Yes, that is it, the ECU knows by the hall pulses how fast the engine is
turning. When it sees the idle switch closed but rpms high it knows you
are coasting in gear and cuts off the injectors.

If you have one of the Ken Lewis O2 meters installed you can see this
happen in real time.

This is part of why a sticking or badly adjusted idle switch can make
the van herky-jerky to drive.

Mark

pdooley wrote:
> Hmmm, I may have an answer for my own question.
> The hall sender could detect RPM below or approaching idle speed and add
> fuel.
>
>
> -----Original Message-----
> From: pdooley [mailto:psdooley@verizon.net]
> Sent: Monday, August 17, 2009 4:45 PM
> To: 'Rocket J Squirrel'; 'vanagon@GERRY.VANAGON.COM'
> Subject: RE: Not quite a backfire . . .
>
> How does this work?
> How does the ECU know to shut fuel completely off when descending but yet
> could add fuel in a split second to prevent stalling if say, you press the
> clutch pedal in?
> The idle switch cannot discern the difference between idling and coasting in
> gear.
> I really don't see how the AFM could perform this function either.
> Somebody enlighten me.
>
>
> <With no pressure on the gas pedal, the fuel is totally shut off, so the
> engine is not firing on any cylinders.....
>