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Vehicles equipped with microprocessor control unit (ECU) generally have fault self diagnosis systems. It can be used for automatic inspection and monitoring of the internal drive system and the control system. When a car fails, the fault indicator, which is mounted on the dashboard, shines to warn the car owner that the car may have a problem. Press a button and the fault code (usually two or three digit numbers represent different faults) is displayed on the dashboard. At the same time, the fault signal will be stored in the memory. Even if the ignition switch is disconnected, the fault is eliminated, and the fault indicator is extinguished, the fault signal will remain in the memory for the maintenance personnel to judge the car's fault. After troubleshooting, the power supply of ECU will be disconnected for 30 seconds, and the fault code will be removed. The auto fault self diagnosis system monitors the operation of the car at all times, even if a small screw is loose, it will also be reflected in order to find out the hidden dangers in time and ensure the safe operation of the car. In particular, the electronic degree of modern automobiles is increasing, which greatly optimizes the performance of automobile technology and makes the control system of automobiles become more and more complex. Once the complex electronic devices fail, it will bring great difficulties. In order to quickly diagnose the fault location and improve the efficiency of maintenance, the world's major auto manufacturers have developed automobile fault self diagnosis system. In the late 70s, in order to further improve the convenience of the use and maintenance of modern automobiles, a special vehicle detector was used to detect the automotive electronic control system. For example, the EEC- I and EEC- II detectors developed by the US Ford Motor Co can be used to monitor the signals of electronically controlled gasoline engines and locate the fault locations. Because this special detector has high technical requirements for operators when diagnosing faults, it has not been popularized. With the arrival of the vehicle diagnosis system in 80s, a new diagnostic system, the vehicle diagnosis system, is introduced by using the micro processing control unit (ECU) to detect and diagnose the components of the electronic control system, and to find out the fault by itself, so it is also called the fault self diagnosis system. Because it can monitor the parameters of automobile electronic control system continuously and record the intermittent faults of the system, it is convenient to find fault in time, so it is more widely used. However, because of the limited memory of the microprocessor, the diagnosis project is limited, and the more complex faults can not be diagnosed. Therefore, people are developing and developing a better diagnosis system. In order to expand the diagnosis capacity and diagnosis function of the auto diagnosis system, the multi-function external diagnosis system was born in the end of 80s, such as the Ford OASIS, TOYOTA's Diaqmonitor diagnosis system, and Nissan's Consult and so on. These system functions are more complete, but the price is more expensive and the technical requirements are high. And the standard is not uniform, so its use and maintenance are also limited. After entering the 90s, some international diagnostic systems, which are suitable for international standards, easy to operate and reasonably priced, have been successfully developed. For example, DOT-21 diagnostics system developed by Dafa in Japan. The self diagnosis system of modern automobile is self-contained and has no universality, so it is not conducive to promotion. It has caused great difficulties for the after-sale service and maintenance of automobiles. Therefore, the diagnostic system must be standard and standard, so its diagnosis mode and diagnosis interface can be unified, and only one instrument can be used to diagnose and detect all kinds of vehicles, which will greatly promote the development of auto diagnosis system. The function of the auto diagnosis system is found to be input to the level signal of the microprocessor, and there is a certain range in the normal state. If the signal outside this range is input, the ECU will diagnose the signal system in an abnormal state. For example, the engine cooling water temperature signal system is specified in normal state, the voltage of the sensor is 0.08-4.8V (-50-+139 degrees centigrade), which is diagnosed as abnormal beyond this range. If the microcomputer itself has a reason barrier, the time limit circuit with an emergency monitoring timer (WDT) is monitored. If the program is abnormal, the time limit circuit is set to stop working so as to use the fault detection method set by the microcomputer. When the fault classification is normal, the abnormal condition of the input signal is detected by the diagnostic program, and then according to the result of the detection, it can be divided into a minor fault, a failure to cause a function decline and a major fault. The fault is classified according to its importance and pre edited in the program. When the microcomputer itself fails, the major fault classification is carried out through WDT. The fault alarm is usually reported to the owner by setting the alarm lights on the dashboard. In a car equipped with a monitor, it is also used to display the content of the alarm