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Air Mass Sensor 2



If you want to measure at the air mass sensor, it is quite simple. There is one condition you may try, namely if you avoid the continuity test and the resistance range. Then you can try two of each of the four or five terminals with the multimeter or oscilloscope. So if you receive a signal that swells up to a maximum of 4.4 volts when you step on the accelerator, then you've come to the right place.

+12 V (Pin 15)
+5 V (Engine Control)
Mass for both
Sensor Signal
Temperature (option)

If you cannot find such a signal, you must proceed more systematically, namely first check the connection with the vehicle electrical system, e.g. between pin 2 and ground or between pin 2 and pin 3. Then the air mass sensor must still have 5 volts from the engine control unit, e.g. between pin 4 and pin 3. If both are present, the air mass sensor must be replaced.

The images coming from an intact sensor have been read from the oscilloscope data. This then has the effect of a filmic representation. A prerequisite is that the tester continuously collects data and saves them in a form that the software can understand. The tests were carried out at BK Simmerath.

The possibility of comparison with other data is pointed out at several places in kfz-tech.de. In most cases, one measurement with a multimeter is sufficient for the workshop. Here for educational purposes once clearly shows how much greater the possibilities for data analysis are when recording with an oscilloscope.

Figure 1 shows the moment when the ignition is switched on. The achieved voltage value can be read exactly in the second magnification. Picture 2 shows the moment when the engine is started. Now you could also read the corresponding speed from the scope image.

The first rotation of the motor by the starter takes between 250 and 300 milliseconds. As this is a four-cylinder engine, you have to take the time during two deflections, because then two cylinders suck in air (= 1 revolution). Times 1000 of milliseconds after seconds and times 60 to minutes result in 150 to 180/min starting speed right at the start.

Figure 3 shows the starting of the motor, recognizable by the increasing frequency of oscillation, not so clearly by the air mass. The idle speed could now be determined in the same way. Figure 4 is exciting because it is a diesel engine whose VTG turbocharger is now starting. The voltage rises slightly more than the frequency.

We cannot yet explain everything conclusively, e.g. the vibrations of the intake-air column. Well known is that of the petrol engine with reversal of motion at the valves and the throttle valve. But with the diesel? Perhaps backflowing air meets incoming air. In picture 6 the partial oscillations can be observed even better.


And in figure 7 we go back to normal idle without loader involvement. Below you can still see the signal of a digital air mass meter. Here only the frequency is changed by different speed or flow. So you see only the result, namely the pressure increase. When the charger starts and whether there are vibrations in the intake manifold, nothing can be seen.









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