Doctors need to "look, listen and ask questions" when they see a doctor, and so does automobile fault diagnosis, among which looking and asking questions are effective methods for quickly diagnosing automobile faults. When a car breaks down, it needs to be diagnosed. When a mechanic sees a car for the first time, it is very important to make a preliminary judgment on the shape and service life of the car, so as to know the shape of the car from the appearance. The service life can be judged by exposing the engine or flipping the cab from the appearance, and experienced maintenance personnel can even judge the car fault at once. When the car needs to be repaired, the maintenance personnel must ask the user and the owner clearly, including the model, service life, maintenance, service, the location and phenomenon of the fault, and what inspections and maintenance have been done after the fault, so as to understand the fault as deeply as possible. This is a shortcut. Knowing the form can reflect the basic structure and performance of the car. If we know the form and structure of the car and have rich maintenance experience, then the diagnosis will be easier. If you don't know enough, you can master it by looking up books and materials. Through in-depth inquiry, you can basically know where the fault occurred. For example, you can ask whether the fault occurred in the engine or the transmission; If it is an engine, we can further understand whether it is an electrical fault or a mechanical fault; If it is a mechanical fault, we can know whether it is a crank-connecting rod mechanism or a valve train, so it is much easier to make further diagnosis. After the fault is determined, it is easy to eliminate and maintain. If the user wants to overhaul the car, he should also ask about repairing the engine powertrain, repairing the car chassis, repairing the car cab and body, repairing the car electrical appliances and air conditioning. Which parts and assemblies are the focus of maintenance, etc. , so as to determine the maintenance plan.

2. Diagnose the fault by empirical method.

As the name implies, the empirical method of fault diagnosis is based on the basic quality and rich experience of drivers and maintenance personnel to diagnose automobile faults quickly and accurately. The so-called basic quality, whether drivers or car maintenance personnel, should learn from books and improve in practice, so as to obtain basic car knowledge and maintenance experience, which is very important. Automobile technology is a comprehensive embodiment of national economic development. Automobile technology is developing faster and faster, and there are more and more new technologies. Therefore, it is impossible to study hard from books and practice. For example, the single-pump oil supply speed regulation technology of automobile diesel engine and the new diesel engine technology abroad all need to learn books and materials on the basis of original knowledge in order to carry out the actual maintenance work. Only practice under the guidance of theory is correct practice, and experience can be summarized and accumulated in practice. The so-called maintenance experience is also very important. With the experience of automobile maintenance, the same fault and similar fault can be solved at once. Personal experience, summing up accumulated experience; There are also experiences learned from books and other ways. Only by combining the two can we accumulate experience and judge automobile faults smoothly. For example, if the diesel engine is broken, the cab should be turned over. If the steering mechanism is stuck for a period of time, the cab cannot be turned over. Experienced people only need to push and pry, and the cab will turn over immediately. For example, when the diesel engine goes out of control, the speed of the diesel engine suddenly rises and the noise becomes louder and louder. Without experience, the diesel engine can't be turned off. Experienced people can turn the fuel change-over valve on the fuel tank by 45 degrees, and the diesel engine will shut down immediately, so as to avoid the occurrence of vicious accidents. It is not difficult to see that this is the result of accumulated experience. Therefore, we should constantly sum up experience, turn it into a powerful weapon for automobile maintenance, constantly arm ourselves with new knowledge and experience, and solve various even very complicated problems in automobiles with experience.

