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I've just bought this MAF sensor for the injection setup I'm going to build, and first I was struggling to figure out how to connect it. It's used for several BMW cars.(E38,E39,Z3) On the connector it says "3 2 1" and the letters Sn and Ag. No idea why that is printed there.

MAF Picture

Pins

I first thought i found out how to connect it. From left to right I connected respectively 12V, ground, and signal. That was wrong. I got a static 'signal' on the ground pin of 2.7V which didn't change when air blew past it.

I later found the right pinout here. That pinout is different; 12V, signal, and ground from left to right. So i formerly had the MAF hooked up with signal pulled to ground, while measuring the ground pin.

Diagram

When hooked up right and 5V applied to it, it gave a steady 680mV signal while no air blew past it, and 70mA of supply current was consumed. When 12V applied to it, it gave a varying signal between 90-200mV, while no air blew past it, and 80mA of supply current was consumed. In both cases, signal voltage increased fairly when I blew air past it.

I'm in doubt about the varying signal at 12V. It may have gone bad because I wired it wrong first, explaining the varying signal when I increased supply to 12V. Is it normal a MAF sensor's signal isn't that steady when there is no air flow, or does this MAF sensor have a supply voltage of 5V? That would be uncommon I believe, but the signal seems steady and in the right range at that voltage.

I can't really test it, because it's a individual sensor that I bought. I don't have the engine that I can use to blow sufficient air through it to test steadiness during airflow. Thanks for your help guys.

Zaid
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Bart
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1 Answers1

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The MAF should be fine

Have a look at this datasheet for the HFM5 series Bosch MAF. It is for a 5-pin MAF (it also measures intake air temperatures) so it's not exactly what you're after, but remains relevant because the fundamental MAF design is more than likely identical:

  • Nominal supply voltage is 14 V
  • Supply-voltage range is 8 - 17 V
  • Output voltage range is 0 - 5 V

Based on the above:

  • Any measurements with 5 V at supply is meaningless
  • The still-air measurements fluctuating between 0.09 V and 0.2 V is not unusual because this is the low end of the usable range (0.1 V fluctuation out of a 5 V range is ≈ 2%, quite possible this would be part of measurement uncertainty)
Zaid
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