With the rapid development of automobile technology, electronic technology has been widely used in automobiles. Since 1980s, automobiles equipped with electronic fuel injection system, electronic ignition system, electronic anti-lock braking system and electronic automatic transmission controlled by microcomputer (ECU) have become the mainstream and direction of automobile industry development. In order to control tail gas emission and save energy, in May of 20001year, China issued a law prohibiting the sale of cars and buses with carburetors with less than 6 seats in 47 major cities. This will play a positive role in promoting the development of China's automobile industry, and also put forward higher requirements for maintenance personnel. Based on the practical experience of daily maintenance of electronically controlled vehicles, I talk about some diagnosis and maintenance skills. Electronic control system 1 maintenance skills. Electronic controller (ECU) is a precise device. Although many failures may be related to ECU, its failure rate is very low, so don't dispose of ECU easily, let alone open its cover casually. 2. Open circuit or poor contact is a common fault of electronic control system. Except for some faults such as broken wires and loose connectors, it is necessary to detect the voltage and resistance of relevant measuring points with high impedance multimeter to determine the fault location, and it is not possible to check whether the line is on or off by scraping fire. Because when the fire is scraped, the instantaneous voltage generated by the self-inductance coil in the circuit will breakdown the electronic components. 3. When the ignition switch is turned on, don't disconnect any electrical equipment, so as to avoid the induced electromotive force generated in the circuit from damaging electronic components. When disconnecting the battery, you must pay attention to the following points: ① The ignition switch must be turned off; ② Check whether the self-diagnosis fault code exists; ③ Remember the password of audio equipment with anti-theft code. 4. After the battery is disconnected and reassembled, if the working condition of the engine is worse than before, don't replace the parts casually, because this situation may be due to the elimination of the "learning correction memory" of E-Cu after the battery is disconnected. When the engine runs for a period of time, the ECU will automatically establish a correction memory, and the bad working conditions of the engine will automatically disappear. 5. When repairing the vehicle by arc welding, be sure to disconnect the electronic control unit from the battery. If welding repair is carried out near the ECU, the ECU box should be removed. Maintenance and diagnosis skills of fuel injection system 1. For electronically controlled fuel injection system, the influence of air leakage in intake system on engine work is far greater than that of carburetor truck. Because in the electronically controlled fuel injection engine, the air leakage is not measured by air flowmeter, which has a great influence on the air-fuel ratio. Therefore, when the engine fails, check whether the air flowmeter, throttle body, auxiliary air valve, idle speed stabilizing valve and exhaust gas recirculation valve are loose, and whether the air hose and its connector are damaged or leaked. 2. After the engine is turned off, there is still a certain pressure of fuel in the fuel pipe, so when disassembling the fuel pipe, it is necessary to prevent the fuel from spraying out and causing danger. 3. The gasket in the tubing is disposable, so it should be replaced again during assembly and cannot be reused. 4. When installing the fuel injector, be careful not to damage the newly replaced O-ring, so as not to affect the sealing performance of the fuel injector. When installing, oil should be used to lubricate the O-ring first, not oil and gear oil. 5. When checking the fuel injection performance of the fuel injector, be sure to know whether the fuel injector is of high resistance type or low resistance type. The resistance of high resistance type is generally 12 ~ 14 ohm, which can be directly connected to the battery to test the fuel injection performance of the injector. However, the resistance of the electromagnetic coil of low-resistance fuel injector is generally only 2 ~ 3 ohms. If it is directly connected to the battery, the fuel injector will be burnt out due to excessive current, so a special connector must be used to connect with the battery. If ordinary wires are used, an 8 ~ 10 ohm resistor should be connected in series. 6. The air flow sensor is a precision component, which has a great influence on the working performance of the engine. When disassembling the air flowmeter, handle it with care and do not disassemble it, so as not to damage or affect its detection accuracy. When cleaning the air flowmeter, do not clean it with water or cleaning solution. 7. The adjusting screw on the air flowmeter is used to adjust the carbon monoxide content at idle speed. Under normal circumstances, it should not be moved. Improper adjustment will lead to a decrease in engine power and an increase in fuel consumption. 8. After long-term use, the performance of the water temperature sensor will change, making the water temperature signal wrong, which will adversely affect the fuel injection, ignition time and the work of the fuel pump. However, the change of performance parameters of water temperature sensor (not short circuit or open circuit) is often not recognized by self-diagnosis system. Therefore, when the engine works abnormally (such as unable to start, unstable idle speed, increased fuel consumption, etc.). ), and the fault self-diagnosis system does not prompt the fault code of the water temperature sensor. Don't ignore the inspection of the water temperature sensor. 