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Diagnosis of difficult faults by waveform method. (Dieselpartsworld)        2016-01-17   

Waveform method is a basic method to eliminate the fault of electronic controlled engine.
Engine failure, sometimes intermittent failure, sometimes without, it is difficult to use data flow analysis and judgment. At the same time in the electronic control system, a lot of sensors and actuators of the signal using voltage, frequency or other forms of digital representation.
In the actual operation of the engine, because the signal changes very quickly, it is difficult to find the problem from the constantly changing figures. However, the waveform can be used to capture the waveform of the fault in the small, intermittent changes. When the normal work of the electronic controlled engine, all sensor signals (including the crankshaft position sensor, cam shaft position sensor, oxygen sensor signal and certain types of air flowmeter signal, injection signal, idle speed motor control signals) described the waveform and fault waveform compared, if there are anomalies, said of the signal control circuit or component itself is a problem.
Waveform analysis can show the need to repair the fault is a kind of what waveform, so that you can see clearly the true existence of the fault, through the analysis you can know whether the real fault.
Waveform analysis in automotive electronic control system fault diagnosis and maintenance, the main application methods have two aspects: one is to determine the operation of the whole system; the second is identified in the whole state of the normal operation of the case, an electrical circuit or whether there is a fault. The most effective and the most effective method of waveform analysis is the analysis of the signal waveform of the oxygen sensor. It can be diagnosed by the oxygen sensor waveform analysis, vacuum leakage, poor ignition, fuel injection imbalance, etc..
For example, a car Shanghai Santana 2000 car, driving more than 80000 kilometers experienced engine idling instability, inability to speed up, and sometimes the tempering phenomenon.
Based on the principle of simple to complex, the ignition coil, high voltage wire, spark plug, electrical appliances were examined at the same time cleaning the throttle, intake manifold and injector, did not rule out failure. According to the failure phenomenon, the ignition system is checked with the emphasis of waveform analysis, especially on the ignition timing.
F98 oscilloscope to test the ignition system, waveform display without exception, ignition system, normal, with car king decoder checkup, the fault code 0561. Its meaning is mixed gas mismatch, which may is the cause of the trouble.
Detection of gas mixture by Santana to the oxygen sensor. The oxygen sensor signal was detected by F98, and the signal voltage was found to have more than 600mV, sometimes down to 100mV, and there was no regular pattern in a certain range. The information of the oxygen sensor detection is the mixture of the mixture. The causes of excessive mixture of gas are: the oil pressure is too high, the fuel injection pulse width is too long, and the combustion is not complete. Detection of oil pressure, idle speed for the 0.25MPa, when the acceleration is 0.29MPa, the oil pressure is normal. Using F98 oscilloscope to detect the first cylinder fuel injection pulse width of 4.3ms, and sometimes for 1.73ms, it appears that the fuel injection pulse width is not normal.
Detection of two or three cylinder also has a similar situation. Check the four cylinder, pulse width for 3.3ms, sometimes reaching 12ms. From the above measured data that the car fuel injection completely out of control, and from the overall look injection quantity uncertainty but also lead to 0561 fault code is the main reason.
Fourth cylinder fuel injection caused by abnormal reasons are: intermittent wiring harness, computer ECU bad, bad computer wire. So carefully check the wiring harness and other details, are not found abnormal, that is, there is a problem with the computer ECU. Replaced the new ECU fault will be ruled out.


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