In-depth understanding of the application of self-diagnosis function of automobile electronic control system Chengdu Panfeng Technology

In-depth understanding of the application of self-diagnosis function of automobile electronic control system Chengdu Panfeng Technology
Some maintenance personnel encountered the following situations when using automobile electronic control system testing equipment: multiple fault codes were read, the fault light came on but When there is no fault code, there is a fault but no corresponding fault code is generated, or there is a fault code but no corresponding fault can be found, people often feel confused and have no idea what to do, and then start to complain about the quality or performance of the testing equipment. In fact, maintenance personnel can effectively use fault diagnosis instruments only after they have a thorough understanding of the principles of automobile electronic control systems, self-diagnosis systems, and automobile electronic control system diagnostic equipment.
Principle of automobile self-diagnosis system
. Abnormal conditions of automobile control system
When the automobile control system is working normally, the input and output signals of the electronic control unit operate within a specified range. When the control system When the circuit signal is abnormal, the diagnostic system in the circuit determines that the circuit signal is faulty. There are three types of abnormal conditions in the circuit:
The first type is that the signal of the circuit exceeds the specified range. For example: when the coolant temperature sensor is working normally, its output voltage is within .~. If it exceeds this range, the diagnostic system will determine it as a fault signal
The second is that the electronic control unit cannot receive the sensor within a period of time. If the signal or received signal does not change for a period of time, the diagnostic system will also determine it as a fault signal. For example: when the oxygen sensor is working normally, its input voltage should fluctuate no less than times/second within .~.
The third is that when the diagnostic system in the electronic control unit accidentally discovers an abnormal input signal, it will not Diagnosed as a fault signal Only abnormal input signals that appear multiple times or last for a certain period of time will be diagnosed as a fault signal. For example: the speed signal () is a pulse signal. When the engine speed is above r/, the loss of several signals will not be judged as a fault. ?
.Fault confirmation method of automobile self-diagnosis system
..Value range determination method
When the input signal received by the electronic control unit exceeds the specified value range, the self-diagnosis system will confirm the There is a problem with the input signal. For example: The water temperature sensor of a certain car is designed to have an output voltage within the normal operating temperature range of -°C (or a larger range). ． So when the electronic control unit detects that the signal voltage is less than. or greater than. It is determined that the water temperature sensor signal system has a short circuit or open circuit failure.
..Time domain determination method
When the electronic control unit detects that an input signal has not changed within a certain period of time or has not changed for a predetermined number of times, the self-diagnosis system will determine that the signal has occurred. Fault. For example: after the oxygen sensor reaches normal operating temperature and the control system enters a closed loop, the electronic control unit cannot detect the output signal of the oxygen sensor for more than a certain period of time or the oxygen sensor signal is under. The self-diagnosis system determines that the oxygen sensor signal system is faulty if the condition exceeds a certain period of time.
..Function Determination Method
When the electronic control unit sends an action command to the actuator, it detects changes in the output parameters of the corresponding sensor. If the sensor output signal does not change according to the parameters specified by the program, confirm that the actuator or circuit has occurred. Fault. For example: Generally, the r-valve height sensor is installed in the r-valve system of a general automobile to detect whether the r-valve is working properly. However, some cars are not equipped with an r-valve height sensor. When the electronic control unit sends a command to open the r-valve, it can also be determined whether the r-valve has moved by detecting whether there is a corresponding change in the output signal of the intake air pressure sensor. If there is no change, confirm the r-valve and circuit. failure.
..Logical determination method
The electronic control unit compares the data of two interconnected sensors. When it finds that the logical relationship between the two sensor signals violates the set conditions, it concludes that there must be a fault. For example, if the electronic control unit detects that the engine speed is greater than a certain speed and the throttle position sensor output signal is less than a certain value, it is determined that the throttle position sensor is faulty.
