With the acceleration of the electromechanical integration of the automobile manufacturing industry in recent years, various electronic control systems such as an engine management system, an integrated electronic chassis system, and an active body control system are widely equipped in automobiles. When people enjoy the comfort brought by high-tech electronic technology, a variety of new automotive diagnostic topics have gradually been placed in front of the majority of car repair technicians. How to make a correct diagnosis of the failure symptoms of these high-tech electronic control systems quickly, effectively, and on the head is a common concern of the current automotive maintenance industry.
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In order to help maintenance technicians in-depth inspection of electronic control systems, automotive maintenance equipment manufacturers have introduced many fault diagnosis and data scanners. By reflecting the sensor data of the electronic control system and the working state parameters of the electronic control unit on the instrument, the analysis of the fault can be facilitated. At the same time, many automobile maintenance books and maintenance manuals also take a large space for the sensor detection of the electronic control system. Introduce. In the minds of many car maintenance personnel, there is a view that the failure of the electronic control system is nothing more than sensor performance degradation, damage or failure of the electronic control unit (computer). From the probability of maintenance inspection examples, it is true that the proportion of faults between the sensor and the computer accounts for a considerable proportion, but the sensor or computer is blindly replaced by subjective ideas, or simply based on the fault content displayed by the fault analyzer. Analytically to guide the maintenance ideas, once you encounter a car after replacing all the sensors or even the computer, the fault still exists, will you be confused?
In view of the above actual situation and personal maintenance experience, we will discuss with you the inconvenience that the power supply system of the vehicle's electronic control system affects the normal operation of the electronic control system.
Electronic control system power supply circuit
The power supply circuit of the electronic control system is an electrical circuit that provides power protection for the normal operation of the electronic control system. For a complete power supply circuit, the power supply line and the ground line are indispensable.
One end of the power supply line is generally connected directly to the battery positive pole of the vehicle or indirectly via the ignition switch, and the other end is directly connected to the electronic control unit (ECU) of the electronic control system or via a relay to the electronic control unit. For some electronic control system components, such as air flow sensors, which do not draw power from the electronic control unit, there are specific power input terminals on these components that are connected to the other end of the power supply line.
One end of the grounding line is always connected to the specific grounding terminal of the electronic control unit or system component, and the other end is connected with the electric conductors such as the vehicle body and the engine casing to form a complete grounding circuit with the negative electrode of the battery.
The importance of the normal voltage range
You may notice that there are often power requirements in the manuals of some household appliances, such as working voltage: 200V ~ 240V or 220V up and down 5%, which reflects the adaptability of the electrical products to fluctuating voltage. However, as a car maintenance personnel, have we paid attention to the normal working power supply voltage range of the in-vehicle electronic control system that we deal with all day? Existing on-board electrical systems are generally designed based on a 12V low-voltage line (except for ignition high-voltage systems), so the voltage range of the system is nothing more than 12V. In the past, when the on-board electrical system was few and simple, the requirements for power supply voltages such as lighting lines, wiper lines, and ignition circuits were generally not too high. For lighting systems, the voltage supplied to 11V and the voltage of 12V were not much. The difference.
Nowadays, the situation is very different. A large number of electronic control systems composed of ultra-large-scale integrated circuits, such as arithmetic circuits, measuring circuits, comparison circuits and even data bus circuits, are increasingly equipped on modern automobiles. These designs require precise electrical system limits on the operating voltage range that are not as sloppy as 11V and 12V. The 12V voltage of the battery is limited to a lower voltage range, such as 5V, through the regulated line in the electronic control unit. Only the processed voltage can be used by the integrated circuit. A little high or low voltage will affect the normal operation of the electronic components. Excessive voltage will cause excessive heat to affect the operating characteristics of the components. Too low voltage will also cause errors in the parameters of the operating points of the system. Logical operation errors, etc. The system voltage required by the MK20IABS system equipped with Superman in Santana 2000 era must not be lower than 10.5V, otherwise the system will work abnormally. All in all, the modern vehicle-mounted electronic control system will become more and more demanding on the system power supply voltage.
Routine detection of power circuits
Although the designers of automotive electronic systems strive to be safe and reliable in the internal design of the electronic control system to the external system, and with the natural extension of the service life of the vehicle, the components of the electronic control system that are under severe working conditions for a long time will inevitably appear. Aging and performance degradation of components. As a wire harness for the nerve system of an automobile electronic system, problems such as hardening of the sheath, looseness of the connector, and adhesion between the wires may occur. Most of these harnesses play the role of the power circuit of each electronic control system (a small part is the signal line). Therefore, after obtaining the fault information through the detector, we should not be busy testing or blindly replacing the sensor or actuator or even the computer pointed to by the fault information. First, we should check the power circuit that supplies the power of these components. The confirmation of the component body can be further carried out after the confirmation is completed.
1. Detection of power supply lines
The power supply line detection is mainly to measure the operating voltage value supplied to the electronic control system. According to the circuit diagram, find the terminal corresponding to the connector end of the power supply harness of the component to be tested, and use the voltmeter to measure the voltage to the negative pole of the battery (this can ensure the accuracy of measuring the voltage at this point and prevent the measurement error caused by the poor selection of the grounding point. ).
