Sick sensor - the detection procedure of the sensor, the precautions in the detection process

The detection procedure of the sensor

1. Symptom Judgment: Infer the location where the malfunction may occur.

2. Decoder detection: Confirm whether the suspected sensor has a fault code in the decoder, and strengthen the judgment in the data stream.

3. Inspection around the sensor: In order to prevent sensor misjudgment that is not caused by the failure of the sensor itself, it is necessary to first conduct an external inspection of the suspected sensor site to see if there is a short circuit, open circuit, dirt, disconnection, wiring, blisters, corrosion, oxidation, poor contact, sensor deformation, etc.

4. Check the external voltage, tie and wiring harness conduction: In order to prevent the active sensor from not working properly due to the lack of power supply, the external power supply should be checked first. For example, a Hall crankshaft position sensor would not have a signal output if it did not have a 12V or 5V voltage supply. If the power supply and tie-up are not normal, the wiring should be checked.

5. Body inspection: mainly visual inspection and resistance inspection, no need to connect external circuits. For sensors that can measure resistance, such as variable resistance sensors and magnetoelectric sensors, resistance measurement can be performed directly. For example, the wheel speed sensor resistance check can turn off the ignition switch, unplug the sensor connector, and check the wheel speed sensor terminal resistance of the front and rear wheels, which should be 1.0-13kΩ. Similarly, the resistance of throttle position sensors and magnetoelectric crankshaft position sensors and the smoothness of resistance changes can be directly measured with the electrical resistance of a multimeter to determine whether the sensor is normal.

6. Output signal check: The output signal check is mainly the process of connecting the sensor to the external line that has been checked to the normal line, or additionally improving the working condition of the sensor. The output signal check should be a step forward from the detection result than the resistance check. This is because the control unit receives the output signal, not the resistance of the sensor itself. The resistance of the sensor itself is normal, and the output signal is not necessarily normal.

As a result, both active and passive sensors can be used to check the output signal under simulated operating conditions. It should be noted that the passive sensor must be properly supplied with a working power supply before the sensor output signal can be detected. The output signal can be checked using the voltage block or current block of the multimeter, but the output signal can be simply judged by using a car special multimeter, and an oscilloscope can be used to judge the signal more accurately.

7. Overhaul and replacement: After the above inspection of the sensor, the quality of the sensor can be basically determined. When replacing the sensor, it is necessary to strictly follow the operating procedures and do not do it recklessly. The ignition switch should be turned off, and it should not be operated with electricity, otherwise it is easy to damage other electronic components. Handle with care when installing.

8. Clear the fault code: After overhauling and replacing the sensor, remember to clear the fault code with a decoder and re-test the operation to simulate the situation of the fault, if the fault phenomenon does not reappear during the test run, and the fault code does not reappear, it means that the judgment is accurate, the installation is correct, and the sensor maintenance operation is completed.

Precautions during the inspection process

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1. Unless otherwise specified during the test, the ECU and sensors cannot be tested with an analog multimeter, and a high-impedance digital multimeter or a special multimeter for vehicles should be used for testing. It is forbidden to use the "scribing method" to check the on/off status of the transistor circuit. Do not use ordinary test lamps to test any electrical devices connected to the ECU to prevent transistor damage, and the pulse circuit should be checked with LED lights or oscilloscopes.

2. When disassembling or installing the inductive sensor, the ignition switch should be unplugged (OFF) to prevent its self-induced electromotive force from damaging the ECU and generating new faults.

3. When arc welding is performed on the car body, the ECU should be unplugged first. Particular attention should be paid when body repair work is carried out close to the ECU or sensors.

4. ECUs and sensors must be protected from moisture. It is not allowed to damage the sealing device of the microcomputer or sensor, and it is not allowed to rinse it with water. The ECU must be protected from severe vibrations.

5. In the electronic control system, it is not the ECU, sensors and actuators that are most faulty, but the connectors. Connectors often have poor contact or transient short circuits due to looseness, desoldering, ablation, rust and dirt, so when a fault occurs, do not easily replace the electronics, but check the condition of the connector first.

6. When unplugging the battery, pay attention to:

a. The ignition switch must be turned off, if the ignition switch is turned on when the battery connection is unplugged, the self-induced electromotive force in the circuit may break down the electronic components;

b. Check whether the self-diagnosis fault code exists, if there is a fault code, the code should be written down and then unplugged the battery;

c. Before unplugging the battery, you should remember the code of the audio equipment with an anti-theft code, otherwise in the next use, the self-locking of the audio system will affect the use.

7. The polarity of the battery must not be wrong, and the negative pole must be taken to the iron. It is strictly forbidden to unplug the battery from the circuit when the engine is rotating at high speed, so as to prevent instantaneous overvoltage from causing damage to the ECU and sensor.

8. When jumping to start other vehicles or using other vehicles to connect to this vehicle, you need to turn off the ignition switch before disassembling and assembling the jumper cable.

9. Do not unplug any electrical equipment when the ignition switch is on, so as to avoid damage to the electronic components caused by the induced electromotive force generated in the circuit.

10. The ECU has a learning function, but once the power circuit of the ECU is cut off (such as removing the battery), the data stored in it during engine operation will disappear, so the battery should be installed after unplugging. If the engine is not working as well as before, do not replace the parts at random, as this may be due to the learning correction amnestics in the ECU after the battery is removed. This is because the ECU learns to correct the system according to the actual situation of the system and controls it according to the data stored in the manufacturer's read-only memory (ROM). If this is the case, the ECU automatically builds up a correction memory after the engine has been running for a certain period of time. If you want the ECU to completely "regain its memory", you need to make the ECU relearn through road tests under different working conditions, and the undesirable conditions of engine operation will automatically disappear.

11. Pay attention to check the condition of the tie wire, and its resistance value should not be greater than 1.5Ω.

12. When the airbag system is overhauled, if the airbag is not operated properly, the airbag will be opened unexpectedly, so it must be carried out in strict accordance with the operating procedures, and when the airbag is overhauled, the ignition switch should be placed in the closed position first, and the negative pole of the battery should be unplugged first, and then operated after 90s to avoid accidents.

13. When overhauling the oxygen sensor, be careful not to let the oxygen sensor fall and collide with other objects, and do not cool it with water. When replacing the oxygen sensor, be sure to brush the threads with a special anti-adhesive solution to avoid difficulties in disassembling next time.

14. The power of some fault alarm lights shall not be changed at will, otherwise there will be abnormal conditions.

15. Pay attention to the shielded wire. It is not comprehensive to determine whether an electromagnetic camshaft position sensor is good or bad by measuring voltage or resistance alone. There are many electromagnetic sensors that measure the resistance and voltage normally, but poor line shielding can also lead to failure.

When the ECU judges that a circuit is faulty, it only provides the nature and scope of the fault, and finally determines that it is the fault of the sensor, actuator or corresponding wiring, and further inspection of the wiring, plug, ECU and related components is required to accurately find the cause of the fault.

When the various sensors of the engine electronic control system are working normally, the signal voltage of the input ECU changes within a certain range. When a signal exceeds the specified range in a sensor circuit, the ECU determines that the signal of the circuit is faulty.

If the ECU does not receive input signals from a sensor for a period of time, the ECU also determines that a fault has occurred. When the engine is working, if there are some abnormal signals occasionally, the ECU does not judge it as a fault. A fault is only judged when an abnormal signal persists for a certain period of time or occurs multiple times.

Sick sensor - the detection procedure of the sensor, the precautions in the detection process

Sick sensor - the detection procedure of the sensor, the precautions in the detection process

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