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.
The main task of electrical equipment repair is to use the residual durability of electrical equipment to ensure the economical and effective repair of automotive electrical appliances and to restore the reliability of their use. The technical conditions of electrical equipment are quite different, so the cost of repairing electrical appliances is also different. At this time, this situation may occur, that is, it is not economically feasible to repair individual fault combinations. Therefore, the economic rationality of electrical repair is the main feature of the electrical condition set divided into various repair process routes. The purpose of the classification of parts to be repaired is to form an electrical repair route that has common characteristics in terms of both process and solution. Therefore, along with the features that describe the condition of the appliance, we must also cite features that can distinguish all faults and their combinations into similar categories of processes. This distinction should be based not only on the commonality of the main repair process, but also on the commonality of the electrical equipment used. When identifying parts, the rationality of their repairs must be considered, which will complicate the work of the inspection and classification section. Because the inspector must not only remember all the fault combinations, but also cannot forget the price standard for the scrap of electrical equipment. When classifying the combination of faults according to the repair route, the most clearly distinguishable features among various faults should be cited. From the point of view of process regulations and organization of electrical equipment repairs, it is helpful to merge the various fault combinations that have been found into a few typical process routes, which greatly simplifies the selection of the best plan and route of the process route. Content. According to certain principles, all the fault combinations should be divided into reasonable categories, and the best plan should be selected to obtain the maximum benefit of electrical equipment repair.
Circuit failure can be divided into gradual failure and sudden failure according to the length of time. A gradual failure has a longer period of occurrence, and the degree of failure ranges from light to heavy and weak to strong. Most of them are caused by friction and wear during the operation of the parts, such as the lack of a cylinder caused by the wear of the ignition breaker cam Fire, start machine, etc. Sudden failures are mostly caused by short circuits or open circuits of the circuit, such as the headlights suddenly turning off and the engine suddenly shut down. Circuit faults can be divided into destructive faults and functional faults according to the degree of their influence on the machine function. Destructive faults are the complete loss of working capacity of electrical assemblies or components due to faults, and the inability to continue working without replacement or overhaul, such as blown bulb filaments, IC regulator breakdown, generator stator coil scorching, etc. Functional failure refers to a non-destructive failure in which the function of the electrical assembly is reduced but the work capability is not completely lost. It is a non-destructive failure, and its function can be restored after adjustment or local repair, such as ablation of the contact of the ignition interrupter, the gap is too large or too small, etc.
Friction, wear or fatigue of machinery in normal operation. If the rotor shaft and sleeve of the starter are lubricated with grease, the driving pinion gear and the flywheel ring gear cannot be properly meshed due to wear and the top teeth are toothed. The circuit has a short circuit or open circuit, poor contact or leakage. If the generator overload causes a short circuit of the rectifier diode; the overvoltage causes the regulator switch tube to break open, the contacts ablate without conduction; the capacitor breaks down and cannot store charge, etc. The electrical components in the circuit rely on the mechanical structure, which results in poor circuit contact due to mechanical wear, looseness, or insufficient spring force. When a car is used in different regions, climates, and terrains, various failures often occur. For example, increased viscosity of lubricating oil and increased starting resistance at low temperature will cause early damage to the battery; automotive electrical appliances will aging plastic parts and insulating materials due to high temperature; acid rain will corrode automotive parts. Illegal driving operation does not maintain, clean and adjust as required, which causes wear of the machine parts; unreasonable design of the machine parts, poor manufacturing and poor assembly will cause failure of circuit components.
Although the types and phenomena of line faults are diverse, their essence can be divided into mechanical faults, electrical faults, and electromechanical faults. These three types of faults are different and related to each other, and cannot be viewed in isolation. For example, bearing wear causes the generator and starter to sweep the bore; the switch cannot be positioned, the spring fails, causing poor contact contact; the shaft bends, causing the amount of runout to be excessive. Mechanical faults will cause electrical appliance faults until a certain period of time. For example, the bore sweep causes the motor armature coil to short-circuit, the contact gap is too large, and the ignition primary circuit cannot be connected.
Electrical failure is mainly caused by short circuit, open circuit, poor contact or leakage in the circuit. For example, the overload of the generator causes a short circuit of the rectifier diode, the over-voltage causes the breakdown of the final switch of the voltage regulator, the contacts ablate without conduction, the capacitor breaks down and cannot store charge, the inductance coil turns or interlayer short circuit or the body Grounding, high-voltage insulation components breakdown leakage, non-conductivity caused by loose or corroded battery pole piles, the power supply voltage is too high or too low, the magnetic flux of the magnetic components is weakened or enhanced, and circuit parameters such as frequency and phase vary. The fundamental solution to the fault of poor circuit contact caused by mechanical reasons is to restore the integrity of the mechanical structure. When judging a circuit failure, people sometimes just focus on the circuit or circuit diagram is not enough, simply pay attention to the circuit and ignore the mechanical structure, resulting in improper handling, mechanical and electrical failures will occur again. In order to improve the accuracy of judging line faults, shorten the time to find the line, and prevent adding new faults, whether it is judged by artificial sense or by means of instrument test lamps and instruments, the following principles should be followed: contact the actual according to the circuit schematic; Identify the symptoms and carefully analyze them; from simple to complex, from the surface to the inside; to prove the structure and combining principles; to segment and compare by system. As long as this is done, the faults can be eliminated one by one.
For faults that are difficult to diagnose and involve a large area, the method of comparing parts can be used to determine the cause and location of the fault by comparing the old and new, the installation direction, and the degree of wear, etc., to determine or reduce the scope of the fault. If the high-voltage spark is weak, if it is suspected that the capacitor is faulty, you can use a qualified capacitor for fire test. If the spark becomes stronger, it means that the original capacitor is damaged, otherwise you should continue to search. Use the method of viewing high-voltage electric sparks to determine the working condition of the ignition system. When the engine is not working well or a few cylinders are not working, you can remove the spark plug end of the high-pressure sub-cylinder line, and test the fire 5 to 7mm away from the spark plug. If the engine operating conditions improve, it indicates that the cylinder is malfunctioning. During the fire test, you can also determine whether the ignition system is working properly by observing the strong, weak, and no fire phenomena of high-pressure sparks. It is called high-voltage electric inspection method to check whether certain electrical parts are damaged by high-voltage electric of ignition system. For example, when checking the fire head, you can lay it flat on the cylinder head, use the high-pressure main fire head to align with the bottom of the fire head hole about 5mm, then turn on the ignition switch, and turn off the power contact to see if the fire head hole Jump fire. If there is no flashover, it indicates that the sub-fire head is well insulated, otherwise it is breakdown and damage to the channel. Use instruments and meters to detect the technical status of automotive electrical appliances and circuits without disassembling their components as much as possible, so as to make scientific judgments or determine faults based on symptoms. For more and more electronic devices in modern automobiles, the instrument detection method has the advantages of time saving, labor saving and accurate diagnosis, but the operator must have the proficiency in the application of instrumentation skills, as well as the principles and standards of automotive electrical components The data can be accurately grasped.
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