Application of the most flaming oscillograph techn

Application of Oscillographic technology in automobile fault detection and diagnosis (II)

III. maintenance examples

1. There is a Santana era Superman GSI car with engine model AJR. The fault phenomenon is that the engine idles unsteadily, when the accelerator is suddenly increased, the air intake backfires, the exhaust is shot, and the high-speed driving performance is poor. Read the fault code with the decoder because the engine Hall sensor has a short circuit/open circuit, which is the same after replacement. After unplugging the hall sensor, the engine can still run. Replace the spark plug, high-voltage wire and electronic fuel pump, and the fault is still not eliminated. Then read the fault code with the decoder, and the instrument displays no fault code. Users come to our company to seek technical support in a helpless situation

after listening to the user's basic introduction, use our company's special oscillographic diagnostic instrument for cars to detect the car sensors When detecting the waveform of the air flow meter sensor, the obvious fault waveform is displayed on the screen, as shown in Figure 1, so it can be judged that the air flow meter is faulty After replacing the air flow meter, the fault disappeared. Using an oscilloscope to detect the waveform of the sensor will often get twice the result with half the effort, and users are very satisfied with this

illustration 1

why can't the engine control unit monitor after the air flow meter is replaced? The air flow meter (map) of Shanghai Santana era Superman GSI car adopts the hot film air flow meter, which is a fourth generation product. Its working principle is that ECU keeps the hot film constant temperature by giving different currents to the hot film. When different flows of air flow through the hot film, they take away different heat. At this time, the current change becomes the measurement of air intake. In the hot film air flow meter, the thermal resistance heated by the current is placed in the intake channel, and the heating resistance maintains a constant temperature. Due to the cooling effect of the intake air flow, the thermal resistance has a downward trend under certain circumstances. In order to keep the temperature constant, the current flowing through the heating resistance changes with the inlet air flow and air temperature. The upper computer adopts the brand computer degree and density. Therefore, the change of the current can measure the amount of inlet air

when the air flow meter fails, especially when the resistance strain gauge inside the fault sensor is not obvious, the engine electronic control unit often fails to monitor the signal of the air flow meter. Of course, the self diagnosis system cannot store or release the fault information. On the contrary, the engine control unit will mistakenly change the fuel injection quantity and ignition advance angle, resulting in unstable idle speed of the engine or intake tempering and exhaust shooting during refueling In this case, only by using oscillograph to detect the waveform of relevant sensor elements, can we get rid of the disease at hand

2. A Jetta GT car is equipped with a 4-cylinder 20 valve AHP EFI engine The idle speed is unstable, there is black smoke when accelerating, and the car is stopped when speeding up

the characteristics of this fault phenomenon are relatively obvious. First, dismantle and inspect the spark plug and find that the electrode gap is too large and there is carbon deposition, which can determine that the mixture is too rich After replacing the spark plug, the ignition energy is very high. At this time, the idle speed of the engine is improved, but the fault phenomenon is still obvious when accelerating

connect the fault diagnosis instrument for detection, and it shows that there is a fault in the throttle body After clearing the fault code, restart the engine, but the fault code reappears After cleaning the throttle body with cleaning agent, set the basic settings of the engine electronic control unit Start the engine again, the acceleration condition is significantly improved, and the idle speed is still unstable During the road test, the phenomenon of sudden acceleration and stopping is still obvious Then use the scan tool to detect, and no fault code is displayed

next, check the fuel system. The pressure of the fuel system is 0.25MPa at idle speed. Unplug the vacuum pipe of the oil pressure regulator, and the oil pressure rise is normal when accelerating. After removing the fuel injector, it was found that there was a lot of carbon deposition at the front end, but no abnormality was found after ultrasonic cleaning and checking the sealing, fuel injection volume and atomization of the fuel injector on the test bench. Therefore, the possibility of fuel system failure can be basically ruled out

after eliminating the ignition and fuel system, how to further determine the location and cause of the fault is the most concerned problem of users Use the special oscillograph diagnostic instrument produced by our company to detect the sensor element. When the air flow meter is detected, a very obvious fault waveform appears, as shown in figure 2 After replacing the air flow meter, the fault disappeared, which solved the problem besetting users

illustration 2

3 The user of a 97 Toyota Camry sxv20l car reported that the engine speed sometimes rises suddenly when the car is running at low speed, and the engine is sluggish, easy to stall and difficult to restart after stalling. However, the exposure draft of the engine catalogue includes two parts: 1. The fault light of the national catalogue of industries encouraging foreign investment is not on

the fault characteristics of the vehicle should be related to the intake pipe system First, check whether each vacuum pipe has leakage and whether the idle speed control valve is stuck After careful inspection, it is confirmed that all vacuum pipelines are in good condition, and the idle speed control valve rotates flexibly except for some carbon deposits After cleaning the carbon deposit on the idle speed control valve, the fault phenomenon did not disappear Connect the scan tool to test the vehicle, but no fault code exists

it is in this case that our company asks for help Based on the above introduction, it is preliminarily confirmed that there is a sensor fault The waveform of the vehicle sensor is detected by the special oscillographic diagnostic instrument produced by our company When the waveform of the water temperature sensor is detected, an obvious fault waveform appears See illustration 3 After replacing the water temperature sensor, the fault disappeared

illustration 3

IV. conclusion

the application of Oscillographic technology in automobile maintenance industry can greatly improve the speed and accuracy of automobile fault diagnosis, especially when using scanning tools to read fault codes, the instrument displays no fault codes, but the characteristics of automobile faults are very obvious. Oscillographic technology adapts to the needs of the development of automotive technology and equipment and is applied to the automotive maintenance industry. In practical work application, it has been popularized and developed because of its accuracy and speed in determining the location and cause of the fault. The post crack oscillographic technology has become a tool that the automotive maintenance industry needs to popularize and master as soon as possible