When the diagnostic system in the electronic control unit detects a fault signal, it immediately stores the fault information in the form of a fault code into the memory and lights up the fault warning light to display the fault information. While electronic control systems improve vehicle performance, they also complicate vehicle fault diagnosis. Automobile maintenance personnel can diagnose the fault and the possible cause and location of the fault by reading the fault code in most cases. If you blindly rely on fault codes to diagnose faults during car maintenance, errors in judgment will often occur and cause unnecessary losses. The fault code is only the conclusion of the electronic control unit program to determine whether the system is normal. In complex and changeable situations, the electronic control unit may not be able to truly identify the location of the fault. Therefore, when repairing electronically controlled vehicles, comprehensive analysis and judgment should be combined with the vehicle's self-diagnosis results and vehicle failure phenomena to find the fault location.
Abnormal diagnosis of automobile fault self-diagnosis system
The fault code diagnosed by the automobile fault self-diagnosis system is stored in the random access memory (r). The fault code can be stored for a long time to clear the fault code. It is necessary to disconnect the dedicated random storage. Connect the circuit to the circuit or disconnect the battery directly. Automobile fault self-diagnosis systems record and store erroneous fault codes, which brings a lot of inconvenience to electronically controlled vehicle maintenance. DTC error messages are likely to appear in the following three situations.
. When the car is running, the fault is obvious. The sensor is faulty but the self-diagnosis system does not detect it.
When the car electronic control unit detects the sensor signal, it can only accept abnormal signals from the sensor within its set range to judge. Whether the sensor is good or bad, whether the fault code is recorded or not, once the fault code is interpreted, you only need to check the corresponding sensor, wire connector, and wire to find and eliminate short circuit and open circuit faults. However, if for some reason the sensor sensitivity decreases, the response is slow, and the output characteristics shift (that is to say, the sensor does not completely fail), the self-diagnostic system cannot detect it. Although the car does have faults, the car's self-diagnostic system outputs a normal system code (the fault indicator light does not flash). In this case, the maintenance personnel will have doubts about the testing equipment or the car. Maintenance personnel should analyze and judge based on the car's fault signs and then conduct targeted detection (data flow, etc.) of the sensor unit in order to find and eliminate sensor faults. For example, when the engine speed stalls and the engine idling is unstable while driving but the self-diagnostic system does not output a fault code, the first thing worth considering and suspecting is that the air flow sensor or intake pressure sensor is faulty because the performance of these two is poor. The damage directly affects the basic fuel injection amount of the controlled engine. Even if the corresponding fault code is not displayed at this time, they should still be checked. For example, when the air flow housing has cracks and leaks, it will cause the air flow sensor to measure inaccurately and cause the engine speed to be out of balance. The self-diagnostic system of the electronic control unit cannot detect this fault phenomenon and does not output a fault code.
. Engine failure phenomena are similar and the self-diagnosis system may display wrong fault codes due to monitoring errors
Volkswagen’s throttle sensor’s sensitivity decreases and slow response causes the engine’s air-fuel ratio imbalance and the air flow meter’s sensitivity decrease. The fault phenomenon of air-fuel ratio imbalance is similar to that when the self-diagnosis system will display the fault code of the throttle sensor or air flow meter. For electronically controlled cars equipped with three-way catalytic converters, once leaded gasoline has been used, such fault characteristics are sometimes more obvious. When the car is inspected, it is often found that the fault code shows that the water temperature sensor is open or short-circuited and the engine cannot speed up. Obviously these faults have little to do with the water temperature sensor. No faults were found after measuring the water temperature sensor. However, when the three-way catalytic converter was removed from the car and opened, it was found that the interior of the three-way catalytic converter was seriously clogged, so it was concluded that the engine failure was caused by this. Therefore, when a fault code appears in the self-diagnosis 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.
Improper maintenance of automobile electronic control systems may also cause wrong fault codes
When performing maintenance on electronically controlled automobiles, improper maintenance or operation errors by maintenance personnel may also cause the automatic change system to output wrong fault codes. For example, if the sensor plug is accidentally pulled out while the engine is running, the self-diagnostic system will record a fault code every time the sensor plug is pulled out. In addition, if the old fault code could not be completely cleared due to improper operation during the last car maintenance, the electronic control unit will also save the original old fault code. Therefore, when repairing the electronically controlled car, care should be taken not to cause any inconvenience. Necessary human fault codes bring confusion and difficulty to maintenance work.