2. Detection of grounding lines
The grounding line detection is mainly to measure and provide the electronic control system to find the corresponding grounding terminal of the wiring harness connector, and measure the voltage between the point and the battery negative pole when the power supply circuit is turned on. A voltage less than 0.5V is acceptable. In the actual test, many people include the ohmic file or the diode conduction buzzer position that I used to use the multimeter in the past to judge the grounding quality by detecting the resistance value between the ground terminal and the battery negative pole. This method is not appropriate, because even if it is a small 0.5Ω, once the current is greater than 5A, the voltage drop is above 2V. The consequence of this would be a drop in the operating voltage of the system, the reference voltage of some measurement circuits being pulled high or the reference low potential between different subsystems within the same module being inconsistent. Especially when using the diode buzzer position, the resistance value of 10Ω up and down can make the buzzer beep, and it is just that we will think that the grounding line is unobstructed.
Electronic control system power circuit fault detection example
Example 1: The engine of the Buick sedan is unstable and the operation is not smooth.
There is a Shanghai Buick New Century sedan with intermittent engine idle speed instability, acceleration and weakness, and engine fault lights sometimes light up. Into the repair station maintenance, the maintenance work fault detector to call out the historical fault code: P0101 (MAF sensor performance degradation). Check the engine operation data under the current operating state of the engine. The frequency of the MAF is about 2000 Hz, and the corresponding intake air flow value is 3 g/s. In the current state, the engine runs smoothly and there are no signs of failure. The car entered the repair station again in a few days, and the repair content remained. At this time, the running of the engine was severely shaken. The fault code was again taken and the engine running data was observed. It was found that the MAF frequency was 0 Hz and the intake air flow rate was also 0 g/s. The repairman immediately determined that the air flow sensor (MAF) was damaged, and the past intermittent faults were reflecting the sensor's failure process. The sensor was replaced, the fault code was cleared, the engine was restarted, and the fault disappeared immediately. However, the car was repeatedly repeated during the test drive in the station. In desperation, the repairman judged that the engine control computer failed, borrowed the computer test of the same type of vehicle, but the fault still exists.
After understanding the above repair process and retrieving relevant fault information, the author found that the fault is still related to the MAF sensor. I think that the power supply to the sensor is normal, so the corresponding sensor power supply and grounding disconnection of the three-pin plug are found according to the circuit diagram. After the ignition switch is turned on, the power supply voltage and the grounding voltage are measured. The power supply voltage is only 4.98V, and the grounding voltage is normal. , is 0.02V. Because the power circuit of the MAF is a 12V circuit that passes through the ignition switch, it is clear that the power failure is the real source of the fault. When the fuse was finally found, it was found that the fuse was in a broken connection state, which caused a voltage of nearly 7V to be pressed at both ends, resulting in a power supply voltage of only 5V for the MAF. After replacing the fuse of 10A, the fault occurred. disappear.
Example 2: Buick ABS does not work, ABS lights are always on.
A Buick sedan suddenly ABS light is on the way, and as soon as the ignition is turned on, the ABS light immediately lights up and no longer goes out. Use the dedicated TECH2 diagnostic device to retrieve the fault information as: no data communication information (Note: all control computers of Buick sedan such as power control module, ABS control module, SDM airbag module, etc. are connected by star network through data lines). Generally, the failure to lose communication information is mostly caused by the open circuit of the data line, and no fault information about other components of the ABS system can be adjusted. According to the circuit diagram, the ABS communication data line connected to the standard OBD-II detection interface was found, but after inspection, the data line was not broken and the connection state was good. In the case of a stalemate in the diagnosis, it is decided to start by checking the power circuit of the ABS system. The power circuit of the ABS system consists of two power supply lines and three ground lines. The power supply and grounding of the control module, solenoid valve group and oil pump motor subsystem are arranged separately. Unplug the harness connector and the power supply line is normal. Because the plug is difficult to handle, measuring the ground voltage is replaced by a simple continuity test. The multimeter selects the buzzer file, and the measurement result: all the grounding wires and the battery negative pole are connected. After checking that the grounding connection point of the ABS system on the vehicle body is very strong, it is rashly considered that the power supply system is fault free. However, after a series of maintenance work and even replacement of the ABS control module with the hydraulic control unit assembly, the ABS fault light still lights up after the ignition switch is turned on. Holding the try, I measured the power system again and measured the grounding line resistance using an ohmic file. One of the grounding terminals to the negative pole of the battery reached 24Ω. To confirm the diagnosis, find the grounding point of the harness connector to the grounding point of the body, and use a jumper wire (one end is directly connected to the negative pole of the battery, and the other end is connected with the wire end of the body grounding point) to bridge the grounding loop. Reinsert the harness connector, turn the ignition on, and the ABS lamp will turn off immediately after a brief illumination. The root cause of the problem was finally found, because the 24 Ω contact resistance between the washer of the grounding wire fixing screw and the body paint layer on the body ground point caused the ABS system to work abnormally. Re-tighten the grounding screw after sanding the paint with sandpaper, and troubleshoot.
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