directly. When the fault is detected, the fault store stores the code of the fault location in the memory. In general, even if the ignition switch is in the disconnected position, the power of the microcomputer and the storage part also keeps on state without causing the storage content to be lost. Only when the battery power is removed or the fuse is removed, the fault code in the memory will be automatically eliminated due to the power cut off of the microcomputer. If malfunction occurs during the operation of the car, in order to prevent the normal driving, it can be controlled by the microcomputer, and use the substitute value (standard value) in the precompiler to keep the basic driving performance, and then the car owners or the maintenance personnel should carry out the corresponding maintenance after parking. The fault self diagnosis system of several different types of vehicles Audi (AUDI) auto fault self diagnosis system, the V6 engine of Audi adopts MPFI multi point fuel system, and its self diagnosis system has 30 different kinds of faults stored in the microcomputer. Its characteristics are: 1. if the fault exists for a certain time, the fault is stored in memory in a stable form. 2. if the fault that has occurred in a certain time no longer appears, the fault is considered an accidental fault. If the engine is started for 50 times, the fault still does not appear again, then the accidental fault will be automatically removed. 3. after closing the ignition switch 150, the microcomputer enters the self holding stage. If the fuel injection and ignition system is overhauled during this period, the fault code that has been stored is then removed and removed. It is necessary to use dedicated instrument --V. A. G1551 fault code reader to call out the stored fault code. The electronic control system of the Chrysler auto fault self diagnosis system (Chrysler Corporation) is called SBEC. When the car fails, the corresponding fault information is stored in the SBEC in the form of code. Each time the ignition switch is opened, the "CHECKENGINE" indicator will shine for a few seconds to show that the indicator works normally. If SBEC receives signals from various sensors that are not normal or receive no signal at all, the "CHECKENGINE" indicator on the dashboard will light up, indicating that the engine has trouble and needs to be overhauled. The method of entering the self diagnosis state is to switch the ignition switch to three times in five seconds, that is, ON, OFF, ON, OFF to ON, and the "CHECKENGINE" indicator on the dashboard will flicker, which can show the stored fault code. The way to remove the fault code is to remove the fault code with the special instrument DRB II. If there is no DRB II, the ignition switch ON/OFF (turn on / off) 50 times, the fault code will be cleared. Volvo auto fault self diagnosis system Volvo series self diagnosis system interface in the right front corner of the car, open the engine cover, the right front headlights have A, B two diagnostic seats, A seat, a diagnostic cross connection, a LED lamp and a button. A and B two diagnostic seats each have six diagnostic ports, each connecting different diagnostic systems. Seat A: No. 1, transmission No. 2, fuel system 3 hole, ABS system 5 hole, turbocharging system 6 hole, 7 hole of ignition system, B seat of instrument diagnosis system: 1 hole, 2 hole of central air conditioning, 5 hole of fixed speed control system, No. 5 air bag, 6 hole of airbag and electric seat train type of Volvo system in self diagnosis function 10 systems can be diagnosed, and in each system diagnosis, it can be divided into six modes: 1. fault code read 2. control element action test; 3. control element action simultaneous control test; 4. set element action instruction control test; 5. numerical reading analysis indication 6. reset microcomputer memory instruction. When the vehicle is diagnosed, the cross wiring on the diagnosis seat can be inserted directly in the different diagnostic holes, and according to the button on the diagnosis seat, a variety of specific diagnostic modes are selected according to 1 to 6 respectively. The method of clearing the fault code is to insert the diagnostic cross wiring into the corresponding diagnosis hole of the system to be diagnosed first, put the ignition switch in ON, read the fault code first, until all the fault codes are displayed, the LED lamp continues to light, and then hold the diagnostic key for more than 5 seconds to clear the fault code. Cadillac auto fault self diagnosis system Cadillac car has a strong self diagnosis function. It shows the fault code, working parameters and working state of the engine through the buttons and displays on the air conditioning control panel (CCP). The anti lock braking system (ABS) is adopted in the new crown car of the ABS fault diagnosis system for the crown automobile. If the system fails, the fault self diagnosis system of the anti lock braking system will identify the fault, and the microcomputer (ABSECU) will store the code for the fault and use the ABS alarm lamp (in the combination instrument). The upper left corner tells the owner of the fault information. The way to clear the fault code is to turn on the ignition switch and connect the Tc and El two line ends of the interface with the jumper connection. Keep the car stationary and press the brake pedal for more than 8 times in 3 seconds. In this way, the fault code in the ABSECU will be cleared.