9. When overhauling the oxygen sensor, be careful not to let the oxygen sensor fall and collide with other objects. When replacing, the thread must be coated with a special anti-sticking brush to avoid the difficulty of disassembly next time. Electronic ignition system 1 maintenance skills. When the engine is started and running, do not touch the ignition coil, high voltage line, distributor, etc. Avoid being shocked by high-voltage electricity with your hands. 2. During the high-voltage fire test, insulating rubber should be used to clamp the high-voltage line, and the high-voltage line should not be directly held by hand to prevent electric shock. At the same time, when testing the working condition of each cylinder by fire breaking method, one end of the high voltage line of the fire breaking cylinder should be grounded. Otherwise, it will produce secondary high voltage and burn wires. 3. The ignition timing has a great influence on the engine. Therefore, don't ignore the inspection of ignition timing when the engine is not working normally or after the engine is overhauled. 4. Pay attention to the following points when checking the ignition signal generator (crankshaft position sensor): ① For the magnetic induction ignition signal generator, be careful not to let the gasket, screw and other metals fall into it when opening the distributor cover; When checking the air gap between the magnetically permeable rotor and stator, use a non-magnetic plug gauge, and be careful not to plug the gauge forcibly; ② For the photoelectric ignition signal generator, do not open the distributor cover easily, and prevent dust from entering when it is really necessary to open the inspection; ② When replacing the distributor assembly, ensure its original installation position, otherwise it will affect the accuracy of ignition time. Electronic anti-lock braking system 1 maintenance skills. The failure rate of the electronic control device of the electronic anti-lock braking system is very low. Therefore, most of the faults of electronic control devices are not caused by electronic components, but by poor line connection or unclean parts. If the fault code indicates that the sensor is faulty, first check whether the connection point of the sensor is in good contact and whether there is corrosion. 2. For the anti-lock braking system with accumulator, when maintaining the hydraulic system, the accumulator should be depressurized first, so as to avoid high-pressure brake fluid from spraying and injuring people. 3. Because the normal operation of the anti-lock braking system must be based on the integrity of the original braking system, the maintenance of the original braking system should also be carried out normally. When replacing the brake pad, unscrew the bleeder screw of the brake caliper first, and then press back the piston, otherwise the scale in the oil cylinder may be pressed into the pipeline, resulting in component failure. Oil return may also make the electronic control device get wrong information, so that the anti-lock braking system can be protected and closed. In the process of maintenance or use, if it is necessary to exhaust the hydraulic braking system, it should be carried out in accordance with the procedures specified in the relevant vehicle manual. Because the deflation procedure of anti-lock braking system with brakes may be somewhat different from that of ordinary braking system, we can't blindly copy the deflation procedure of the original braking system. Maintenance skills of electronically controlled automatic transmission 1. The characteristic of fault diagnosis of electronically controlled automatic transmission is to first determine whether the fault is in the circuit part or the mechanical part. If the fault light is on, it can be considered that the fault is in the circuit part, otherwise it is in the mechanical part. 2. When maintaining the vehicle, turn off the ignition switch, that is, turn it to the "LOCK" position. Only when the negative terminal of the battery is disconnected for more than 20 seconds can the electronic control system be overhauled. 3. When replacing the oil seal and repairing the valve body and solenoid valve, drain the transmission oil as much as possible. When removing nuts and sensors, use special tools whenever possible. When installing, it should be tightened according to the specified torque. 4. The accuracy of automatic transmission parts is relatively high, so all kinds of tests cannot be carried out after disassembly, and tests can only be carried out after comprehensive overhaul and reassembly, so a comprehensive inspection must be carried out before disassembly and maintenance. 5. When disassembling and repairing automatic transmission parts, it is best to disassemble and repair only one component group at a time, and then repair the other group after this group is assembled, so as to avoid wrong assembly. All disassembled parts should be cleaned with transmission oil or kerosene and dried during assembly. Oil passages and oil holes shall be purged with compressed air. Do not use a rag. 6. Before replacing the friction plates of new brakes and clutches, soak them in transmission oil for at least 15 minutes. All removed sealing gaskets and sealing skirts should be replaced with new ones, and transmission oil should be applied during assembly. 7. When the valve body is overhauled and disassembled, pay attention to the disassembly method to avoid the ball valve on the valve body falling off without knowing the installation position. Each control valve and spring should be carefully disassembled, and do not touch the surface of the control valve when disassembling. 8. When removing and installing the transmission on the vehicle, ensure that the hydraulic torque converter is installed in place. And the necessary inspection and adjustment can be made to the transmission before starting the test. Source: www.lw372 1.com