3. Diagnose the fault through observation.

The so-called observation method is that the automobile mechanic carefully observes the fault phenomenon according to the fault position pointed out by the automobile user, and then makes a judgment on the fault. This is the most basic and effective fault diagnosis method. For example, the failure of engine exhaust pipe emitting blue smoke can be judged by the phenomenon of emitting blue smoke. For example, the engine runs for a long time because of blue smoke during use. Generally, it can be judged that the cylinder or piston ring is worn, which leads to excessive fit clearance, and the oil in the oil pan escapes into the combustion chamber through the gap between the piston ring and the cylinder wall. If the blue smoke only comes out when the engine starts, and then gradually fades, it can generally be judged that the oil baffle on the engine valve stem is aging or the inner hole is worn, which makes the oil baffle function invalid and a small amount of oil leaks into the cylinder along the valve stem. Experienced people can accurately judge, and inexperienced people should further observe. In the process of observation, we should use experience and theory to make careful thinking and deduction, and we should not be hasty and confused by superficial phenomena. Some phenomena can't be seen clearly by experienced people at once, so we need to look at them several times and observe them carefully before we can see through the fault phenomena from the outside to the inside.

4. Diagnose the fault by auditory method

It is a common and simple method to diagnose automobile and engine faults by hearing. When the car is running, the engine runs under different working conditions, and the car and the engine as a whole make noisy but regular sounds. When a part fails, there will be abnormal sound, and experienced people can immediately judge the car failure according to the abnormal sound. For example, the noise of engine crankshaft and connecting rod mechanism, main transmission and transmission shaft is easy to judge. A good driver should exercise his hearing while driving, listen to the sounds made by various parts of the car clearly, and judge abnormal sounds and faults from them. When the car and engine break down and need to be repaired, the car maintenance personnel often start the engine in the parking state, let the engine run at different speeds, and check and diagnose the engine failure through hearing; For the faults of chassis and transmission, road test is often used to let the car run under different working conditions to check and auscultate the faults of the car; For the difficult faults of the engine, you can also use stethoscope and simple instruments for auscultation. For example, you can use a long stethoscope to auscultate the sound of crankshaft and connecting rod mechanism, and you can hear the sound of valve train. A rubber hose can be inserted into the hole of the dipstick, and the crankshaft noise above the oil level of the oil pan can be heard at the lower end, and the air leakage sound at the piston ring can be heard. When the engine stops, check the brakes. You can step on the brake pedal when the engine is turned off, and you can obviously hear the sound of brake locking. When you lift the brake pedal, you can hear the brake shoe return to its original position. Therefore, when a well-trained driver uses the foot brake while driving, he can not only hear the working sound of the brake according to the reaction of automobile deceleration, but also comprehensively evaluate whether the brake works normally. Therefore, as long as the car and engine are running, you should have good hearing. In the process of driving, they should always monitor the sounds made by various parts of the car and the engine. With the change of vehicle speed, the noise is different everywhere, so they can hear the normal sound clearly, judge the abnormal sound in the normal sound and judge the fault in the abnormal sound. Of course, it must be guided by theory and experience.

5. Diagnose the fault by testing.

It is one of the common methods to diagnose automobile and engine faults by test. Under the condition of not disassembling the automobile or less, the function of the automobile and engine is checked by test to achieve the purpose of fault diagnosis. The so-called test is to verify through the test. If the judgment is not clear, try. For example, if the owner reflects that the brake system of the car is not working, he can step on the brake pedal at the parking position of the car, and the brake system will immediately send out a set of braking actions, and the tester can judge the fault of the brake system according to the noise emitted by each brake; If you can't judge clearly at the moment, you can also take a road test. Press the brake pedal at a certain speed, and the brake system will work. The tester can comprehensively judge the fault of the braking system according to the reaction of the car after braking and the noise emitted by each braking. Similarly, for the fault of steering system, the tester can turn the steering wheel in situ, and a set of steering actions from steering wheel to wheel rotation can judge the fault of steering system. If the judgment is unclear, try to drive, consciously hit the steering wheel on the corner, and the steering system works. The tester can judge the fault of the steering system according to the steering response and abnormal sound somewhere. For engine failure, it is necessary to check the engine operation. Testers can generally run the engine through different speeds or acceleration and deceleration, observe the operation of the engine by experience, and auscultate the noise of the engine by experience to find faults. When the lighting lamp is off and it is suspected that there is no electricity in the circuit, a wire can be shorted to the grounding terminal to try to check the fire. If there is fire, it can be judged that the circuit has electricity, and then check whether there is fire. However, there are many fuses and relays on the line, so be careful when testing the fire. The normal operation of cars and engines is regular. If it is not normal, it means there is a fault. If it is not normal, you can test it. The judgment of normality and abnormality should be guided by theory and experience.

