Symptom: a Toyota VIA car dashboard on the engine fault warning lights are bright, and the fuel consumption is too high. Before the speed of 20km / h or more, the four doors will automatically lock, but now when the speed reaches 20km / h or more, 4 doors will not automatically lock.
    Troubleshooting: After receiving the car, the first connection to the Toyota-specific detector to retrieve the engine fault code, there is a fault code P0500, meaning the vehicle speed sensor circuit failure. The customer reflects that the car has ever replaced the vehicle speed sensor, but did not troubleshoot the problem. Before proceeding with the maintenance work, we should be aware of the work of the Vistage car speed sensor. The speed sensor is mounted on the transmission, and the transmission output shaft is rotated by the rotor shaft of the speed sensor driven by the driven gear. When the rotor shaft rotates, the pulse signal is generated. The pulse signal enters the combination meter and is converted into the actual speed. In addition, the waveform shaping circuit in the combination instrument can change the pulse signal inputted by the vehicle speed sensor into a more accurate square wave, and then output the square wave to the engine control unit. The engine control unit refers to this signal to select the optimum fuel injection. The square wave signal of the combined instrument is also output to the anti-theft system control unit. The anti-theft system control unit locks the four doors according to the signal when the vehicle speed exceeds 20km / h.
    According to the above work of the vehicle speed sensor, combined with the fault code P0500, you can determine the fault point may be in the vehicle speed sensor, the combination of instruments, engine control unit and the relevant wiring harness. Clear the fault code after the road test, through the detector to read the engine dynamic data flow, focusing on observation speed, maintenance personnel found that the data flow speed value is always 0km / h. Observe the speed of the instrument on the display, the instrument shows the speed and actual speed consistent. During the test, the engine fault warning light is lit, when the instrument shows the speed of more than 20km / h, 4 doors will not automatically lock. Retrieve the fault code, still P0500.
    After the above information, the maintenance personnel believe that the car shows the normal speed of the instrument, which indicates that the speed sensor, instrument and the speed sensor and the instrument line between the connection is no problem. In addition to the speed of the data in the engine control unit, other data items are normal, indicating that the engine control unit is basically no problem, the next step need to focus on checking the instrument and the engine control unit between the line connection.
    Unplug the meter harness connection plug C8 and the engine control unit harness connection plug E4, measuring the resistance between the 15th pin of the plug C8 and the 9th pin of the plug E4 is less than 1Ω, indicating that the pin 15 of the plug C8 is connected to the plug E4 The connection between the two purple / white lines between the 9th and the 9th junction is intact and the point of failure should be on the connection of the plug C8 or plug E4 to the corresponding socket. Check the plug C8, instrument socket, plug E4 and engine control unit socket, the results are in good condition. The vehicle speed signal line of the control unit of the anti-theft system is also connected to the instrument wiring harness connection plug C8, and when the speed exceeds 20km / h, the four doors are not automatically locked, so the connection between the instrument wiring harness connection plug C8 and the instrument socket is the largest The The C8 and E4 plugs are inserted and the removed parts are fully assembled and tested. The dynamic data flow of the engine is observed. The value of the vehicle speed data item is the same as that of the instrument. If the speed is more than 20km / h The door will automatically lock, so the fault is completely ruled out.
    Review the process of troubleshooting, due to the wiring harness connector on the instrument C8 and the instrument socket between the poor contact, resulting in the instrument speed signal can not be output to the engine control unit, making the engine control unit can not be adjusted according to the actual speed of the engine work, resulting in increased fuel consumption The At the same time, because the anti-theft control unit can not get the speed signal issued by the instrument, resulting in the car when the car can not automatically lock at high speed. In addition, the instrument speed signal is also output to the sound system and GPS satellite navigation system, the sound system automatically adjusts the volume according to the speed signal to obtain the best sound effect, GPS satellite navigation system based on the vehicle speed signal to calculate the actual driving distance of the vehicle.
    In the troubleshooting of this case, the maintenance personnel use the detector to read the data stream, while the working principle of the relevant parts have a clear understanding, thus eliminating some of the possibilities, quickly determine the scope of the fault point. In the actual maintenance work, many times can be read through the data flow to find doubt, so as to help determine the point of failure, maintenance personnel should dig deep data mining meaning, and should not only stay in the data surface.