Several misunderstandings used in car diagnostics

Automotive diagnostic connector

With the rapid development of automotive technology, more and more related electronic control systems are used in automobiles, such as engine electronic control systems. , Automatic transmission, SRS airbag, electronic suspension and cruise control system, etc. At the same time, the role of car diagnostics in diagnosing car faults has become increasingly important. However, many maintenance personnel have a lot of misunderstandings when using car diagnostic procedures, which greatly affects the effectiveness of the diagnostic procedures. Generally speaking, the misunderstanding has the following aspects.

1, some maintenance personnel only think that the fault code represents the actual fault.

The development and application of an automatic fault self-diagnosis system provides convenience for timely fault detection and repair. By interpreting the fault code, maintenance personnel can usually determine the possible cause and location of the fault. Fault codes are divided into read fault codes and clear fault codes. It is the core function of the car diagnostic instrument and one of the important selling points of the diagnostic instrument. The diagnostic instrument queries the fault code by sending the read fault code command to the ECU on the bus, and then converts the data returned by the ECU to display specific car fault information. The entire process is usually called decoding. However, when repairing a car, if you only rely on the fault code to find the fault, errors often occur in the judgment. In fact, the fault code is just the definition of “yes” or “no” recognized by the electronically controlled automotive computer (ECU), not necessarily the actual fault location of the car. When repairing electronically controlled vehicles, comprehensive analysis and judgment should be made in conjunction with the fault phenomenon to find out the fault location. For example, to make an electronically controlled engine run normally, the first thing to emphasize is that the mechanical components of the engine itself and electrical appliances and circuits not related to the electronic control system must be kept in good condition, otherwise, it is futile to check the electronic control system anyway.

2, when a fault code occurs, the maintenance personnel will not judge the signal.

The sensors of the control system cannot work properly and will be displayed in the form of digital codes. As long as the fault code appears in the maintenance test, first check whether the sensor signal displayed by the code is normal. Compare the detected value with the standard value, analyze the possible causes, and then repair until the code disappears, and then perform other repairs. Maintenance personnel should try to avoid blindly judging maintenance to improve maintenance efficiency.

3, when there is no fault code output from the testing equipment, I do n’t know where to start.

When a sensor signal is detected, the electronic control unit can only accept the sensor abnormal signal within its setting range to judge the quality of the sensor and decide whether to record the sensor. The fault code, and the sensor’s sensitivity drop due to some reason, no response and output characteristic deviation, the self-diagnosis system can not detect, of course, there is no fault code output, but the engine will display obvious fault symptoms, such as common black smoke Unstable idle speed, poor acceleration, etc. Because there is no fault code, some maintenance personnel will not know where to start or how to deal with it. During maintenance, this type of failure is difficult to judge. Under certain conditions, the fault indicator will flash. At this time, analysis and judgment should be based on the fault symptoms of the engine, and then use other diagnostic tools, such as oscilloscope, engine analyzer and oil pressure gauge. Wait for sensor target detection to find and eliminate sensor failure. For example, when the engine speed is misaligned and the unstable idling is not accompanied by the output of the fault code during driving, the air flow sensor and the intake pressure sensor have to be considered and suspected first. The performance of these two sensors directly affects the basic fuel injection quantity of the engine controlled by the ECU. Although the corresponding fault codes are not displayed at this time, they should also be checked. As another example, when the crack of the airfoil air flow meter casing leaks, it will cause the air flow meter to measure incorrectly, resulting in engine failure, but the ECU’s self-diagnosis system cannot detect this failure. .

4, the maintenance personnel cannot correctly judge the wrong fault code.

Due to the similarity of certain fault phenomena, the ECU monitors the error, so the self-diagnostic system may display the wrong fault code. For example, for an electronically controlled car equipped with a three-way catalytic converter, once leaded gasoline is used, during the maintenance process, the fault code usually shows “water temperature sensor open and short circuit fault”, but the engine’s symptom is that the engine is cold or hot Unable to start smoothly under conditions, accompanied by unstable idle speed and “tempering”. The engine speed cannot always be increased. Obviously, the relationship between these fault symptoms and the water temperature sensor is not very close, and a measurement of the water temperature sensor will not find any fault. However, when the three-way catalytic converter is removed from the car and cut open, it will be found that the interior of the three-way catalytic converter is severely blocked, and it can be concluded that the car has started. Machine failure is caused by this. Therefore, when the fault code appears in the self-diagnosis system, it should be compared with the actual fault symptoms of the engine, make a correct and reasonable judgment, and then perform maintenance.

5, maintenance personnel did not operate according to regulations, resulting in inaccurate diagnostic data.

In actual work, the mentality of some maintenance personnel is “the decoder is easy to use and does not need to operate according to regulations”. Therefore, when repairing EFI vehicles, the operation method is too arbitrary, and as a result the diagnostic system outputs an error code. For example, some maintenance personnel will unplug the sensor at will during the operation of the engine, and each time the sensor plug is unplugged, the self-diagnosis system will record the fault code. In addition, if the old fault code cannot be completely cleared due to improper operation during the last maintenance, the ECU will save the old fault code in it, which will mislead the fault judgment.

Maintenance personnel should avoid the misunderstandings outlined above as much as possible, and make full use of car diagnostic procedures to improve work efficiency.