6. Diagnose the fault by touching.

The human body and hands and feet are sensitive sensory organs, which can be used to diagnose automobile and engine faults, just like pulse diagnosis in traditional Chinese medicine. Judging from the feeling that the car conveys to the human body. People can judge the damage of suspension system and shock absorber by the vibration when the car is running. The average driver is the most sensitive and often drives, and the driver can feel it after the shock absorber fails. When a car wants to go to expressway or drive at a high speed, the driver usually checks four wheels and kicks the tires with his feet. The tire pressure can be judged by the elasticity of the tire, and the fastening degree of the hub bearing can be judged by the deflection and shimmy of the tire, which is a typical method of touching with the feet. After the car runs on the expressway, the driver can touch the temperature of the tire with his hand. Be careful if the tires are hot in summer. You can touch the brake drum by hand and try the temperature of the brake drum; Or pour water on the brake drum, take a look at the evaporation of water, and judge whether the brake drum is delayed. When it is found that the engine is overheated and the cooling system has coolant, you can touch the upper and lower parts of the radiator by hand to determine whether the thermostat is damaged or the radiator water inlet is blocked. Touch the hose at the outlet of the water pump, and you can feel the fluctuation of water pressure, indicating that the water pump is working normally. It is often encountered to check the belt tightness with finger pressure, feel the work of the fuel pump with fingers, and check the oil supply of the high-pressure oil pipe with hands. During maintenance, check the wear of the friction surface by hand; It is ubiquitous to feel the tightness of friction pairs with your hands. In short, the hand is an important organ of the human body. The work is done by hand, and there is a feeling in the work, and this feeling is the best instrument for fault diagnosis.

7. Diagnose the fault by olfactory method.

Smell is a sensitive sensory organ. Some people say that people who take fewer cars have a sharper sense of smell than drivers. In fact, drivers' sense of smell is often weakened by the smell of cars. As the saying goes, "you can't smell it if you live in a bad city for a long time." Although perfume is put at the front desk of the dashboard to offset the odor inside the car, the driver can smell all kinds of odors on the car with a little attention. The smell of diesel oil is one of the common car smells. It's not surprising that people can smell it when adding diesel. It is strange that the diesel in the car is ambiguous, that is, the diesel pipe leaks oil or the engine is not completely burned, so we should take it seriously. The smell of engine exhaust gas indicates that the engine burns engine oil and the engine burns incompletely. If the car has a strong and obvious smell when braking, the engine should be adjusted or repaired. The special smell of non-metallic materials burning indicates that the clutch friction plate is burnt or the wire is burnt. Please carefully check whether there is smoke or touch somewhere to determine the fault location. Oil leaks into the running engine, and the engine temperature is high, which will produce odor; Oil dripping on the exhaust pipe will produce a stronger smell; The smell of the engine can easily enter the cab from the air conditioner, which can be clearly smelled. The water (electrolyte) in the leakage pool of automobile battery will give off an unpleasant smell; If the battery consumes too much water, the generator will forcibly charge the battery when the car is driving, which will overheat the battery, emit white smoke and even make people faint. Anyone can judge. Leakage of other working media on the vehicle, such as power steering oil and transmission oil, will give off peculiar smell, but be sure to smell it carefully; Attention should also be paid to the leakage of articles or inflammable and explosive articles on the car. In a word, once the automobile smells in operation, or when the smell is large, it should stop for further inspection, find out the root cause of the fault, and take corresponding measures to eliminate the smell. If it is a car fault, it should be eliminated or sent for repair.

