The characteristics of modern automotive electrical appliances and electronic equipment are mainly reflected in functional intensification (combination), control electronics and connection standardization. When analyzing the fault of an electronic circuit, since it is always associated with related electrical equipment, it is necessary to understand the general characteristics of electrical and electronic equipment. Features that should be noted before analyzing and repairing electronic circuits: Cars generally have a total power switch, and most of them are electromagnetic. There are many places on the car with fusible wires to protect the harness, not to protect a particular appliance. It differs from a fuse in that it has a slower blown response and is in the form of a wire. For some reason, after its protective blown, it can't be easily found like a fuse, and some even in the wire bundle, pay more attention when analyzing faults. Except in a few cases, all imported cars are connected by a single wire, and the metal structure of the body is used as another common wire. All electrical appliances are connected in the form of "grounding". In principle, the appliances used are low voltage and high current devices. Even the same model of the same manufacturer will have some improvements due to different factory years.
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The characteristics of electrical faults on modern cars can be linked to their use characteristics one by one. Generally, electronic components are very sensitive to overvoltage and temperature. For example, the PN junction of a transistor is susceptible to overvoltage breakdown. The leakage of the electrolytic capacitor increases when the temperature rises, and the thyristor component is 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, and the like. Breakdown sometimes manifests itself as a short circuit, sometimes as a form of open circuit. Overvoltage and overcurrent breakdown due to circuit faults are often unrecoverable.
b. Component aging or performance degradation. This includes many aspects, such as capacitor capacity reduction, insulation resistance drop, transistor leakage increase, resistance value change, adjustable resistance value can not be continuously changed, relay contact ablation. For components such as relays, there are often faults due to insulation aging, coil burnout, turn-to-turn shorts, contact jitter, and even the inability 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. This type of fault is generally independent of the component.
Maintenance points for the above faults:
a. To analyze the circuit principle, to understand the overall circuit and contact. Once you encounter unfamiliar models and lines, you often have to do it yourself, analyze the circuit principles, and even map the necessary circuit diagrams. Therefore, automotive electronic circuit maintenance will involve circuit analysis methods.
b. Exclude first and then internally, and finally determine its technical status. Many electronic circuits in automobiles often use non-removable packages, such as thick-film package regulators and solid-sealed ignition circuits, for performance reasons and technical protection. If a fault may involve its internals, it is often difficult to judge. It is necessary to first exclude them from the periphery and finally determine if they are damaged.
c. Pay attention to the feasibility of component replacement. For example, some electronic circuits on imported cars, although they can be disassembled, often lack the replacement of the same type of discrete components, so it is often necessary to try to replace them with domestic or other imported components. This involves the feasibility of component replacement.
d. It is not allowed to use the “testing fire†method to identify the fault location and cause. In the maintenance method, the failure of the traditional automobile electrical system can often be used to determine the fault location and cause one by one. Although this method is not very safe and reliable, and has certain harm to the battery, it is still feasible in the traditional maintenance method. This method is not allowed on imported cars with electronic circuits. Because "fire test" produces an overcurrent, it can cause unexpected damage to certain circuits or components. Therefore, when repairing imported automobile electrical appliances, it is necessary to use some instruments and tools to carry out according to certain methods.
e. Prevent current overload. It is not allowed to use the ohmmeter and multimeter's Rx100 low-resistance ohmic file to detect small power transistors, so as not to damage the current overload.
f. Beware of static electricity breaking through the triode. When replacing the triode, the base should be connected first; when disassembling, the base should be finally removed. For metal oxide semiconductor tubes, the core should be electrostatically breakdownd. When soldering, remove the soldering iron plug from the power supply. Prevent the soldering iron from burning the component. When removing and installing components, the power should be turned off. Unless otherwise specified, the component pin should be more than 10mm from the solder joint to avoid soldering the soldering iron. Use a soldering iron with a constant temperature or power less than 75W.
Modern automobile electronic control system works under the complicated and varied conditions like other assemblies and components. In addition to the reasons of design and manufacture, after a certain mileage, there will inevitably be one or the other problems, that is, circuit failure. Partial or total loss of working ability. In the automotive electrical equipment repair process, it is determined whether the electrical equipment can be reapplied and which method of troubleshooting is selected, based on the performance and damage level of the electrical equipment damage. When repairing at the factory according to the process route of electrical equipment repair, the selection of the repair method and the determination of the repair process are important to the overall failure of the various repair routes. Therefore, it is necessary not only to study the distribution of electrical equipment damage, but also to understand the statistical laws of the actual combination of various faults, and to prepare the repair process route of electrical equipment according to certain principles.
The main task of electrical equipment repair is to use the remaining durability of electrical equipment to ensure the reliability of economically and effectively repairing automotive electrical appliances and restoring their use. The technical situation of electrical equipment is very different, so the cost of electrical repair is also different. At this time, it may happen that it is economically uneconomical to repair individual fault combinations. Therefore, the economic rationality of electrical repair is the main feature of the electrical condition collection 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 features, whether it is a process problem or a solution. Therefore, along with the features describing the condition of the appliance, features that distinguish all faults and their combinations into process-like categories are also cited. This distinction must be based on the commonality of the main processes of repair and the commonality of the electrical equipment used. When identifying a part, it is necessary to consider the rationality of its repair, which will complicate the work of the inspection and classification section. Because the inspector must not only remember all the fault combinations, but also can not forget the price standard for the scrapping of electrical equipment. When classifying fault combinations by repair route, the most distinguishing features among the various faults should be cited. From the point of view of the organization of electrical equipment repairs, it helps to combine the various fault combinations that have been discovered into one of the few typical process routes, which greatly simplifies the best solution and route for selecting the route. Content. According to certain principles, the whole fault combination should be divided into reasonable categories, and the best solution should be selected to obtain the maximum benefit of electrical equipment repair.
