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


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.


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.

  • It's strange by the way, by the look of it i'd say it is a hot wire air mass meter, not a hot film. There's a copper plate what could be the film, but two wires run across it, one having the look of a resistor, the other is a blueish one which i'd think is the hot wire. It would be better if actually is a hot film, because those are less prone to get dirty and give poor performance. – Bart Nov 1 '16 at 9:00
  • I cleaned up my comments. Good job on finding that! How sure are you that "no air blew past it"? The air around us is constantly moving. Maybe at 5V it had its sensitivity "turned down" too much. Maybe at 12V it was detecting the faint breezes that humans can't feel. A test that I (who knows nothing) would try would be to wrap it in Saran wrap and see if it still bounces around at 12V. – Zach Mierzejewski Nov 4 '16 at 11:14
  • @ZachMierzejewski Thanks but I didn't put much effort in it, I just stumbled upon it while googling. Well, i mean i didn't blow through the maf sensor. There may be some environmental air movement, but that is nothing on the scale of air displacement by an engine. So the signal also shouldn't change noticably i suppose. I know the reaction isn't necessarily linear, but environmental air movement is out of range of the sensor i suppose. Your idea to wrap it completely rules that possibilty out, so i'll try that to be sure. – Bart Nov 4 '16 at 11:26

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)
  • Thanks. If you say these measurements are acceptable, i will take that as an answer. An error of 2% in the range of 5V would still be acceptable. The point is that if i'll only ever use 1.5V of the signal(in WOT situation) due too little air flow, the error becomes significantly bigger, and the MAF would be unusable at idling. Another thing is that the signal voltage gets higher, when the supply voltage is lower. That surprises me. Furhtermore, every MAF sensor graph i have seen now starts at 1V at still air, including the one you posted. Mine starts at ~0.15V. Weird. Thanks anyway. – Bart Nov 4 '16 at 12:08
  • @Bart that's why it's important to select the right sized MAF. Too large a MAF and your measurement resolution suffers, too small and your MAF would be inappropriate for WOT. – Zaid Nov 4 '16 at 16:30
  • That's right. I selected this MAF sensor, because the cars that have it, have an engine that is comparable to mine regarding displacement. I don't know how much of the signal that cars use though. – Bart Nov 6 '16 at 11:40

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