speed signal and DRAC

[Shannen Durphey]

> >>
> >> Hi I'm hoping some one can give me some info.I'm building a pulse generator
> >> with a 555 to make aake VSS signal. I order to feed this signal to the ECM
> >> do I have to go thru the DRAC (its a 7749 Syclone computer) or can I
> feed it
> >> directly to the ECM?
> >>
> >> Thanks
> >>
> >> Nolan
> >

The P4 paper (on the FTP site) describes two different types of speed
signal input to that ECM.  You need to know whether the original app
feeds the speed signal directly to the pcm, or to the DRAC first.  By
using Promgrammer and looking at a Typhoon calibration, it looks like
the Ty VSS isn't wired directly to the ECM.  My guess is DRAC output
is connected to the optical sensor input of the computer, DRAC output
being 2002 pulses per mile. Promgrammer is shareware, you can look
through your Syclone calibration for KAFOPT3 to see if the magnetic
speed sensor is enabled.  Best bet is to get a copy of the wiring
diagram for the Syclone.  Oh yeah, the DRAC is an A/D converter as
well as a frequency converter.
Shannen

> That is what a DRAC is... <g> Some GM vehicles use the DRAC to feed just
> the speedometer, and the TCM/ECM is fed from another sender. Other vehicles
> feed the output of the DRAC to the speedometer and the TCM/ECM. You need to
> know which configuration your unit uses.
> 
> Q: why not use RPM (Engine) to simulate the speed?
> 
> At 10:07 AM 6/23/99 -0500, you wrote:
> >Search for drac in the diy_efi archives ...
> >
> >I did and what I read leads me to believe that the DRAC is a frequency
> >converter that is used to drive the speedometer.  Do you have the
> >schematic?  If so just do what they did.  Just give the ECM the
> >frequency it expects, and be aware that some count pulses and some count
> >zero crossings.  YMMV
> >
> >--steve
> >