AFM vs. MAF. 30 years and going strong?

[sugarwood]

I have meant to ask this question for some time.
Our 911 AFM never seems to be replaced. In fact, it seems like an NLA part.
Modern cars commonly need a new MAF after 10 years or 100k miles.

I assumed MAF and AFM were the same thing.

So, I found this online. Why don't AFMs ever need service? Is the AFM door flap mechanism is a mechanical, reliable potentiometer where a MAF has a delicate filament that is prone to failure?

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.

[DaveMcKenz]

They do occasionally need service. The moving door has a contact wiper that can wear out its curved contact path, leading to unreliable measurement of airflow. It can be addressed, but like all mechanical devices it does wear. OTOH it it an extremely reliable and durable analog device.
Dave

[ischmitz]

Not true, search around here. Lots of poor-running is traced back to the AFM. The wiper that forms the variable resistor with a track wears out the track it moves on. As a result the AFM signal is no longer continuous but has distinct jumps and that messes with the mixture.

Here is one example: [3.2 Motronic] Air Flow Meter wiper arm re-tracked - WOW!

There is a simple test with a 9V block battery and a volt meter to check the response of the AFM throughout its range. Again, the search function is your friend.

Ingo

[mysocal911]

Originally Posted by sugarwood

Modern cars commonly need a new MAF after 10 years or 100k miles.
Our 911 AFM never seems to be replaced. In fact, it seems like an NLA part.

I have meant to ask this question for some time.
I assumed MAF and AFM were the same thing.

So, I found this online. Why don't AFMs ever need service? Is the AFM door flap mechanism is a mechanical, reliable potentiometer where a MAF has a delicate filament that is prone to failure?

Good assumption, as long as they:

1. Don't get damaged during an engine backfire where the flapper is bent.
2. Are maintained with the carbon track and the flapper pivot points lubricated with
a light oil every 5+ years.

[KNS]

^^ Any DIYs on maintenance of the AFM, oiling, etc?

[sugarwood]

^^ Ingo's link above is a start.