Re: 4wd range selector / vss in 747 application

[Shannen Durphey <shannen@grolen.com>]

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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