Re: 4wd range selector / vss in 747 application

[Shannen Durphey <shannen@grolen.com>]

Hey, now.  Things aren't considered "old" around here till ya get back
into the forties. : )
Shannen

Programmer wrote:
> 
> Yep--you're right. Affects 87 and 88 5.0 and 5.7L trucks...Thanks Shannon.
> Guess I
> haven't had a real "old" one in for a while...
> 
> Lyndon.
> -----Original Message-----
> From: Shannen Durphey <shannen@grolen.com>
> To: gmecm@diy-efi.org <gmecm@diy-efi.org>
> Date: August 17, 2000 7:28 PM
> Subject: Re: 4wd range selector / vss in 747 application
> 
> >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@grolen.com>
> >> To: gmecm@diy-efi.org <gmecm@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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