Basic knowledge of electronically controlled vehicle maintenance

1. Modern electronic controlcarCharacteristics of electrical and electronic equipment

The characteristics of modern automobile electrical and electronic equipment are mainly reflected in the intensification of functions (combination), control electronics and connection standardization. When analyzing the fault of an electronic circuit, because it is always connected with related electrical equipment, or it depends on the work of other electrical equipment to reflect the function of the electronic circuit, it is necessary to understand the general characteristics of electrical and electronic equipment.

(1) General characteristics of electrical equipment

Electrical equipment has its own structural principle and characteristics depending on the nature and type of its work. Here are a few issues that should be paid attention to before analyzing and repairing electronic circuits.

1. Hyundai cars generally have a total power supplyswitch, And mostly electromagnetic.

2. There are many places on modern cars equipped with fusible wires (notfuse) To protectWire harness, Instead of protecting a particular appliance. The difference between it and the fuse is that its fusing response is slower and it is in the form of a wire. After a protective fuse due to some reason, it cannot be found as easily as a fuse, and some are even in the wiring harness. Pay more attention when analyzing the fault.

3. Except for very rare cases, all electronically controlled cars are connected by a single-wire system, and the metal structure of the body is used as a common wire, and all electrical appliances are connected to it in the form of "grounding".

4. In principle, all electrical appliances used are low-voltage and high-current devices.

5. Even if it is the same model of the same manufacturer, there will be some improvements due to the difference in the year or even the month of the factory.

(2) Use characteristics of electronic equipment

1. Temperature and temperature

The change of temperature includes two basic aspects: one is the temperature in and out of use, that is, the ambient temperature. In our country, the range of this change is about -40 to +50°C (under the sun). The second is the conditional use temperature, which is closely related to the length of the car's working time, the location of the electronic circuit layout and its own heating and heat dissipation conditions.Generally speaking, the temperature of the engine can reach above 100℃, and the temperature of the inner wall of the instrument panel can reach above 60℃.exhaust pipeThe internal temperature can reach above 600℃ (exhaust gas containsOxygen SensorThat is placed here). For electronic components, such a high operating temperature is often one of the main causes of overheating damage. In addition, the temperature gradient of a car working in a cold area changes greatly. For example, when the car is running in a cold area immediately after starting, the temperature of each part changes sharply, and the coolant temperature rises from -30°C outdoors to 10 minutes after starting. When it reaches about +80°C, the engine oil temperature also rises to about 80°C after 30 minutes of starting. Therefore, the installation of electronic equipment should take into account the temperature environment of the installation location.

The increase in humidity will increase the infiltration of water molecules to electronic components, which will reduce the insulation performance of electronic components and accelerate aging.

2. Voltage fluctuation

Voltage fluctuations come from two aspects:

(1) Power supply voltage fluctuation

Under normal circumstances, the car’s power supply fluctuates, or before the engine startsRotating speedWhen it is lower than a certain value, byAccumulatorPower supply; when the engine speed exceeds a certain speed,DynamoExternal power supply, used for electrical equipment and charging batteries. Due to the different levels of battery discharge, the output voltage varies greatly. At the same time, the generator regulator controls the generator excitation current by means of on and off, and the output voltage fluctuates around the standard voltage. This fluctuation range should be from the battery terminal voltage to the voltage at which the regulator works. For example, in a car with a 12V power supply, the battery terminal voltage can be as low as 6-8V when starting at a low temperature, and it can be up to when the generator is running at high speed. 14.5V.

(2) Instantaneous overvoltage

Transient overvoltage refers to the higher voltage generated in a short period of time due to electromagnetic induction, also known as pulse voltage. There are many factors that cause instantaneous overvoltage, mainly due to the switching process, contact disconnection, ignition pulse, etc. when the electrical appliance is working. Although the peak value of instantaneous overvoltage is very high, but the duration is very short, it is not harmful to strong current equipment (such as starter, electric horn, etc.), but it is more harmful to microelectronic equipment and its components. Therefore, when using a car with an electronic control device, special attention should be paid to the generation and prevention of instantaneous overvoltage.

3. Radio interference

(1) Mutual interference of electrical devices

The various electrical appliances on modern cars work differently, so they invade each other in different ways. The above-mentioned pulses in the form of ignition and switches are a kind of interference. Generally, the ability of all automotive electrical appliances to work together in the car without interfering with the normal operation of other electrical appliances, and at the same time, the ability to resist interference from other electrical appliances is called the compatibility of automotive electrical appliances.