The use of diagnostic instruments for electronic control units is only limited to code reading and code clearing, ignoring the most important detection method of data flow detection. In fact, when diagnosing vehicle faults, sometimes a fault does not necessarily lead to a fault code as mentioned above. At this time, we can use the data flow analysis method to make judgments. At this time, maintenance personnel need to flexibly use basic and theoretical knowledge of automotive expertise.
By analyzing the data flow, it is easy to determine the faulty component.
Examples of abnormal diagnosis of automobile fault self-diagnosis system
. The first example: the fault phenomenon is that the Santana or Jetta engine is not easy to start when the car is cold, the idle speed is unstable after starting, and the acceleration is poor after the car is warm
.. Fault diagnosis analysis and treatment:
The hot car accelerates poorly and it is difficult to increase speed. Perform normal maintenance and replace spark plugs. Use the decoder to read the fault code and there are the following fault codes: Mixture adaptation exceeds the adaptation limit air flow meter sensor.
..The read data stream is displayed as follows:
) Group: The measured value of the mixture component is lower than the standard value - (equivalent to -.+.)
) Group- .-. -.
) group- .-. . .
) group-
) group- .-. .
) group.-. . -.
) group % -.-.
) Group- .
) Group. .Idle speed matching error
..Data flow analysis is as follows:
) The measured value of the mixture composition in the group is low At the standard value - indicates that the mixture ratio is abnormal and corresponds to the fault code recorded by the control unit and the mixture exceeds the specified value.
) The engine load, throttle angle, and ignition advance angle values in the group are within the specified values.
) The engine load and air quality measurement values in the group are within the specified values.
) The battery voltage, water temperature, and intake air temperature values in the group are within the specified values.
) The idle speed control value and intake air volume value in the group are within the specified values.
) The control value of the excess air coefficient of the group mixture is within the specified value.
) The oxygen sensor voltage in the group is within the specified value.
) The status value in the group indicates that the throttle matching is complete and the adjustment is normal.
) The group data stream shows a throttle matching error. If the basic setup is completed normally, the fourth bit of data in the group should be. This is inconsistent with group data.
..Analysis of the preparation of the gas mixture of a gasoline engine:
...About .kg of air is required to completely burn 1 kg of gasoline. Let the excess air coefficient d= at this standard air-fuel ratio. When the mixture is lean (d=.), more air is inhaled, while when the mixture is rich (d=.), the air is inhaled less. The maximum power and better working conditions of the engine are in the rich mixture area, but in order to reduce fuel consumption, it is hoped that the engine will work in the lean mixture area. The engine controller calculates the control voltage signal based on the throttle opening and engine speed. In addition, it also requires accurate regulation by the oxygen sensor. Therefore, whether the air volume ratio is normal or not depends on the value of the oxygen sensor. The following working conditions electronic control unit relies on throttle signal, air volume signal, temperature, speed, knock and other signals to implement.
...Mixture enrichment: It affects cold start warm-up operation, idling operation and full load conditions as follows:
) Cold start: During cold start, the electronic control unit instructs the nozzle to operate according to the water temperature. Extra fuel is injected into the intake manifold when starting to facilitate cold starts.
) Warm-up operation: After a cold start, a rich mixture must be supplied in order to warm up the engine. The warm-up valve regulating device changes the control pressure with changes in temperature and time. Under the condition of equal air flow, the reduction of control pressure allows the valve plate to have a larger stroke and obtain a properly enriched mixture.
)Idle speed: The electronic control unit causes the valve plate in the throttle to deflect slightly. This slight deflection generates the air flow required for idle speed supply.
d) Full load: In order to make the engine output maximum power at full load, the electronic control unit sends an enrichment signal based on the throttle sensor signal and speed signal to enrich the mixture.
