4wd range selector / vss in 747 application

[Programmer]

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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