1 Introduction of oscilloscope
An oscilloscope is a tool used to measure the fluctuation of voltage or current in a circuit. It can reflect the working condition of the device in real time. In circuit analysis, it is used to measure the input and output waveforms, and by the observer through analysis and research, the excellent condition or problem of this circuit performance is obtained.
2 Importance of sensor waveform analysis in automobile fault diagnosis
With the development of modern automotive technology, a large number of sensors are used in automobiles. When the sensors feel the changes in physical quantities in their working environment, they send the changes to the car ECU in the form of current or voltage, and the EZCU receives After receiving the signal from the sensor, make the corresponding judgment, drive the related equipment to work, and adjust the working state of the car.
The sensors used in modern automobiles can be divided into: temperature, speed, pressure, oxygen content, vibration and position sensors, which generate a variety of voltage or current signals, and oscilloscopes can reflect the changes of these signals in a waveform manner come out. When the sensed physical quantity changes normally or abnormally, it can be reflected by the change of the waveform. By comparing with the normal waveform, the fault location can be determined.
What I am talking about here is not to say that an oscilloscope can solve all problems in car repairs, but only provides a method for judging faults and a method for dealing with problems, just like a stethoscope used by a doctor.
3 Case study
Failure phenomenon A Volkswagen Passat 1.8T car has a failure phenomenon that is not easy to start. It must be fired multiple times each time to start. The last time it died during driving, it could not be fired and could only be dragged to the 4S shop. service.
After the fault diagnosis arrives at the store, it can be started from time to time but not from time to time. The fault is indicated by the 1552 diagnostic instrument as the crankshaft position sensor is damaged. So replace and start again, the fault still remains. So I changed the camshaft position sensor and tried again. The fault still exists, and the maintenance is in a deadlock.
Fault analysis and testing uses dual trace oscilloscope to simultaneously measure the waveforms of the crankshaft position sensor and camshaft position sensor on the faulty car as shown in Figure 1, and the waveforms of the camshaft position sensor and crankshaft position sensor measured on the same type of normal car are shown in the figure 2 shows.
By comparing Figure 1 and Figure 2, it is found that the waveform of the crankshaft position sensor is different. Why is there such a difference, is it normal or abnormal? After analyzing and studying the structure of the crankshaft position sensor, it can be seen from the structure of the crankshaft position sensor in FIG. 3 that it is a ring of notched teeth. Compare the crankshaft in FIG. 2 According to the waveform of the position sensor, it can be concluded that there are 60 notch teeth in this ring, of which one notch occupies the position of two teeth. In the waveform of the crankshaft position sensor in FIG. 2, 59 sine waves can be calculated, which The ring has 59 notched teeth, and the position with a larger spacing in the waveform of the camshaft position sensor in FIG. 2 corresponds to the position of the two missing teeth of the notched ring. The waveform generated by the crankshaft position sensor has a starting position relationship with the waveform of the camshaft position sensor in FIG. 2, and this relationship can be more clearly seen through FIG. 4.
It can also be seen from Figure 4 that the crankshaft position sensor waveform only appears twice before it coincides with the camshaft position sensor waveform once. The relationship is consistent with the relationship between the crankshaft speed of the vehicle and the camshaft speed ratio of 2: 1, which is also just It is the position of the top dead center of one cylinder, that is, the reference position of the ignition timing of one cylinder, and also determines the ignition timing of the other three cylinders. This signal is transmitted to the foot, and the ECU can issue an ignition signal to start the engine. Otherwise, the ECU will not issue an ignition signal and the engine will not start.
Troubleshooting After replacing the crankshaft position sensor and camshaft position sensor, the problem cannot be solved, so the idea is transferred to the notch ring of the crankshaft position sensor. The waveform at the left third of the camshaft position sensor waveform in Figure 1 and other The place is different, and the waveform is different from the normal car. There is no problem with the sensor, only the notch ring of the crankshaft position sensor is problematic, and it is at a position 15 teeth away from the large notch. At first, it was suspected that there was a foreign object on the 15th tooth, which caused the sensor signal to be bad. No foreign object could be seen through the sensor installation, so the oil pan could only be removed. After removing the oil pan, it turned to the 15th notch tooth position As shown in Figure 3, the notched ring gear at the indicated position of the iron rod is not a foreign body, but a little deformation. After the deformation, the distance between this position and the sensor becomes larger, so that the generated signal is not strong enough, and an abnormal small waveform is generated. After replacing the crankshaft, the problem of this car was completely solved.
Failure summary Why does this car have intermittent starting difficulties? Let's look at the waveform of the crankshaft position sensor in Figures 1 and 2 again. If a horizontal line is drawn at the top and bottom of the waveform of the crankshaft position sensor in Figure 1, Draw a horizontal line at the same position on the waveform of the crankshaft position sensor in Figure 2, according to this line
With or without a waveform, two waveforms in Figure 5 can be obtained. The ECU can only send an ignition signal to start the engine if it obtains the waveform in the bottom figure of Figure 5, and the waveform on Figure 5 cannot be sent by the ECU to start the engine, but it fails at an occasional time. When the waveform generated by the position just reaches the qualified bottom line, the engine can be started. After starting, the ECU's detection of the crankshaft position sensor waveform is not so strict, so the engine will not stall after starting. That is the root cause of this car failure.
Another tip: This fault reflects the strict start of the engine to protect the engine; but when the engine is started, the design concept of maintaining the operation of the engine as much as possible.
4 Summary
The fault diagnosis of this car makes full use of the dual-track display function of the dual-track oscilloscope, so that the waveform and speed relationship between the crankshaft and the camshaft can be clearly reflected, so that the fault can be judged and located more quickly.
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