In normal operation, the voltage value of the input and output signals of the electronic control unit ECU has a certain variation range. When the voltage value of a signal exceeds this range, and this phenomenon will not disappear for some time, ECU will be judged as part of the failure. ECU put this fault in the form of code into the internal random access memory (RAM), while lighting fault inspection lights (such as ENGINE SRS, CHECK, ABS, etc.), this is the basic principle of fault diagnosis. When a circuit has a fault, the signal can not be used as the control parameters of the engine.
In order to maintain the operation of the engine, ECU from its program memory (ROM), the transfer of a fixed value, as the engine emergency parameters, to ensure that the engine can continue to run. When the microcomputer system in ECU failure, ECU automatically enabled backup control loop for simple control of the engine, the car can drive home or to a nearby garage for repairs, this is the function of fault operation, also known as the "limp" mode. On the other hand, when the ECU detects an actuator failure, for safety reasons, some safety measures are taken. This feature is called fault insurance.
The ECU fault diagnosis is based on the sensor, microcomputer system and actuator in the system. When the sensor and the computer failure, often take the fault operation mode. When the actuator failure, often take the failure of insurance measures.
The sensor itself is generated signals, therefore, the fault diagnosis of sensors do not need special lines, but only in the software, the realization of fault diagnosis of sensor sensor input signal recognition program can be compiled. The normal input voltage of the water temperature sensor is 0.3-4.7V, corresponding to the engine cooling water temperature is -30-120. So, when the ECU detects the voltage signal beyond this range, if it is once in a while, ECU's diagnosis procedure does not consider it to be a fault. But if the signal is not normal for some time, then the diagnostic procedure is to determine the cooling water temperature sensor or the circuit is faulty. ECU will be in the form of code (this code is designed to be a good representative of the water temperature sensor signal abnormal fault digital code) in the form of random access memory. At the same time, by checking the engine warning light "ENGINE CHECK", notify the driver and maintenance personnel, engine control system failure. When the ECU found that the water temperature sensor is not normal, it uses a set of a predetermined constant as a substitute for the water temperature signal, so that the system works in the running state.
If the computer system fails, the control program can not run normally, and the computer is in an abnormal working state. This will cause the car to control system failure and can not drive. In order to ensure that the car can continue to operate in the computer failure, the design of the control system, the design of the backup circuit (standby integrated circuit system). When the microcomputer in the ECU failure, ECU automatically call backup loop to complete the control task, into a simple control operation state, with a fixed control signal, so that the vehicle continues to travel. Because all the simple function of the system only have to maintain the engine running and cannot replace the computer, so the reserve back road work is also known as "limp" mode. When the standby system is used, the fault indicator light is on. Whether the computer work is normal or not is monitored by a circuit known as a monitoring circuit. The monitoring circuit is provided with a counter independent of the microcomputer system. In normal operation of the computer, the operation procedure of the microcomputer is cleared to reset the counter. In this way, the numerical value of the counter in the monitoring circuit is never an overflow phenomenon. When the computer system does not operate normally, the microcomputer can not be cleared to the counter, which causes the overflow of the monitoring counter. The output level of the output from the monitor counter is changed to a high level by the low level (this output is generally the carry flag of the counter. When the counter reaches its maximum value, and then add a counting pulse, the counter will overflow. At this point, the level of the overflow of the counter will be changed to a high level by the low level; at the same time, the counter is cleared. The change of the output level of the counter will directly trigger the standby circuit. The standby circuit can control the fuel injector and the ignition device according to the starting signal and the idle contact closure condition, and the fuel injection timing and the ignition advance angle are controlled by the constant injection duration and the ignition advance angle.
In the automotive electronic control system, the actuator is the main device which decides the engine running and the safety of the automobile driving. When the actuator fails, it will cause a certain impact on the vehicle's driving. Therefore, the processing method of actuator fault is usually recognized as: when the actuator failure is carried out by ECU to take appropriate security measures according to the severity of the fault, in the control system, and specially designed fault insurance system.
Because the ECU performs the control operation, the control signal is the output signal. Therefore, to work on the actuator diagnosis to additional circuit fault diagnosis, the ECU sends a control signal to the actuator, the actuator must have a special loop feedback on the implementation to the ECU. In the engine electronic control system, the typical components of fault diagnosis of the actuator are the ignition device. Under normal circumstances, when the ECU is controlled by the ignition device, the ignition is carried out once every time, and the ignition confirmation circuit is carried out by the ignition confirmation circuit in the ignition device.
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