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Digital Mass Air Flow Sensor 2

Allow me a comment before we go on: Modern digital measuring devices and multimeters don't actually fit together very well. Certainly, there are a few crutch-like solutions, but one can find the error much more effectively, e.g., by using an oscilloscope. Neither is a multimeter with a frequency display a comprehensive substitute.

Let's have a look e.g., at a Hot Film sensor Measuring device from the year 2002 with the name HFM6, or the newer HFM7/HFM8 devices from the Bosch company. These are capable of supplying the control device with a digital signal, with the latter there is also the option of an analogue signal.

We however, will remain on the track of the digital signal. Thus, if the air-mass and the air-temperature data are transferred in this manner, it can occur with increasing-, dropping or with pulse width modulation. Indeed, you mustn't necessarily assign this to each manufacturer. With an oscilloscope you can see that already.

Should you, despite having given gas or having differing air-temperatures, not be able to detect a change in the signal, that means you're very close to finding the fault. It's astonishing, that no LIN- or even CAN-Bus picks it up, although, perhaps the signal only interests the engine management.

There are nearly always four outlets for the current supply, the two digital cables and the earth. There may be another one for the analogue transfer of the air-temperature or one which serves as a frequency generator. In this case, the synchronisation through the control device is easier. This can however, also be triggered by the temperature signal.

If your suspicion points to the mass air flow meter, then, the current supply can read off from the oscilloscope. Should this not be reached, there may be a short-circuit in the mass air flow meter. That means, disconnect and try again. Should it be OK, you will indeed then need a multimeter to check the current. You already know about the rest of the checking from our previous pages. 04/15

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