Inspection Scheme of Power Line of Vehicle Electronic Control System
With the acceleration of electromechanical integration in the automobile manufacturing industry in recent years, various electronic control systems such as engine management systems, integrated electronic chassis systems, and active body control systems have been widely equipped in automobiles. While people are enjoying the comfort brought by high-tech electronic technology, a variety of new automotive diagnostic topics are gradually placed in front of the majority of automotive repair technicians. How to quickly, effectively and accurately diagnose the fault symptoms of these high-tech electronic control systems is a common concern of people in the automotive maintenance industry.
In order to help maintenance technicians to conduct in-depth inspection of the electronic control system, auto repair equipment manufacturers have launched many fault diagnostic instruments 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, it is easy to analyze the fault; at the same time, many car maintenance books and maintenance manuals also spend a lot of space on the sensor detection of the electronic control system Introduce. In the minds of many automobile maintenance personnel, there is such a point of view: any failure of the electronic control system is nothing more than a decline in sensor performance, damage or failure of the electronic control unit (computer). From the perspective of the probability of maintenance detection examples, it is true that the failure rate of the sensor and the computer accounts for a considerable proportion, but the sensor or computer is replaced blindly based on the subjective idea, or simply according to the failure content displayed by the failure analyzer, without adding Analytically guide maintenance ideas. Once you encounter a car that has failed after replacing all sensors or even computers, will you be in chaos?
In view of the above realities combined with personal maintenance experience, we will discuss with you the intractable diseases related to the power circuit of the vehicle's electronic control system that affects the normal operation of the electronic control system.
Power supply circuit of electronic control system
The power supply circuit of the electronic control system is an electrical circuit that provides energy protection for the normal operation of the electronic control system. For a complete set of power supply circuits, the power supply circuit and the ground circuit are indispensable.
One end of the power supply line is generally directly connected to the positive electrode of the battery of the car or indirectly connected to the battery through the ignition switch, and the other end is directly connected to the electronic control unit (ECU) of the electronic control system or connected to the electronic control unit via a relay. For some electronic control system components, such as air flow sensors, which do not obtain power from the electronic control unit, these components have specific power input terminals 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 forms a complete grounding loop with the negative electrode of the battery through the connection with the conductors such as the body and the engine casing.
The importance of the normal voltage range
Maybe you will notice that there are often power requirements in the manuals of some household appliances, such as working voltage: 200V ~ 240V or 220V floating up and down 5%, which reflects the electrical product's adaptability to fluctuating voltage. But as car maintenance personnel, have we paid attention to the normal working power supply voltage range of the vehicle electronic control system that we deal with all day? Existing in-vehicle electrical systems are generally designed based on 12V low-voltage lines (except for ignition high-voltage systems), so the voltage range in which the system works is nothing more than 12V. In the past, when there were fewer and simpler on-board electrical systems, the power supply voltage requirements such as lighting circuits, wiper circuits, and ignition circuits were generally not too high. For lighting systems, the supply voltage of 11V and 12V is not much. Difference.
Now the situation is very different. A large number of computing circuits, measuring circuits, comparison circuits and even data bus circuits and other electronic control systems composed of very large scale integrated circuits are increasingly equipped in modern automobiles. These designs require a precise electrical system to limit the operating voltage range in no way as 11V and 12V can be sloppyly equivalent. The 12V voltage of the battery is limited to a lower voltage range such as 5V through the voltage stabilizing circuit in the electronic control unit, and only the processed voltage can be used by the integrated circuit. A little high or low voltage will affect the normal operation of electronic components. Excessively high voltages cause excessive heat generated during component operation to affect the operating characteristics of the components. Too low voltages will also cause errors in the parameters of each operating point of the system or Logic operation error, etc. For example, the system voltage required by the MK20IABS system equipped by Superman in the Santana 2000 era must not be lower than 10.5V, otherwise the system will malfunction. All in all, the requirements of the modern vehicle electrical control system on the system power supply voltage will become more and more stringent.
Routine testing of power circuits
Although the designers of automotive electronic systems strive to be safe and reliable from the internal design of the electronic control system to the layout of the external system, with the natural extension of the use period of the car, the components of the electronic control system under long-term harsh working conditions will inevitably appear Component aging and performance degradation. The wiring harness that is the nerve of the automotive electronic system naturally also has problems such as skin hardening, loose connectors, and adhesion between wires. Most of these wiring harnesses play the role of power circuits of various electronic control systems (a small part are signal lines). Therefore, after we have obtained the fault information through the tester, do n’t be busy with detecting 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 provides power for these components. Only after confirming that it is intact can further diagnose the component body.
1. Detection of energy supply lines
The detection of energy supply line is mainly to measure the working voltage value supplied to the electric control system. According to the circuit diagram, find the corresponding terminal of the power supply harness connector end of the component to be tested, and use a voltmeter to measure the voltage to the negative electrode of the battery (this can ensure the accuracy of measuring the voltage at this point and prevent the measurement error caused by the poor grounding point selection). ).