Circuit faults can be divided into progressive faults and sudden faults according to the length of time. The period of occurrence of gradual failure is long, and the degree of failure is from light to heavy, from weak to strong. They are mostly caused by friction and wear during the operation of the parts. For example, the wear of the ignition breaker cam causes a cylinder shortage. Fire, starter broom, etc. Sudden faults are mostly caused by short circuit or open circuit of the circuit, such as the headlight suddenly does not light and the engine suddenly turns off. Circuit faults can be classified into destructive faults and functional faults according to their degree of influence on machine functions. Destructive failure is the complete loss of working capacity of the electrical assembly or component due to failure, no replacement or overhaul can not continue to work, such as bulb filament burnout, integrated circuit regulator breakdown, generator stator coil burnt. Functional fault refers to the function of the electrical assembly is reduced but not completely lost. It is a non-destructive fault. It can be restored after adjustment or partial maintenance, such as ablation of the ignition breaker contact, excessive or too small gap.
Friction, wear or fatigue of the machine during normal operation. For example, the rotor shaft and the bushing of the starter are lubricated with grease. The wear pinion and the flywheel ring gear are not properly meshed due to wear and the top teeth are toothed. Short circuit or open circuit, poor contact or leakage occurs on the circuit. If the generator is overloaded, the rectifier diode is short-circuited; the over-voltage causes the regulator switch tube to break through, the contact ablate without conduction; the capacitor breaks down and cannot store the charge. The electrical components in the circuit are based on the mechanical structure, resulting in poor circuit contact due to mechanical wear, looseness or insufficient spring force. Cars are used in different regions, climates, and terrain conditions, and various failures often occur. For example, if the viscosity of the lubricating oil increases at a low temperature and the starting resistance increases, it will cause early damage of the battery; the automobile electrical appliance will be aged due to high temperature, and the plastic parts will cause corrosion of the automobile parts. The illegal driving operation does not cause maintenance, cleaning and adjustment of the parts, resulting in wear and tear; the design of the machine is unreasonable, and the manufacturing is inferior and the assembly is poor, which may lead to the failure of the circuit components.
Although the types and phenomena of line faults are various, their essence can be divided into mechanical faults, electrical faults, and electromechanical faults. These three types of faults are different from each other and are related to each other and cannot be viewed in isolation. For example, the bearing wear causes the generator and the starter to sweep; the switch cannot be positioned, the spring fails, causing poor contact of the contacts; the shaft is bent, causing excessive runout. Mechanical failures can cause electrical failures for a certain period of time. For example, the broom causes the motor armature coil to be short-circuited, and the contact gap is too large to make the ignition primary circuit unable to be connected.
Electrical faults are mainly caused by short circuits, open circuits, poor contact or leakage. For example, the generator overload causes the rectifier diode to be short-circuited, and the overvoltage causes the final stage of the voltage regulator to break through, the contact ablate without conduction, the capacitor breaks down and cannot store the charge, and the inductor coil is short-circuited between the turns or the body. Grounding, high-voltage insulation components breakdown leakage, battery poles loose or corroded causing non-conduction, power supply voltage is too high or too low, the magnetic flux of magnetic components is weakened or enhanced, and circuit parameters such as frequency and phase are mutated. The fundamental solution to the problem of poor electrical contact caused by mechanical reasons is to restore the integrity of the mechanical structure. When judging circuit failure, people sometimes pay attention to the circuit or circuit diagram is not enough, simply pay attention to the circuit and ignore the mechanical structure, resulting in improper handling, will re-generate mechanical and electrical comprehensive faults. In order to improve the accuracy of judging line faults, shorten the time to find the line, and prevent the addition of new faults, whether it is judged by manual feeling or by instrument light, instrument to detect, the following principles should be followed: according to the circuit schematic diagram to contact the actual; Find out the symptoms and analyze them carefully; from simple to complex, from the table to the inside; to find out the structure, combine the principle; As long as you do this, the faults can be eliminated one by one.
For faults that are difficult to diagnose and involve large faces, the method of replacing the parts can be used to determine the cause and location of the fault by comparing the old and new comparisons, the comparison of the installation direction, and the degree of wear, to determine or narrow the fault range. If the high-pressure spark is weak, if it is suspected that the capacitor is faulty, it can be replaced with a qualified capacitor for fire test. If the spark becomes strong, the original capacitor is damaged, otherwise it should continue to be searched. Use the method of viewing high-voltage electric spark to judge the working condition of the ignition system. When the engine is not working properly or a few cylinders are not working, the high-speed split cylinder line spark plug end can be removed, and the spark plug is 5~7mm. If the engine condition improves, it indicates that the cylinder is malfunctioning. During the test fire, it is also possible to judge whether the ignition system is working properly by observing the strong, weak, and non-fire phenomena of the high-pressure spark. It is called high voltage electric test to check whether some electrical parts are damaged by the high voltage of the ignition system. For example, when checking the splitter head, it can be placed flat on the cylinder head, and the high-pressure total fuse head is aligned with the bottom of the split burner hole by about 5mm, then the ignition switch is turned on, and the power-off contact is turned to see if there is a hole in the split-head hole. Jumping fire. If there is no fire, it means that the fire head is well insulated, otherwise it will damage the electricity. Use instrumentation to detect the technical condition of automotive electrical appliances and circuits without disassembling their components as much as possible, so as to make scientific judgments or to determine problems based on symptoms. For more and more electronic equipment on modern cars, the instrument detection method has the advantages of time saving, labor saving and accurate diagnosis, but requires the operator to have the operational skills of skilled instrumentation and the principle and standard of automotive electrical components. The data can be accurately grasped.
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