Probability analysis method I in fault diagnosis o

Probability analysis method in automobile fault diagnosis (1)

Advanced sedan you and thousands of young people will participate in the whole process. There are many kinds of vehicle faults. 5. Mask synthetic blood penetration tester; Almost all parts of the vehicle, mechanical, electrical, oil, gas and other aspects of the fault are complex. In terms of fault judgment, if there is no scientific and reasonable judgment method, it is difficult to quickly determine the location of the fault. For such a complex system as automobile, the possibility of failure mainly depends on the product quality. For products with high reliability, the failure position is often conceivable by normal thinking; However, for vehicles with defective product quality, due to problems in materials and processing technology, the fault location often appears in places that people cannot think of in normal thinking. Of course, this situation is not absolute. The following examples can illustrate this situation. When listing the failure probability, we use the method of sequencing, that is, from the most likely parts and causes. A Mercedes Benz 560sel car suffered from severe idling jitter after replacing a new rocker arm due to abnormal valve noise. According to the general thinking process, only the rocker arm and camshaft have been disassembled and assembled. These parts should be considered first to find the fault. If you follow this clue, you may be able to eliminate the fault soon. Unfortunately, the repairman found that at least three cylinders were not working well during the fire break test. Of course, he thought of replacing the rocker arm of only one cylinder. Even if the software was reinstalled and the backup files were copied to the new installation path for replacement, the failure would not cause the three cylinders to fail. The cause of the failure may lie in other aspects. According to experience, the possible causes are listed as follows:

(1) exhaust gas recirculation (EGR) system failure, and a large amount of exhaust gas enters the cylinder (this is the most likely)

(2) the air intake system leaks, the mixture is too thin, and the idle speed works poorly (this possibility is in the middle)

(3) the model (quality) of the replacement rocker arm is defective (this is the least likely)

first, check the exhaust gas recirculation system. Remove the vacuum pipe on the EGR valve, and the fault remains; When the EGR valve was removed from the engine, it was found that the valve was seriously rusted, the exhaust gas channel and intake channel were not connected at all, and the exhaust gas did not enter the cylinder, making it impossible for the most likely one. Next, check the air intake system, and no air leakage is found. The second possibility is also ruled out. For the third possibility, even if it is finally found that it is the problem of the rocker arm, it cannot be regarded as a successful maintenance for this maintenance, because by this time, the maintenance work has been carried out for half a day, and the owner has been dissatisfied with this. Of course, it is finally found that the rocker arm model is indeed wrong. The new rocker arm on the contact surface with the valve is about 2mm higher than the old rocker arm. After grinding it for a period, it is corrected to the standard value, and the engine idles smoothly after reinstallation, Troubleshooting. Due to the wrong judgment of the possibility of failure, the delivery time of the car was delayed, which should be a lesson worth learning for maintenance personnel. If the vehicle is overhauled after disassembly and adjustment, this part should be checked first, rather than according to the conventional steps. Another example is a new Toyota (carmy2.2) car, which came into the factory for maintenance due to cylinder tamping and engine damage when driving. After replacing the new cylinder block and other components, the engine failed to start after the trial run. When the starter drives the engine to run, the engine is either backfired or fired. It seems that the ignition is disordered, and the high-voltage line is not inserted incorrectly (the engine is directly ignited). Learn from the last lesson and don't check blindly. First ask the repairman what is abnormal when disassembling the engine. The repairman replied that there is a gear shaped sensor on the crankshaft. When disassembling the engine, it can't be removed from the crankshaft due to rust. After forcibly prying it off, it was found that there is a tooth cracking, which was glued with 502 glue and then installed. As a result, the above fault occurred. According to the information obtained, ask the repairman to remove the crankshaft position sensor rotor from the car and carefully check it. During the training, no obvious abnormality is found, and there is almost no trace at the bonding place; However, for the faults and parts defects that people can't see, the computer may not be able to detect them, because the conditions of the repair shop are limited, and the output waveform of the sensor can't be observed with an oscilloscope. However, for the faults described in this car, from the perspective of probability, I still think that the rotor of the crankshaft position sensor is damaged with the greatest possibility. The second possibility is that the computer is damaged. But as we all know, even if the computer can output normal code, it cannot be absolutely considered that the computer must be normal, but this possibility is small. The third possibility is insufficient cylinder pressure. However, when the valve timing is correct, the possibility of insufficient pressure in four cylinders at the same time is also small