I'm having some mysterious engine problems, and I thought I'd traced it down to the potentiometer in the air flow meter. I followed the testing procedure in Bentley: measure the resistance across pins 1 and 2, and see if it varies as you move the vane (they didn't say what the resistance should be, just that it should change smoothly). The resistance didn't change at all, so I figured the meter was bad and ordered a rebuilt one. The resistance on that didn't change either. I sent it back, and ordered another from a different company - same thing.
I guess it's possible that I got two bum units in a row from two different sources, but it seems unlikely. Which makes me suspect that maybe I'm doing something wrong. If anybody who's reading has an M42, a multimeter, and a few spare minutes, can you see what results you get? You just need to unplug the cable from the AFM (the pins are numbered - if not, then 1 and 2 are the ones at the bottom) and unclip the top of the air cleaner box so you can reach inside the AFM and move the vane.
I found this thread, which describes the same problem, but doesn't really answer anything:
http://www.318ti.org/forum/showthread.php?t=11740