In fact, since the mutual interference between automobile electrical appliances is inevitable, electromagnetic compatibility is important for automobile electronic circuits. Oscillations in the circuit triggered by any factor will be emitted in the form of electromagnetic waves through wires, etc., which will not only interfere with radios and communication equipment, but also cause electromagnetic interference to the electronic systems on the car with high-frequency response characteristics. Therefore, the application of computers (controllers) in automobiles should have good electromagnetic shielding measures. Once the shielding is damaged, it will also cause abnormal operation.

(2) Interference outside the vehicle

Radio waves emitted by radio equipment such as transceivers, radars, and radio stations outside the car will interfere with the instruments on the car and make the electronic control device out of control.

4. Other environment

Vibration and shock are the characteristics of car driving. The damage to electronic equipment is mechanical, which can cause disconnection, soldering, contact vibration, and poor grounding. In addition, they are also harmed by water, salt, oil and other chemical substances. Therefore, electronic components must be tested in the following environment and can be installed only after being qualified:

(1) Water immersion and icing test: test the resistance of electronic components to water immersion and freezing;

(2) Salt resistance test: Carry out 5% salt water spray test on automotive electronic components to test the corrosion resistance to salt;

(3) Dust and sand dust will cause the disconnection of the breaker, or absorb moisture and attach to the components to cause leakage.

(4) Oil and other chemical substances: It is required that the electronic components used in automobiles cannot be caused by oil, oil additives, gasoline andantifreezeThe impact of the impact and affect the function.

Two, modern car electrical maintenance methods

(1) Faults and maintenance features

1. Failure characteristics

The characteristics of electrical faults on modern cars can be linked to their use characteristics one by one.Generally, electronic components are very sensitive to overvoltage and temperature. For example, the PN junction of transistors is prone to overvoltage breakdown and electrolysis.capacitanceWhen the temperature rises, the leakage of the device also increases, and the SCR components are sensitive to overcurrent. The characteristics of these failures are summarized as follows:

(1) Component breakdown

Breakdown includes overvoltage breakdown, overcurrent, and thermal breakdown caused by overheating. The breakdown sometimes manifests as a short circuit form, and sometimes it manifests as an open circuit form. Overvoltage and overcurrent breakdown caused by circuit faults are often irrecoverable.

According to statistics, about 85% of the damage of automobile capacitors is caused by dielectric breakdown, and about 70% of breakdown faults occur in new cars, that is, within the first few hundred hours of operation, because if the capacitor is defective , It will be broken down in the first few hundred hours of use.When the capacitor breaks down, it often burns outresistanceelement.

The breakdown of the PN junction of the transistor is the main fault phenomenon. A fault with poor thermal stability should be regarded as a component quality problem. The electronic components on some electronically controlled cars often lead to a "thermal short circuit" (or "thermal penetration" similar to a breakdown fault) due to their poor thermal stability. ")Phenomenon.

(2) Components aging or performance degradation

This includes many aspects, such as the decrease of the capacity of the capacitor, the decrease of the insulation resistance, the increase of the leakage of the transistor, the change of the resistance of the resistor, and the resistance of the adjustable resistor cannot be continuously changed.RelayContact ablation, etc. Components such as relays often suffer from failures such as insulation aging, coil burnout, short circuit between turns, contact jitter, and even failure to adjust the initial operating current.

(3) Line failure

Such failures include loose wiring, poor contact, poor insulation caused by moisture, corrosion, etc., short circuits, bypasses, etc.

Such failures are generally not related to components.

2. Maintenance features

(l) For the maintenance of electronic circuits in electronically controlled automobiles, the current outstanding problem is the lack of information and the difficulty of spare parts. Once you encounter unfamiliar car models and circuits, you often have to do it yourself, analyze the circuit principle, and even survey and map the necessary circuit diagrams to clarify the overall circuit and connections, and then analyze the faulty circuit. Therefore, the maintenance of electronic circuits in electronically controlled automobiles will involve circuit analysis methods.

(2) Many electronic circuits of electronically controlled automobiles often adopt non-detachable packages for performance requirements and technical protection and other reasons, such as thick-film package regulators and solid-sealed ignition circuits. If a fault may involve their internals, it is often difficult to judge, and it is necessary to drain them one by one from the periphery first, and finally determine whether they are damaged.

(3) Although the electronic circuits on some electronically controlled cars are detachable and detachable, they often lack the replacement of discrete components of the same model, so it is often necessary to try to replace them with domestic or other imported components. This involves the feasibility of component replacement.

