Re: 4wd range selector / vss in 747 application

["Programmer" <nwester@eidnet.org>]

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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