Air Flow Meter (AFM) vs. Mass Air Flow (MAF) Sensor

General E28 FAQs.
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Shawn D.
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Air Flow Meter (AFM) vs. Mass Air Flow (MAF) Sensor

Post by Shawn D. »

Problem: Not knowing the difference between an Air Flow Meter (AFM) and a Mass Air Flow (MAF) sensor.

Cause: Lack of knowledge/understanding.

How to fix: Read the explanation below.

The air metering device on E28s is an AFM, not a MAF -- no E28s came stock with a MAF. While the objective is to end up with a measurement of the total amount (by mass) of air that is going in, the function and method of signal interpretation is entirely different.

An AFM is not a "mass flow device." Air flowing through the AFM causes a spring-loaded "flapper" door to deflect an amount proportional to the dynamic pressure of the flow. The AFM sends both flapper door angle and temperature to the ECU in separate signals. The ECU interprets the flapper door angle as dynamic pressure and combines this with the temperature reading to determine the mass flow.

A MAF is a "mass flow device." The ECU interprets the cooling of a heated wire element in the MAF to determine the mass flow. The lower the temperature, the greater the density, and/or the greater the volume passing over the heated element, the greater the cooling. Again, no E28s came stock with a MAF.
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