When the sensor is performing a test, it should be performed in the following seven steps.
1. Judging by signs
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
To prevent sensor misjudgments not caused by sensor malfunction itself, the suspected sensor part should first be externally inspected to check for short circuits, open circuits, dirt, disconnection, wiring problems, water damage, corrosion, oxidation, poor contact, sensor deformation, etc.
4. Check external voltage, grounding, and wiring harness continuity.
To prevent passive sensors from malfunctioning due to lack of power, the external power supply should be checked first. For example, a Hall effect crankshaft position sensor will not output a signal without a 12V or 5V power supply. If the power supply or grounding is abnormal, the problem must be traced back to the wiring.
5. Ontology Check
The main checks are visual inspection and resistance testing; no external circuitry is required. For sensors capable of resistance measurement, such as variable resistance sensors and magnetoelectric sensors, resistance can be measured directly. For example, to check the resistance of a wheel speed sensor, turn off the ignition switch, disconnect the sensor connector, and check the resistance at the terminals of the front and rear wheel speed sensors; it should be between 1.0 and 13 kΩ. Similarly, the resistance and stability of the resistance change of the throttle position sensor and magnetoelectric crankshaft position sensor can be directly measured using the resistance setting of a multimeter to determine if the sensor is functioning correctly.
6. Output signal check
Output signal checking primarily involves connecting the sensor to an external circuit that has already been checked and confirmed to be functioning correctly, or by further enhancing the sensor's operating conditions, to examine the sensor's output signal. Output signal checking represents a further step beyond resistance checking. This is because the computer receives the output signal, not the sensor's own resistance. Even if the sensor's resistance is normal, the output signal may not be.
Therefore, both active and passive sensors can have their output signals checked under simulated operating conditions. It should be noted that passive sensors require a proper power supply before their output signals can be tested. Output signal checks can be performed using the voltage or current settings of a multimeter; however, using an automotive multimeter only provides a basic assessment. For more precise output signal analysis, an oscilloscope is recommended.
01 Analog DC signal
For example, the throttle position sensor can be tested using a DC voltage range meter specifically designed for automobiles.
02 Analog AC signal
ABS wheel speed sensors and magnetoelectric crankshaft position sensors can be tested using an automotive-grade multimeter with AC voltage range, which meets the requirements.
03 Electronic signals of pulse width modulation/frequency modulation signals
While a multimeter can be used, the results are not accurate enough. To clearly observe the specific changes, an oscilloscope is necessary. For example, the Karman eddy current air flow sensor used in Mitsubishi automobiles outputs a signal of 2.2~3.2V at idle speed. This voltage is the average voltage of the frequency-modulated signal, but an oscilloscope can easily show whether the frequency and amplitude of the air flow sensor signal meet the specifications.
7. Repair and Replacement
After performing the above checks on the sensor, its condition can be basically determined. When replacing the sensor, strictly follow the operating procedures and avoid brute force. The ignition switch must be turned off, and no operation should be performed while the power is on, otherwise other electronic components may be damaged. Handle with care during installation.
After repairing or replacing the sensor, remember to use a decoder to clear the fault code and test drive the vehicle again to simulate the fault situation. If the fault phenomenon does not reappear during the test drive and the fault code does not reappear, it means that the diagnosis is accurate, the installation is correct, and the sensor repair operation is complete.