..Ideal working condition analysis:
The main reference signals for the above working conditions are: rotation speed, empty gauge, and knock signal. The speed signal provides basic reference parameters for fuel injection and ignition, and the knock signal corrects the ignition signal. Assuming there is no problem with the engine cylinder pressure, the fuel injection amount provided by the air volume meter is correct (reaching the theoretical air-fuel ratio), the ignition advance angle provided by the speed signal is OK, and the ignition energy can ignite the combustible mixture in time. Theoretically, the combustible mixture can completely burn oxygen. The oxygen content in the exhaust gas detected by the sensor is zero.
.. Abnormal working condition analysis:
Assume that the air supply is abnormal, the fuel supply is abnormal, and the ignition supply is abnormal and the oxygen content in the exhaust gas detected by the oxygen sensor exceeds the standard. The electronic control unit adjusts the fuel supply amount according to the oxygen sensor detection signal. If the reduced or increased fuel supply volume still cannot make the combustible mixture in the engine fully combust, the electronic control unit relies on the oxygen sensor signal to try to obtain information from the changes in oxygen content to participate in the mixture. The gas is adjusted again but the mixture adjustment exceeds the adaptive value and is too high or too low. If the value is relatively large, it cannot be adjusted to the ideal value according to this signal value, so a fault code that is adjusted to the limit value is generated. The ignition system should be checked and replaced. Only by deeply understanding and analyzing the principles of the electronic control unit can we draw correct diagnostic conclusions.
..Troubleshooting:
Check the starting voltage of the battery and the water temperature sensor. If there is a problem, clear the code after replacement and make a basic setting. Check the air flow meter again and replace the oxygen sensor. Start the engine and keep it running at idle speed. The display group observes the oxygen sensor feedback signal voltage. The signal voltage can be in. ． fluctuates but the frequency of change is very slow. Carry out oil pressure test and the oil pressure gauge at idle speed shows: . When the gas pedal is pressed, the oil pressure gauge pointer is on. -. swing between. After turning off the ignition switch, the fuel system maintains a pressure of. . If the oil pressure values all meet the standards, it can be determined that the fuel pump is working well and the oil pressure regulator is normal.
) Clean the fuel system and throttle body and perform basic settings but still no results.
) Check and clean the air flow meter and the fault persists after replacing the oxygen sensor.
) Check the ignition system and find that the cylinder spark plug spark is weak. Considering that the car and cylinder share the same ignition coil, the fault was completely eliminated after replacing the ignition coil.
..Troubleshooting results:
It is known from this that the root cause of the fault is the poor operation of the ignition module and insufficient cylinder ignition energy, which leads to the deterioration of the mixture combustion condition. Components causing similar failures include oil pressure, air flow meter, throttle position sensor, and oxygen sensor. Therefore, to correctly understand the automobile self-diagnosis system, we should correctly understand the setting conditions of the diagnostic system:
...from the initial simple monitoring of the line voltage of the output and input components, such as when the monitored voltage is in a short circuit state, the low potential is in the open circuit When the high potential in the state and the sudden change of the line voltage exceed the voltage threshold set within the self-diagnostic system, the self-diagnostic system corresponds to the code number originally set in the read-only memory r according to the line port and fault symptom corresponding to the monitored voltage. Set the corresponding fault code. Early self-diagnosis systems could only identify or set a small number of fault codes, and the contents of the fault codes were limited to open circuits, loss of short-circuit signals, abnormal changes in the current of incompletely working actuating components, and the like.
...Due to the requirements for control accuracy and the diagnostic requirements of each monitoring and diagnosis system, the content of different fault codes is no longer limited to simple expressions such as excessive voltage, low voltage, or signal non-existence. Newly emerged The fuel trim system is too rich for a long time, the performance is degraded, the r system position deviation and other fault codes that are difficult to understand at first glance. Moreover, the settings of these faults often have specific conditions according to the special requirements of the monitoring system. After retrieving the fault diagnosis code, it is very important to carefully read the maintenance manual to find the corresponding fault information, fault setting conditions, and strategies after fault setting.