2. Detection of grounding line
The detection of the grounding line is mainly to measure the grounding terminal of the corresponding harness connector on the electrical control system, and measure the voltage between this point and the negative electrode of the battery when the power circuit is turned on. Voltages less than 0.5V are acceptable. In actual testing, many people, including myself, used to use the ohm or buzzer gear of the multimeter to judge the quality of the ground by detecting the line resistance between the ground terminal and the negative electrode of the battery. This method is not appropriate, because even a small 0.5Ω, once the current passed is greater than 5A, its voltage drop is above 2V. The consequence of this will be a drop in the operating voltage of the system, the reference voltage of some measurement circuits is pulled up or the reference low potentials of different subsystems in the same module are not uniform. Especially when using the diode buzzer gear, the resistance value above and below 10Ω can make the buzzer beep, and it is precisely that when we encounter this situation, we will think that the ground line is smooth.
Examples of fault detection of power supply circuits in electronic control systems
Example 1: Buick car engine has unstable idle speed and is not running smoothly.
A Shanghai Buick New Century sedan experienced intermittent engine idling instability, weak acceleration, and engine failure lights sometimes lit up. Enter the maintenance station for repair, and the maintenance worker uses the fault detector to call up the historical fault code: P0101 (the performance of the MAF sensor has deteriorated). Check the engine operating data under the current engine operating state. 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 without any signs of failure. The car entered the repair station again within a few days, and the contents of the repairs remained the same. At this time, the operation of the engine shakes severely. Recall the fault code and observe the engine operating data. It is found that the MAF frequency is 0 Hz, and the intake air flow is also 0 g / s. The repairman immediately determined that the air flow sensor (MAF) was damaged, and the past intermittent failures are reflecting the sensor's failure process. The sensor was replaced, the fault code was cleared, and the engine was restarted. The fault disappeared immediately. However, the car's failure occurred again during the test run in the station. In desperation, the repairman judged that the engine control computer may be faulty and borrowed the computer test of the same car, but the fault still exists.
After understanding the above repair process and retrieving the relevant fault information, the author found that the fault is still related to the MAF sensor. Imagine whether the power supply to the sensor is normal, so I found the corresponding sensor power supply and ground breaker of the three-pin plug according to the circuit diagram. After turning on the ignition switch, the power supply voltage and ground voltage were measured. The power supply voltage was only 4.98V, and the ground voltage was normal , Which is 0.02V. Because the power circuit of the MAF is a 12V circuit that comes through the ignition switch, it is clear that the power supply failure is the real source of the failure. When the fuse was finally found, it was found that the fuse was in the state of fuse connection of the broken wire, which caused a voltage of nearly 7V to be pressed down at both ends, resulting in the actual supply voltage of MAF being only about 5V. After replacing the 10A fuse, the failure disappear.
Example 2: Buick car ABS does not work, ABS light is always on.
A Buick sedan suddenly turned on while driving, and as soon as the ignition was turned on, the ABS light immediately turned on and did not go out. The fault information retrieved using the special TECH2 diagnostic instrument is: No data communication information (Note: All control computers of Buick cars such as power control module, ABS control module, SDM air bag module, etc. are connected in a star network through the data line). Generally, the failure to lose communication information is mostly caused by the open circuit of the data line, and no failure information about other components of the ABS system can be called up. According to the circuit diagram, I found the ABS communication data line connected to the standard OBD-Ⅱ detection interface, but after inspection, the data line was not broken and the connection was in good condition. In the case of a deadlock in the diagnosis work, the decision is to start with checking the power circuit of the ABS system. The power supply circuit of this ABS system is composed of 2 energy supply lines and 3 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 to measure, the power supply line is normal; because the plug is difficult to start, measuring the ground voltage is replaced by a simple continuity test. The multimeter selects the buzzer file, and the measurement result: all ground wires are connected to the negative electrode of the battery. After checking that the grounding connection points of the ABS system on the car body are very solid, it is sloppy that the power system is not faulty. However, after a series of maintenance work and even replacement of the ABS control module and hydraulic control component assembly, the ABS fault lamp remained lit immediately after the ignition switch was turned on. With a sense of giving it a try, the power supply system was measured again, and the resistance of the ground line was measured using an ohm range. Among them, the resistance from one ground terminal to the negative electrode of the battery reached 24Ω. To diagnose the fault, find the grounding point of the harness connector's grounding end to the body, and use a jumper (one end is directly connected to the negative pole of the battery, and the other end is connected to the end of the body grounding point) to connect the grounding loop. Reinsert the harness connector, turn on the ignition switch, and the ABS lamp goes out immediately after a short light. The source of the fault was finally found, because the 24Ω contact resistance between the washer of the grounding wire fixing screw on the body grounding point and the body paint layer caused the ABS system to work abnormally. After sanding the paint layer with sandpaper, re-tighten the ground screw to eliminate the fault.
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