TBI to TPI

[David Sagers]

Shannen

Thanks for responding, I really appreciate your help as it sounds like you 
really know your stuff on this subject.  In fact, you are way over my head!  
Mind if I ask a few questions.

As you suggested, I looked at the archives a searched for DRAC, found about 
5 messages and the most useful is:

Formula to find divide ratio when using General Motors  DRAC speed sensor 
and changing gear ratios, tire sizes, and etc.

Input Ratio=63360/RC*XR*P/128000

63360= #of inches in a mile
RC= tire circumference in inches
XR= axle ratio
P= # of speed sensor output pulses per output shaft revolution.  The 700R4 
with VSS (not speedo cable) is 40 pulse/rev.

128,000 pulses per mile is DRAC output to RWAL module in fullsize pickup 
application.

Rolling circumference is preferred method of obtaining tire circumference.  
Mark line on tire and corresponding line on floor.  Roll vehicle back until 
tire makes complete revolution.  Mark
2nd line on floor and measure distance between 2 lines.  This is actual 
rolling tire circumference.

(Circumference=(Pi)(Diameter) for estimating purposes.)

Look up input ratio on chart to find proper jumper settings.

Notes:
DRAC is designed to use AC input signal.
73.8" rolling circumference and 3.73 axle ratio yeild 1.00 divide ratio.
747 ECM receives 1.11hz/mph signal

Sounds like I need to try to reprogram the signal to the ECM?  This might be 
more than I have enough experience to pull off.  So I'm facing two routes.

1 - Don't try to reporgram the DRAC signal to the ECM.  What happens if I if 
I just follow the instructions for the aftermarket harness?  Basically the 
instructions for the VSS say there are two wires for the VSS.  If you 
connect them correctly the speedo will work.  If the speedo doesn't work 
then the wires are connected backwards.  Will I lose the ABS function of the 
rear brakes?  Will I lose the ABS for the front brakes?  Any other features 
on the truck I'll lose?

2 - Dig into the DRAC and try to figure out the programming.  In order to 
change this do I need to use a chip programmer?  What I'm saying is that I 
read and re-read your message and can't quite figure out how to pull off the 
change.  Any help in explaining would be very much appreciated!!

>Ok, let's get that pained salesman straightened out.  The
>computer is looking for a 2k or 4k pulse per mile speed signal input.
>The VSS in a 90 suburban outputs 40 pulses per driveshaft revolution.
>Now without knowing your tire size, gear ratio, or driving habits I
>can definitely say that this is not compatible. However...
>Have you seen the term DRAC mentioned on the list?  Maybe in the
>archives? (hint, hint)  Well, the DRAC's job is to take that 40
>pulse/rev ac signal and turn it into 2k and 4k (and 128k) pulse/mile
>square
>wave output.
>Are things looking better?
>
>Now you'll need to locate one.  Well, if you remove your
>glovebox and reach into the dash toward the driver's side, you should
>find a small white box about 3" X 2 1/2" taped or velcroed in.  This
>is the beast you need.
>So what to do next?
>
>You'll need some wiring diagrams to connect it to the ecm.  The
>schematic for the 747 and the 727 are in "incoming", and there may be
>a 730 pinout there also.  Now that I've gotten around to it, you'll
>also find a file called dracnfo.zip which has unconfirmed drac
>pinouts, some pictures, and some instructions for using the DRAC with
>non-original configurations.  For anyone who may have a version of
>this that's about 2 Meg, this one's a "lite" version, with some small
>corrections in the conversion instructions.
>Shannen


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