4wd range selector / vss in 747 application

[Shannen Durphey]

See Chevrolet bulletin 88-180-6E.  Also, I know it can happen on
manual trans vehicle.  It was pretty easy to set the code on the
hoist, by running the engine at higher rpm while the rear wheels were
spinning, but the trans was not in gear.  The bulletin talks about new
calibrations to prevent it, prolly an issue with 88, 89 trucks.
Shannen

Programmer wrote:
> 
> Shannon,
> I don't recall ever seeing a VSS code unless the PRNDL switch input showed
> P/N. Otherwise, no codes "should " set, even in low range...
> Lyndon.
> -----Original Message-----
> From: Shannen Durphey <shannen at grolen.com>
> To: gmecm at diy-efi.org <gmecm at diy-efi.org>
> Date: August 16, 2000 5:03 AM
> Subject: Re: 4wd range selector / vss in 747 application
> 
> >Niinikoski, Juha wrote:
> >>
> >> I don't have any 4wd truck wiring diagrams so can anybody help me on
> this. I
> >> don't know if a switch is needed to inform 747 ecm if the transfer case
> is
> >> in low range. If it is what is the input pin for this information ? Are
> >> there other tricks how ecm handles lo range ? In my conversion easiest
> place
> >> to build vss is inside speedo. This way it is always going to represent
> >> absolute vehicle speed.
> >There is no low range input to the 7747.
> >ftp://diy-efi.org/incoming/747auto_pinout.zip
> >That is where the VSS is usually placed.  4X4 pickups have a VSS in
> >the transfer case.  There are no tricks, except that the ECM can set a
> >VSS code if low range is used.  If the engine rpm stays above a
> >certain level, Vacuum is high, and vss signal is below a certain
> >speed, a code 24 will be set.
> >>
> >> What I have understood so far is that vss input is important for ecm to
> >> handle idle adjustments and prevent stall on decelerating conditions.
> What
> >> is not clear for me how ecm handles same situations in lo range. Going
> >> downhill you may have very low speed but same engine conditions than in
> road
> >> speeds.
> >Low range is handled by the same maps as high range.
> >Shannen
> >>
> >> Thanks Juha
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