[Diy_efi] 1227747 VSS signal??

[Ludis Langens]

frederic wrote:
> 
> > I am installing a TBI system out of a 88 chev 1/2 ton into a 75 Nova.  I have a 4000 pulse/mile speed pulse generator installed on the speedo cable.  How do I get this thing to work with my computer?  I think the instrument cluster "cleaned up" the signal in the truck?  Will it work to just hook up the speed sensor wires directly to the ECM?  Any help would be greatly appricated...
> 
> I wasn't able to get my 4k per mile speed sensor to talk directly to the
> 749 ECM for whatever reason, so I put a simple op-amp between the two
> wired up for zero gain (using two 5.6K resistors).  Worked fine.  You
> might have to do the same.

The 747 for his TBI wants a 2000 ppm "optical" signal.  Electrically,
the ECM is expecting a "switch" to ground that opens and closes.  The
donor vehicle should have had a box that did the needed conversion.  It
might have labeled as a "4 OUT BUFFER".  BTW, the "box" does the 4000 to
2000 conversion too.

Your 749 can use two different types of VSS signal depending on a mode
bit in the EPROM.  One setting will accept the same signal as the 747. 
The opposite setting wants a 4000 ppm "magnetic" signal to be applied
across a pair of ECM pins.  The "magnetic" signal can be created via a
DC motor run backwards (aka generator) or a reluctor setup.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/
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