(4) In terms of inspection and repair methods, traditional automobile electrical failures can often be used to "fire test" to identify the location and cause of the failure one by one. Although this method is not very safe and reliable, and has certain hazards to the battery, it is still feasible in the traditional maintenance method. These methods are not allowed on electronically controlled cars equipped with electronic circuits. Because of the overcurrent generated by the "fire test", it will bring unexpected damage to some circuits or components. Therefore, some instruments and tools must be used to repair electronically controlled automobile electrical appliances according to certain methods.

(5) It is not allowed to use ohmmeters and multimeters with low resistance ohms below R×100 to detect low-power transistors, so as not to damage them due to current overload.

(6) When replacing the triode, the base should be connected first, and when disassembling, the base should be disassembled last. For metal oxide semiconductor tubes (MOS), it should be electrostatically broken down, and the soldering iron plug should be unplugged from the power supply when soldering.

(7) When removing and installing components, the power supply should be cut off. If there is no special instructions, the distance between the component leads and the solder joints should be more than 10mm to prevent the soldering iron from damaging the components, and an electric soldering iron with a constant temperature or a power of less than 75W should be used.

(8) After repairing, ensure good contact between the components with heat sink and the heat sink to ensure good heat transfer.

(9) Other necessary maintenance experience. All of the above characteristics should require maintenance personnel to have a certain foundation of electrical engineering and electronics and the ability to analyze circuit principles and use basic instrument tools.

(2) General procedures for troubleshooting

When troubleshooting, you can use the "five-step method" described below.

1. Verify the user's feedback. Power on all the components in the problematic circuit and try to see if the user's feedback is true. At the same time, pay attention to observe various phenomena after power-on. Before disassembling or testing, the setting range of the cause of the accident should be minimized.

2. Analyze the circuit schematic diagram. Draw the problematic circuit on the circuit diagram, analyze the path of the current from the power load to the ground, and clarify the working principle of the circuit. If you are not sure about the circuit principle, you should carefully read the circuit description and related materials until you are clear. The related lines of the problematic lines should also be checked. Each circuit diagram gives the names of related lines that share a fuse, a grounding point, or a switch. For the related lines that were missed in the first step of the procedure, try it. If the related lines work normally, it means that the shared part is okay, and the cause of the fault is limited to the line with the problem. If several lines fail at the same time, the cause is mostly the fuse or the grounding line.

3. Check the circuits/components where the problem is concentrated

Test the circuit to verify the inference made in step 2. The key to the speed and success of troubleshooting is that the troubleshooting procedures are simple, clear and reasonable. Highlight the most probable causes in the system fault diagnosis table, test them first, and test the easiest places first.

4. Make repairs

Once the problem is identified, the necessary repairs can be carried out.

5. Check whether the test circuit is back to normal

Perform another system check on the line to see if the problem has been resolved. If the fault is a blown fuse, each circuit that uses the fuse should be tested.

(3) Precautions for electrical maintenance of modern cars

One of the principles of repairing electrical systems is not to replace wires or electrical equipment at will. This operation may damage the car or cause a fire due to a short circuit or overload. At the same time, pay attention to the following items:

1. When removing the battery, always remove the negative (-) cable first; when installing the battery, always connect the negative (-) cable last.

2. When replacing the burned insurance, the insurance of the same specification should be used. Using an insurance larger than the specified capacity can cause damage to electrical components or fire.

3. When removing or installing the battery cable, make sureIgnition SwitchOr other switches have been turned off, otherwise it will cause damage to the semiconductor components.

4. The wiring harness near the vibrating component (such as the engine) should be fixed with a clip, and the slack part should be tightened to prevent the wiring harness from contacting other components due to vibration.

5. Don't treat electrical components roughly, and don't throw them around at will. No matter the device is good or bad, it should be handled with care.

6. The part of the harness that is rubbed against the sharp edge should be wrapped with tape to avoid wear.

7. When installing fixed parts, make sure that the wire harness is not clamped or damaged.

8. When installing, make sure that the plug is connected firmly.

9. During maintenance, if the temperature exceeds 80°C (such as during welding), the temperature-sensitive parts (such as relays and ECU) should be removed first.

(4) Misunderstandings in self-diagnosis of electronically controlled vehicles

The development and application of modern electronically controlled automobile fault self-diagnosis system enables automobile maintenance personnel to discover faults in a timely manner while the automobile is running.car repairIt provides convenience for fault query. By interpreting the fault code, most automobile maintenance personnel can identify the possible cause and location of the fault. However, only relying on the fault code to find the fault often leads to errors in judgment. In fact, the fault code is only a yes or no conclusion recognized by the electronically controlled vehicle computer (ECU). When the electronically controlled vehicle is repaired, a comprehensive analysis and judgment should be made to find the fault location based on the phenomenon of the vehicle failure.

