1 and/or V 3 . Likewise the correct operation of the switch, that is it closes and makes a good connection, would be confirmed by a very low reading on V 1 . What is often described as a ‘bad earth’ (when what is meant is a high resistance to earth), could equally be determined by the reading on V 3 . To further narrow the cause of a volt drop down a bit, simply measure across a smaller area. The voltmeter V 4 , for example, would only assess the condition of the switch contacts. 2.5.5 Testing for short circuits to earth This fault will normally blow a fuse – or burn out the wiring completely! To trace a short circuit is very different from looking for a high resistance connection or an open circuit. The volt drop testing above will trace an open circuit or a high resistance connection. My preferred method of tracing a short, after looking for the obvious signs of trapped wires, is to connect a bulb or test lamp across the blown fuse and switch on the circuit. The bulb will light because on one side it is connected to the supply for the fuse and on the other side it is connected to earth via the fault. Now disconnect small sec- tions of the circuit one at a time until the test lamp goes out. This will indicate the particular circuit section that has shorted out. tải về 7.07 Mb. Chia sẻ với bạn bè của bạn:
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