Failure phenomenon:
When a millennium Sea Lion car equipped with a 4Y engine travels between 60 and 70 km / h, the speedometer hands often jump.
Troubleshooting:
According to the information provided by the owner, the speedometer is normal when the speed is less than 60km / h, and when it exceeds 60km / h, the hands of the watch rise at the same time as there is a jump. 10 consecutive test runs, this phenomenon occurs 3 times, and the amplitude of the 3 jumps varies. First read the fault code with Kinder K81 decoder, the result is no fault code display.
The extraction of the speedometer signal of Hiace 4Y engine is completed by a Hall sensor. The sensor is inserted into the output shaft of the transmission, and the Hall element is driven by the helical gear to output a square wave signal. The magnetic field force pushes the pointer to rotate by an angle, thereby displaying the vehicle speed value. From the point of view of the nature of the fault, this is an irregular gap fault. For this fault phenomenon, we must pay attention to the timing when measuring. Only when the fault phenomenon occurs, the detected data is meaningful. The car's speed sensor has 3 wires, one wire is the 8V sensor power supply, one wire is the sensor ground wire, and the third wire is the 4.5V sensor signal wire. The speedometer controller calculates the speed of the vehicle through the pulses on the signal line.
After analysis, it is believed that this jitter may be caused by a certain interference signal, and the strongest interference signal on the car is the ignition system. To determine whether it is the interference generated by the ignition system, you can only ask the Kode K80 oscilloscope to "drive the mirror". Is here. If it is the interference of the ignition system, the waveform when the fault occurs should be as shown in Figure 1. However, the actual measured waveform when no fault occurs is shown in Figure 2, and the waveform when the fault occurs is shown in Figure 3 and Figure 4.
Waveform analysis:
If the 5V signal line of the speedometer sensor is intermittently grounded, the signal waveform should go down from 5V, and the abrupt part will be at a right angle.
If the sensor power line, ground line, and signal line are intermittently broken, the sensor signal waveform should return to the 5V position, and the abrupt waveform should be at a right angle or burr.
If the accuracy of the transistor in the sensor is not high or the performance is poor, there will be rounded corners in the signal waveform, which is caused by the characteristics of the transistor, and the transistor is working in the saturation region at this time.
The Hall-type speed sensor used by the car has two rounded corners in the waveforms 3 and 4 when the fault is actually detected. This shows that the additional rounded corner signal is generated by the transistor of the Hall-type speed sensor In other words, the fault waveform is caused by poor performance of the vehicle speed sensor.
Trouble shooting after replacing the speed sensor.
Maintenance experience:
In practice, the most common method for maintenance personnel to use a faulty vehicle is to use the decoder to query the fault code, then repair the fault code, and finally clear the fault code. However, if the fault reported for repair cannot be ruled out, some maintenance personnel have no idea what to do next. In fact, some of the fault codes of electronically controlled cars may be the poor performance of the component itself, or the circuit may be open or short-circuited, or it may be caused by other sensor faults or interfering signals. Use an oscilloscope to diagnose these accidental or difficult faults The effect is very good. The oscilloscope can display the change relationship between the measured voltage and time in real time with a waveform, which is widely used in automobile diagnosis. Diagnosis is now the top priority of car maintenance. Car diagnosis can be understood as follows: "diagnosis" is to obtain useful data through "look, smell, ask, cut, sniff", or use a variety of testing equipment or instruments. This process is able to Accurately analyze the cause of the failure and find the premise and basis of the fault location. However, if the detection instrument or equipment is not displayed correctly, like some domestic car decoders display false ignition waveforms and false data streams, then the correct diagnosis result becomes The root of the wood, the passive water. In addition, "off" is to analyze and judge, based on the correct detection data of the previous "diagnosis", correct judgment is obtained after correct thinking.
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