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The crankshaft position sensor, also known as the engine speed sensor, is one of the most important sensors in an engine’s control system. It detects signals related to the piston’s top dead center and the crankshaft’s rotation angle, and sends these signals to the engine ECU. These signals are used to control the timing of ignition and fuel injection. Additionally, it serves as a source of signals for measuring the engine’s speed. The engine control unit determines the position of the crankshaft based on the signals provided by the crankshaft position sensor, ensuring that the fuel injection timing and ignition timing are accurate. At the same time, the 1° signal from the crankshaft position sensor can also provide the engine speed signal, and the engine control unit determines the basic fuel injection amount based on the air flow meter signal and the engine speed signal. The crankshaft position sensor is usually installed at the front or rear end of the crankshaft. Common crankshaft sensors include inductive and Hall effect types. When using an oscilloscope for measurement, connect a BNC-to-banana plug to channel 1; connect the black banana plug to a alligator clip for grounding, and connect the red banana plug to the probe tip in order to link it to the sensor’s signal terminal. The channel attenuation ratio is set to 1X, the vertical scaling is set to 2V/div, and the time base can be adjusted accordingly based on the engine speed. As shown in the figure below, this is the waveform of an inductive crankshaft position sensor. The frequency of this waveform should correspond to the engine’s rotational speed. The characteristics of this waveform are as follows: 1. A change in the duty cycle of the Hall effect sensor indicates that rotor blades of different widths are passing by the sensor. Anything other than this change between pulses indicates a fault. 2. Check the consistency of the waveform shape and examine the corners at the upper and lower edges of the waveform. Since the supply voltage to the sensor remains constant, the height (amplitude) of all wave peaks should be equal. In practical applications, some waveforms may have defects or slightly irregular shapes in their upper and lower parts; this might be normal. What’s important here is consistency. 3. If the waveform shows a straight line at 0V, it should be confirmed that the waveform detection device is properly connected to the sensor ； Verify that the relevant parts (crankshaft, camshaft, etc.) are all rotating ； Use an oscilloscope to check the power circuit of the sensor, as well as the power and ground circuits of the engine ECU ； Check the power supply voltage and sensor reference voltage. 4. If the waveform shows a straight line at the supply voltage, the integrity of the sensor’s grounding circuit should be checked ； Verify that the relevant parts (crankshaft, camshaft, etc.) are all rotating ； If the sensor has a proper power supply and grounding, and the waveform detection device shows a straight line at the voltage supplied to the sensor, it is likely that the sensor is damaged. 5. If pulse signals are present, the criteria for determining the amplitude, frequency, and shape from one pulse to another should be identified. The amplitude of the digital pulses must be high enough (usually equal to the voltage supplied to the sensor at startup), the time between two pulses remains constant (except for synchronized pulses), and their shape is repetitive and predictable. Effect of the top dead center signal on engine operation: 1. If there is no 1° signal or the signal is very weak, in such a case the engine control unit will be unable to determine the position of the crankshaft and cannot issue accurate ignition commands. If the engine control unit does not receive the 1° signal, the engine cannot start. 2. There is no cylinder detection signal or the signal is very weak; in such cases, the impact on the engine depends on the specific function of the cylinder detection signal. If the cylinder judgment signal is only an injection reference signal rather than an ignition reference signal, the engine can operate with no obvious fault symptoms. It has a certain impact on the engine’s economy and power. 3. The upper limit signal is absent or very weak; in such cases, it is also necessary to consider what role the upper limit signal actually plays. Generally speaking, the top dead center signal is used as the ignition reference signal. Therefore, when the top-end signal is absent or very weak, the engine will not start.