8. Use the replacement method to diagnose the fault

The so-called replacement method is that auto mechanics try to replace damaged assemblies and parts with qualified ones according to the parts pointed out by auto users, which is a simplified and effective fault diagnosis method. It is worth pointing out that the replacement spare parts should be tested and reliable, or the new parts must be qualified. If you accidentally replace the bad parts with bad parts, you will not find the fault, but will cheat and increase the difficulty of diagnosis. If the oil pressure indicator system is out of order, when the pressure sensing plug is suspected to be damaged, the pressure sensing plug on the original car can be replaced with a good standby pressure sensing plug, and then try again. If the replacement was not good before, the problem will be solved immediately after the replacement, and it is obvious that this part is faulty and will be repaired immediately. After maintenance, spare parts can be removed and replaced with new ones. If the replaced good parts are still not good during trial operation, then the fault may not be here, and then think of other ways. There are many parts that can be replaced for difficult faults. For example, for the failure of insufficient engine power, a new air filter can be replaced and re-tested; For the failure of the oil supply system, if it is suspected that the pump oil pressure is insufficient, a new fuel pump can be replaced. For the fault of the braking system, if it is difficult to find out, you can try to change a master brake cylinder (master cylinder). After the system is readjusted, the fault of the braking system will be eliminated. For power steering system failure, if the steering is heavy, you can try to replace a power steering booster. After the system is re-debugged, the power steering system fault will be eliminated. Then, through the above example, it can be explained that the fault occurred in the replacement part. For the replacement assembly, there is no need to disassemble it, so even the maintenance procedure is saved, which is also a matter of killing two birds with one stone. Some auto mechanics have some commonly used spare parts, such as headlights, small lights, relays, fuses and so on. When they encounter some faults, they can be replaced as soon as possible, which is a simple, effective and reliable fault judgment method. In order to apply the replacement method reasonably, we must be cautious when using the replacement method, and the fault judgments should be close to each other. Avoid blind changes and increase workload.

9. Diagnose the fault with the instrument method

Instruments and meters are indispensable tools for diagnosing automobile faults, and should be used as much as possible when conditions permit. On-board instruments and indicator lights can effectively indicate automobile faults. For example, the fuel gauge shows the amount of fuel. When the car is stationary and the fuel gauge indicates zero, it means that the car is out of fuel, not a fault. The brake warning light is on, indicating that the brake system is faulty and needs to be further searched. The voltmeter used in the automobile indicates the voltage value of the automobile electrical system, and can also accurately judge the power generation of the generator and the charging of the battery during driving; Whenever the electrical equipment is switched on and off, the voltmeter will reflect it, that is, it can be judged whether the electrical equipment is working normally. The non-contact tachometer for automobile can accurately measure the instantaneous engine speed, indicate the engine speed while driving, and indicate the change of engine speed when shifting gears. With the tachometer, some faults of the engine can also be reflected by the tachometer. The cylinder pressure gauge for maintenance can measure the cylinder pressure and pressure difference of each cylinder, and can also measure the air leakage of each cylinder. Multimeter can easily judge the faults of automobile electrical system. The front wheel aligner can also measure the front wheel alignment parameters of the automobile; The sound level meter can measure the noise of cars and engines; Smoke meter, five-wheel instrument, brake test bench, automobile drum test bench, etc. Whether all instruments and test equipment should be used in automobile maintenance, and use these special equipment if necessary. It is difficult to master the knowledge of instruments and electronic diagnostic equipment. Only by having basic knowledge and studying hard can we master and diagnose the difficult faults in automobiles.