.Second example: The fault phenomenon is that the Santana or Jetta engine has unstable idle speed and emits black smoke when accelerating.
..Fault diagnosis:
) Read the fault code: mixture adaptive over-limit and water temperature sensor open circuit/short circuit to positive, sensor open circuit/short circuit to ground. After writing down the fault code, clear the code and re-read the code only for the water temperature sensor. After checking, the water temperature sensor was found to be faulty after replacement. At this point, the function of the fault code has been exhausted.
) Diagnostic method for not reading the data stream: Unstable idle speed, clean the throttle body, and redo the basic settings to see if there is black smoke and check if the oil pressure is normal. Clean the fuel injectors and replace the fuel filter. When I started it again, I found that black smoke was still coming out but the idle speed had become smoother. I replaced the oxygen sensor because it was still smoking, but it didn't work. Check the spark plug and high-voltage line. Normally, the spark plug gap is large and black. After replacing the spark plug and test-running, the fault phenomenon subsided but black smoke still emitted when accelerating. Suspected damage or damage to the ignition coil, improper valve timing or damage to the air flow meter. In line with the procedure from simple to complex, never changing parts to parts, check that the gas distribution timing is good, replace the ignition coil if it is invalid, and the fault is eliminated after replacing the air flow meter.
) Reading data stream diagnostic method (taking Santana as an example):
For an engine with black smoke coming out of the exhaust pipe and unstable idle speed, the data stream of group and group can be read.
) Read from the group: mixture d control -% (normally -%~%) d sensor voltage.~.(normally.~.). This shows that the mixture is indeed too rich and far exceeds the ability of D control.
) Read from the group: engine load. (Normally it is ~.) Engine cycle injection flow is ./ (Normally it is .~./).
) The throttle valve opening angle read from the group is ~ (normal is ~), although it is not over the limit, it is still too large.
.. Troubleshooting:
During idling, the d control exceeds the limit because the throttle valve is in the idle position and the engine speed is adjusted according to the idle speed. Excessive intake air flow is considered to be due to high engine load and does not reduce the amount of fuel injection (ie, injection duration), causing the idle speed to fluctuate high and low. Because the fuel injection volume at idle speed is large, the fuel injection volume is larger when accelerating, causing black smoke to emit from the exhaust pipe. The fault was eliminated after cleaning the throttle body and replacing the air flow meter. After reading the data stream and conducting quantitative analysis, relevant components can be detected and replaced in a targeted manner. Using the method of reading the data stream to replace the spark plug and ignition coil reduces fault diagnosis time and saves labor and materials.
Self-diagnosis system and limp system
When the car is running normally, the voltage values of the input and output signals of the electronic control unit have a certain range of change. When the voltage value of a certain signal exceeds this range and this phenomenon does not disappear within a period of time, it is judged that this part is faulty. Store this fault in the internal random access memory (r) in the form of a code and light up the fault check light. When a circuit fails, its signal cannot be used as an engine control parameter.
In order to maintain the operation of the engine, a certain fixed value is called out from its program memory (r) as the emergency parameter of the engine to ensure that the engine can continue to operate. When the electronic control unit fails, it automatically activates the backup control loop to simply control the engine so that the car can be driven home or repaired at a nearby auto repair shop. This function is fault operation, also known as limp mode. On the other hand, when a malfunction of an actuator is detected, some safety measures are taken for safety reasons. This feature is called failsafe.
Fault diagnosis is performed on the sensors, electronic control units and actuators in the system. When the sensor and electronic control unit fail, fault operation mode is often adopted and when the actuator fails, fail-safe measures are often adopted.
