RFD Keeper Thread, Anyone?

[Garfield Willis]

Once upon a long time ago, I asked the following:

>Would anyone onlist be willing to take a stab at explaining how
>the analog stuff on the memcal is used in limp mode, electrically
>speakin? The only thing I've heard so far is the resistive network
>sets the cyl-count, but that's about it as far as my understanding
>goes.  Just exactly what else does the memcal provide to the ecm in
>limp mode, and how does the ecm use it. In short, say more please!

After some catching up with his email, Walt sent me this offlist:

On Fri, 15 Sep 2000 18:31:25 -0700, "Walter Sherwin"
<wsherwin at home.com> wrote:

>... Within the MEMCAL
>there will typically be two resistive network chips located under the
>clear-gooy-coated  SNEF structure.  Two of those resistors (usually R28 and
>R29) will hardware mode the PCM/ECM into a certain injector firing scheme
>based upon cylinder count and intended firing schedule, and will usually be
>isolated from the other resistors in the network.  The other integrated
>resistors participate in an analog network, to communally shape a
>rudimentary fuel curve based upon CTS, RPM and TPS (or MAP).  I've read
>through the documentation on same, and don't see a clear path as to how to
>post-calibrate the analog feature (refered to by GM as the Redundant Fuel
>Device (RFD)).  Also, the OE RFD cal tended to be shared across many
>platforms, and was NOT very accurate when called upon (kinda like it was an
>afterthought).

Walt mentioned this has been discussed briefly before onlist, but
didn't ever get fleshed out fully. Can we try jointly to fill in the
missing pieces? Surely there must be some onlist that know how these
res networks are used by the ECM.

My first question would be, what are the inputs feeding these analog
cal res networks? I expect that the portion that dictates
#ofCyls/firingMode is likely just a voltage that has a few select
values for 4, 6, or 8 cyl memcals; I kinda remember this part being
discussed before. But what about the "rudimentary analog fuel curve";
anyone have a schematic that shows what if any sensors feed this
network, as well as how the ECM reads this network, or does it even
read it directly with an A/D port.

If it doesn't happen naturely, I'll be happy to summarize and compile
as we go. Hey, maybe it's really dirt-simple, and I'm the only one in
the dark on this. If so, please enlighten me by telling all you know.
Enquirer minds wanna know. :)

Gar


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