10. measurement fault diagnosis

According to national standards, it is an indispensable method for fault diagnosis and debugging to measure the effective parts and parameters of automobiles with gauges and instruments. Metric rulers are used to measure length, as well as micrometer, micrometer, feeler and tape measure. Used to measure length, including diameter, gap and displacement. Force and gravity are measured by dynamometer and gravimeter. Pressure gauges and vacuum gauges are used to measure pressure and vacuum. Various measuring instruments of automobiles also have measuring units, for example, the decibel value of sound pressure level should be used for sound pressure measurement; Voltage is used for voltage measurement. For the steering system failure, the toe-in value of the steering wheel should be measured with toe-in meter and front wheel aligner. Measure the tire pressure with a tire barometer; If necessary, you can also measure the angle of the steering wheel with an angle ruler, so as to accurately judge the cause of the failure. For engine failure, it is necessary to measure the cylinder diameter, piston ring diameter, thickness and opening clearance after engine disintegration, so as to find out the exact cause of the failure and repair the engine. Even during the repair, the diameter of the cylinder barrel should be re-measured, and the piston and piston ring should be selected according to the requirements of grouping, so as to repair the engine. When diagnosing the fault of electronic control system, measurement is indispensable. For example, when the engine is unstable or the power is low, it is suspected that the oil supply pressure is insufficient, and the system pressure should be measured with a pressure gauge; When it is suspected that there is something wrong with the electronic control system, it is necessary to measure the voltage and resistance with a digital multimeter. Measure frequency and waveform amplitude with frequency meter and oscilloscope. Therefore, all kinds of measuring instruments and rulers are extensions of the eyes and hands of maintenance personnel. Only the correct measurement can accurately judge, and the feeling is only superficial. The combination of measurement method and instrument method should make the diagnosis more accurate.

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The so-called subsection inspection method means that the automobile mechanic checks the faults according to the systematic lines, pipelines and working routes on the automobile. According to the inspector's experience, the inspection can be based on the route from the power supply to the actuator, from back to front or from the middle. If you can find it from the actuator at once, of course, otherwise you have to look back. For example, if the lighting and indicating system fails, in principle, the circuit of power supply-switch-fuse-relay-wire-bulb should be found from front to back. Experienced people can check the fuse first, some people may check the light bulb first, and some people may check the relay first. When the fault cannot be found from front to back or from back to front, the problem may appear in the middle, the combination switch may be broken, or the wire may be broken somewhere. In principle, check the brake pedal, vacuum booster, brake master cylinder, brake pipe, sabs, brake pipe and wheel brake for brake system failure. For experienced people, you can also start with wheel braking or brake master cylinder first, and then check other parts; However, for the difficult faults of the braking system, the measuring instrument should be used to check and measure from front to back. Find out the reason for the failure. In principle, the fault of steering transmission system should be checked in the order of steering wheel-steering gear-steering gear-steering wheel. For convenience, it can also be disassembled from somewhere in the steering gear, which makes it easier to judge whether the fault is in the steering gear or at the rear of the transmission mechanism. Using the existing theoretical knowledge and methodically using the sectional inspection method, the root cause of the fault can be found.

12. Fault diagnosis by partial disassembly method

The so-called partial disassembly method refers to the method that the automobile mechanic can disassemble a functional part for inspection and then reinstall it according to the working principle of the assembly after determining that a fault has occurred on an assembly, but it is not yet accurate to determine where the fault occurred. If the method is used properly, the fault location can be determined immediately. Therefore, the partial disassembly method is a simple and easy method to diagnose automobile faults quickly. There are many similarities between the partial disassembly method and the test method, but the difference is that the partial disassembly method is a fault diagnosis method, which is mainly to disassemble and try again after disassembly. For example, when it is suspected that a certain cylinder of the engine is not working, it can be checked by the method of cutting off the oil in a single cylinder. Partial disassembly is to remove the high-pressure oil pipe joint of this cylinder, and the change of speed and noise when the engine is running indicates that this cylinder is working normally; And no response is abnormal work. When the engine power is insufficient and it is suspected that the air filter is blocked, you can remove the air filter core and try the engine again. If the power supply is restored without an air filter, this is a fault. In fact, the partial disassembly method makes the normal engine or circuit system lose its original function and act under abnormal working conditions. Therefore, disassembly and assembly must be cautious, and corresponding safety measures should be taken when safety projects are involved. All fault diagnosis methods are complementary, and the purpose is to find the fault of the car. Using these fault diagnosis methods flexibly, we can find automobile faults, carry out targeted maintenance on automobiles and engines, and restore normal use functions.