. Sensor fault self-diagnosis and fault operation
Since the sensor itself generates electrical signals, the sensor fault diagnosis does not require special circuits but only needs to compile the sensor input signal recognition program in the software. Realize fault diagnosis of sensors. The normal input voltage value of the water temperature sensor is -. The corresponding engine cooling water temperature is -℃-℃. So when the detected voltage signal exceeds this range occasionally, the diagnostic program does not consider it a fault. However, if the abnormal signal continues for a period of time, the diagnostic program will determine that the cooling water temperature sensor or its circuit is faulty. This situation is stored in the random access memory in the form of a code (this code is a digital code that has been agreed upon during the design to represent abnormal faults in the water temperature sensor signal). At the same time, the driver and maintenance personnel are notified of a fault in the engine electronic control system through the check fault warning light. When the water temperature sensor is found to be abnormal, a preset constant is used as a substitute value for the water temperature signal to keep the system in operation.
.Fault self-diagnosis and backup circuit of the electronic control unit
If the electronic control unit fails, the control program will not be able to operate normally. The electronic control unit is in an abnormal working state. This would render the car unable to drive due to a malfunction in the engine control system. In order to ensure that the car can continue to run when the electronic control unit fails, a backup loop (standby integrated circuit system) is designed in the control system engineering. When the electronic control unit fails, it automatically calls the backup loop to complete the control task and enters the simple control operation state, using a fixed control signal to allow the vehicle to continue driving. Since this system only has the simple function of maintaining engine operation and cannot replace all the work of the electronic control unit, the work of this backup loop is also called limp mode. The fault indicator light comes on when the backup system is working. Whether the electronic control unit is working properly or not is checked by a circuit called a control loop. ****The circuit is equipped with a counter that is independent of the electronic control unit. When the electronic control unit is running normally, the counter is cleared regularly by the operating program of the electronic control unit. In this way, the value of the counter in the circuit will never overflow. When the electronic control unit operates abnormally, the electronic control unit cannot clear the counter regularly, causing the counter to overflow. ****When the counter overflows, the output level changes from low level to high level (this output is generally the carry flag of the counter. When the counter reaches its maximum value and another counting pulse is added, the counter will overflow. At this time The level of the overflow terminal of the counter will change from low level to high level and the counter will be cleared at the same time). This change in the counter output level will directly trigger the backup loop. The backup circuit only controls the injector and igniter with a constant injection duration and ignition advance angle based on the start signal and idle contact closure.
.Fault diagnosis and fault insurance of actuators
In automobile electronic control systems, actuators are the main components that determine engine operation and vehicle driving safety. When an actuator fails, it will often cause certain consequences to the driving of the vehicle. Influence. Therefore, the handling method for actuator failure is usually as follows: when an actuator failure is confirmed, appropriate safety measures are taken according to the severity of the failure, and a failsafe system is specially designed in the control system.
Since the actuator is controlled, the control signal is an output signal. Therefore, in order to diagnose the working conditions of each actuator, it is generally necessary to add a fault diagnosis circuit, that is, to send a control signal to the actuator. The actuator must have a dedicated loop to feedback its execution status. A typical component in an engine electronic control system for troubleshooting actuators is the igniter. Under normal circumstances, when the igniter is controlled, the ignition confirmation circuit in the igniter will feedback the ignition execution status in the form of an electrical signal every time the igniter performs ignition. When the ignition circuit or igniter fails, the ignition control command is issued and no feedback signal is obtained. At this time, it is considered that the igniter is no longer working properly. Because if the ignition system fails when the engine is working, unburned mixture will enter the exhaust device and exhaust pipe. The catalyst temperature in the exhaust purification device will greatly exceed the allowable value. At the same time, excessive accumulation of unburned mixture in the exhaust pipe can cause an explosion in the exhaust system. For this reason, a fail-safe system is used to immediately cut off the power supply of the fuel injection system to stop fuel injection when the ignition confirmation signal is not received.
In short, using the automotive electronic control system diagnostic instrument, maintenance personnel can quickly, conveniently and accurately locate the fault and eliminate it smoothly. However, no matter how powerful the instrument is, its effectiveness still depends on the initiative of the maintenance personnel.