At present, some maintenance personnel still judge faults by experience, but with the rapid development of automobile technology today, this method often leads to some misjudgments and detours in maintenance work. Of course, the important role of maintenance experience in the process of automobile maintenance is undeniable, but relying on experience alone is not enough. On the basis of mastering the working principle of the system, maintenance personnel should use data analysis methods and combine maintenance experience to find the key points to solve the problem. This will not only get twice the result with half the effort, but also effectively avoid the losses caused by detours. The following fault cases are the author's practical experience in data analysis.

Fault symptoms: A Shunchi 1.4TSI sedan produced by 20 10 is equipped with an automatic manual dry direct shift gearbox with a mileage of 40,000 km. Users report that the car often appears serious jitter in a certain driving state. Before this, the control unit and double clutch assembly of the electro-hydraulic transmission have been replaced, but the problem has not been solved.

Inspection and analysis: road test after the maintenance personnel pick up the car. The fault rule is that when the vehicle speed is 30 ~ 50 km/h and the engine speed is 1700 r/min, if you want to accelerate the vehicle moderately, you will immediately feel a violent jitter coming from the engine compartment. The scan tool detects that there are no fault codes in the engine and transmission control unit.

As far as the test run is concerned, the fault phenomenon is very similar to the fault performance of clutch jitter in manual transmission vehicles. According to the characteristics of direct shift transmission, its power transmission is inseparable from the dual clutch assembly (Figure 1). However, if the engagement between the clutch pressure plate and the clutch plate is not smooth, it will inevitably make the vehicle shake in the transition stage of clutch engagement, just like the manual transmission model. The double clutch assembly of this car adopts dry clutch, and there are two main reasons for the jitter of dry clutch: first, the friction coefficient between clutch friction plate and pressure plate is uneven, and its friction force is jumping under a certain engagement pressure; The other is the unstable engagement pressure of the clutch. If the engagement pressure is not a smooth transition but a jump, it will also lead to the jitter of power transmission. The clutch engagement pressure is controlled by the hydraulic control unit (Figure 2). If the control oil pressure is unstable, it may affect the clutch engagement pressure.

Through the above analysis, the problem once again points to the dual clutch assembly and transmission control unit. So, are there any problems with the parts replaced last time? This possibility cannot be ruled out. Besides replacing these parts again, is there any other way to rule out the possibility of problems with the parts? After careful consideration, the maintenance personnel gave up the idea of replacing parts again and decided to look for the answer from the data first.

Clutch jitter is essentially the irregularity of speed transmission relationship between driving wheel and driven wheel. If the vehicle jitter is really caused by clutch engagement jitter, it will inevitably be reflected from the input shaft speed and output shaft speed of the transmission. To this end, the maintenance personnel connected the scanning tool for road test.

The road test confirmed that the fault occurred in the second gear driving state. Play back the data flow when the fault occurs (Figure 3), and compare the commissioning process with the data. The fault occurred after the 1 gear was shifted to the second gear and before the accelerator pedal was released for the first time. And the time when the fault occurs is closer to the moment when the accelerator pedal is released for the first time. As can be seen from the figure, during the fault period, the rotation speed of the output shaft and the input shaft of the transmission changes smoothly, and the actual rotation speed completely coincides with the target rotation speed, so the fault does not occur when the transmission shifts gears. In this way, the failure has nothing to do with the engagement process of the dual clutch, so the possibility of problems with the dual clutch assembly and the transmission control unit can be ruled out. So, where did the violent shaking feel during the test run come from?

Although the speed of the input shaft and the output shaft of the transmission does not jump, it cannot be denied that there is pulsation in its acceleration. If the acceleration generated by the input torque of the transmission is pulsating, it will inevitably bring the feeling of impact vibration. However, this impact will be smoothed away by the inertia of the vehicle, so the speed fluctuation can not be seen from the transmission part alone. It can be inferred that the transmission, as a transmission device, bears some kind of torque impact when the fault occurs. The direct shift transmission is rigidly connected with the engine crankshaft, so it is inferred that the torque impact on the transmission should come from the engine.

Connect the scan tool again for a test run, this time focusing on the engine. The playback data (Figure 4) shows that the engine speed, intake pressure, fuel injection pulse width, throttle opening and ignition advance angle have no obvious changes when the fault occurs, but the load rate and misfire intensity have obviously increased. When the misfire intensity increases, the actual output torque of the engine is lower than the normal torque corresponding to the intake at that time, which will naturally reduce its power reserve and increase its load rate. When the load rate is too high, the misfire intensity is too high, and engine jitter is inevitable. It can be seen that the abnormal misfire of the engine leads to the pulsation of its output torque.

Considering that the engine of this car adopts in-cylinder direct injection technology, the nozzle hole of its injector is extremely small and easy to block, which is probably the cause of the fire. After removing the fuel injector and cleaning it, test run it and observe the data flow of the engine again (Figure 5). At this time, it was found that the period of increasing flameout intensity appeared after the throttle was closed, which should be produced after the engine flameout. It can be considered as an active fire, the purpose of which is to produce engine braking effect, which is a normal phenomenon. Comparing the data before and after cleaning the injector, it is found that the misfire intensity before cleaning appears at the moment when the throttle opening increases, while the misfire after cleaning appears after the throttle is closed. After cleaning the injector, the fire situation changed. Obviously, the problem is the syringe.

So why does the fault always happen in the second gear of the transmission and the low speed of the engine? The reason is that when the engine is running at low speed, due to the inertia of airflow, its inflation efficiency has almost no potential for sudden improvement. When the vehicle needs to accelerate, the load borne by the engine suddenly increases. If the throttle opening is not large at this time, the inflation of the engine is limited and the load rate will increase sharply. In this case, the poor combustion of the engine caused by the lean mixture will be manifested in the form of unstable torque output, so the impact of the crankshaft will appear. From the data of fire detection, this situation has also been fully reflected. When the engine speed increases or the throttle opening is large enough, the potential of torque increase increases and the load rate decreases, and the fault phenomenon will be covered up by the smooth action of the dual-mass flywheel.

Troubleshooting: The performance of spark plugs will be affected to some extent, considering that the vehicle is driving under the condition of lean mixture and different fire for a long time. Therefore, the spark plug was replaced. After replacing the spark plug, the vehicle was tested, and it was found that the acceleration performance of the vehicle was obviously improved, and the vehicle jitter would never occur again under various working conditions, and the fault was completely eliminated.

Review and summary: For vehicles with in-cylinder direct injection and direct shift technology, maintenance personnel are often influenced by the previous vehicle test experience and have some misjudgments because of the novelty of the technology. In this case, the maintenance work will inevitably take a detour. In the process of diagnosis, the focus of this fault case is on the transmission at the beginning, and the thinking is far from the real fault point. Through the continuous analysis of the test data, we gradually turned our attention to the engine and finally found the correct diagnosis direction. This seems to be a big detour, but compared with the previous maintenance personnel who mistakenly replaced the dual clutch assembly and the hydraulic electronic integrated transmission control unit, the maintenance risk between them is not the same.

It is not difficult to see that the development trend of automobile maintenance technology is analysis plus experience, so we should pay attention to mastering the system principle and cultivating the ability of data analysis. In automobile fault diagnosis, using data analysis method will not cause too much loss even if it takes a detour. The detour taken by maintenance personnel in data analysis sometimes becomes a catalyst for deepening the understanding of the working principle of the system, so this "detour" is an effective way to learn and master modern automobile technology. Through such a tortuous experience, you will be handy when you encounter the same type of fault next time.