The fault self-diagnosis system of electronically controlled automobile generally consists of fault identification and fault operation control software, fault monitoring circuit and fault operation backup circuit in the electronic controller (ECU). The fault detection items of the fault self-diagnosis system of the cars produced by different manufacturers are different, and the fault code storage and display methods are also different. The fault code is stored in the random memory (RAM). The random memory is directly connected to the battery. The fault code can be stored for a long time. To clear the fault code, you need to disconnect the special random memory connection circuit or directly disconnect the battery.

At present, most of the fault codes of electronically controlled vehicles are obtained in three ways.One is to read by the number of flashes of the fault indicator on the dashboard; the second is to read the fault directly with the help of a dedicated car decoder; the third is to rely on the fault code produced by a domestic manufacturerAnalyzer, Read the Chinese text description of the fault code in Chinese display mode. Obviously, obtaining the meaning of the fault code in Chinese characters is a method generally favored by the majority of auto repairers. For the first two reading methods, you need to consult relevant information to understand the meaning of the fault code. However, no matter what method is used to interpret the fault code, once the EFI vehicle's control computer records and stores the wrong fault code, it will bring a lot of inconvenience to vehicle maintenance. In the following three cases, error messages are prone to appear in the fault code, and you hope to attract attention.

1. The fault is obvious when the car is running,sensorThere is a fault but the self-diagnostic system does not detect it.

Electronically controlled car control computer (ECU) pairsensorWhen the signal is detected, it can only accept the abnormal signal of the sensor outside the set range, so as to judge the quality of the sensor, and record or not record the fault code.Once the fault code is interpreted, you only need to check the corresponding sensor, wireConnector, Check the wire, find and eliminate the fault of short circuit and open circuit. However, if for some reason the sensitivity of the sensor decreases, the sensor responds slowly, and the output characteristic shifts, the self-diagnostic system will not be able to detect it. Although the engine does show a fault, the self-diagnostic system has output a normal no fault code (the fault indicator does not flash). At this time, it should be analyzed and judged based on the symptoms of engine failure, and then the sensor unit should be tested specifically to find and eliminate the sensor failure.For example, when the engine speed is accompanied by unstable engine idling during driving, but the self-diagnosis system has no fault code output, the first thing worth considering and suspicion is the air flow sensor or intake air.Pressure SensorA malfunction, because the performance of these two sensors directly affects the basic fuel injection volume of the engine controlled by the ECU. Although the corresponding fault codes are not displayed at this time, they should be checked. For example, when the wing-plate air flow housing has cracks and leaks, it will cause the air flow sensor to measure inaccurately, causing the engine to run out of adjustment, and the self-diagnostic system of the control computer ECU cannot detect this failure phenomenon. Therefore, no error code is output.

2. Due to the similar phenomenon of engine operating conditions, ECU monitoring errors, the self-diagnostic system may show wrong fault codes. For example, for an electronically controlled car equipped with a three-way catalytic converter, if leaded gasoline has been used, this type of failure characteristic is sometimes more obvious. When the car is overhauled, it is often found that the fault code shows "water temperature sensor open or short circuit" fault, but the engine fault symptom is: whether the engine is in a cold state or a hot state, it is not easy to start, and it is accompanied by Instability of idling speed and backfire phenomenon, the engine speed always does not increase. Obviously, these faults are not very closely related to the water temperature sensor, and no faults were found after the water temperature sensor was measured individually. However, when the three-way catalytic converter was removed from the car and cut open, it was found that the inside of the three-way catalytic converter was seriously blocked, so it can be concluded that the engine failure was caused by this. Therefore, when a fault code appears in the self-diagnostic system, it should be analyzed and compared with the actual fault symptoms of the engine to obtain a correct and reasonable judgment. The fault code should not be used as the only basis for troubleshooting.

3. Improper use and maintenance of electronically controlled vehicles can also cause wrong fault codes.

When performing maintenance on electronically controlled vehicles, improper maintenance or operating errors by the maintenance personnel will also cause the self-diagnostic system to output an incorrect fault code. For example, when the engine is running, the sensor plug is unplugged randomly or unintentionally. Each time the sensor plug is unplugged, the self-diagnostic system will record a fault code. In addition, if the old fault code was not completely cleared due to improper operation during the last car repair, the computer will also save the old fault code in it. Therefore, it should be added when repairing the electronically controlled car. Pay attention, so as not to cause unnecessary man-made fault codes and bring confusion and difficulties to the maintenance work.