The fault characteristics of electrical appliances on modern cars can be related to their use characteristics one by one. General electronic components are very sensitive to overvoltage and temperature. For example, the PN junction of a transistor is prone to overvoltage breakdown, the leakage of electrolytic capacitors increases when the temperature rises, and the thyristor components are sensitive to overcurrent. The characteristics of these faults are summarized as follows:
a. Component breakdown. Breakdown includes overvoltage breakdown or overcurrent, thermal breakdown caused by overheating, etc. Breakdown sometimes appears as a short circuit, sometimes as a broken circuit. Overvoltage and overcurrent breakdown caused by circuit failure are often unrecoverable.
b. Component aging or performance degradation. This includes many aspects, such as reduced capacity of capacitors, decreased insulation resistance, increased leakage of transistors, changes in resistance, changes in resistance of adjustable resistors that cannot be continuously changed, and ablation of relay contacts. Components such as relays often have faults due to insulation aging, coil burnout, inter-turn short circuit, contact jitter, and even unable to adjust the initial operating current.
c. Line failure. Such faults include poor insulation, short circuit, bypass, etc. caused by loose wiring, poor contact, moisture, corrosion, etc. Such failures are generally not related to components.
Main points for maintenance of the above faults:
a. To analyze the circuit principle, clarify the overall circuit and connections. Once you encounter an unfamiliar car model and circuit, you often have to do it yourself, analyze the circuit principle, and even map the necessary circuit diagrams. Therefore, the maintenance of automotive electronic circuits will involve circuit analysis methods.
b. One by one, and then one by one, and finally determine its technical status. Many electronic circuits in automobiles often use non-removable packaging, such as thick film packaging regulators, solid sealing ignition circuits, etc. for various reasons such as performance requirements and technical protection. If a certain fault may involve its inside, it is often difficult to judge, it needs to be eliminated from the periphery one by one, and finally determine whether they are damaged.
c. Pay attention to the feasibility of component replacement. For example, although some electronic circuits on imported cars can be disassembled, they often lack the replacement of discrete components of the same model, so they often need to be replaced with domestic or other imported components. This involves the feasibility of component replacement.
d. It is not allowed to use the "fire test" method to identify the fault location and cause. In terms of overhaul methods, traditional automotive electrical failures can often be identified one by one by the "fire test" method. Although this method is not very safe and reliable, and has a certain harm to the battery, it is still feasible in the traditional maintenance method. This method is not allowed on imported cars equipped with electronic circuits. Because "fire test" generates overcurrent, it will cause unexpected damage to some circuits or components. Therefore, when repairing imported automobile electrical appliances, some instruments and tools must be used to perform certain methods.
e. Prevent current overload. It is not allowed to use ohmmeters and multimeters to test low-power transistors with low-resistance ohms below Rx100, so as not to damage the current overload.
f. Be careful of static electricity breaking through the transistor. When replacing the triode, the base should be connected first; when removing, the base should be removed last. For metal oxide semiconductor tubes, it should be electrostatically broken down. When soldering, unplug the soldering iron from the power supply. Prevent the iron from scalding the components. When removing and installing components, the power supply should be cut off. If there is no special instruction, the lead of the component should be more than 10mm away from the solder joint, so as not to scald the component. An electric soldering iron with constant temperature or power less than 75W should be used.
Modern automotive electronic control systems work under complex and changing conditions like other assemblies and components. In addition to design and manufacturing reasons, after a certain mileage, one or other problems will inevitably occur, that is, circuit failures It is partially or entirely incapacitated. In the repair process of automotive electrical equipment, the decision whether electrical equipment can be used again, and which troubleshooting method to choose, should be based on the performance of the electrical equipment damage and the degree of damage. When repairing in the factory according to the repair route of electrical equipment, the selection of repair methods and the determination of the repair process have an important influence on the totality of various faults that form the repair route. Therefore, not only should we study the distribution of electrical equipment damage, but also to understand the statistical rules that form the actual combination of various faults, and compile the repair process route of electrical equipment according to certain principles.
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