VSS and DRAC questions

[Shannen Durphey]

Mark S. Riley wrote:
> 
> 95 LT1 engine, harness, and PCM in a 88 5spd pickup truck. Studying the
> DRAC2.pdf on incoming along with what I have in the Alldata. The truck vss
> supplys a 40 pulse per driveshaft revolution to the DRAC which then supplys
> the PCM with a 4000 pulse per mile signal? 

I've messed around with these, though it's been a few years now.
2000 and 4000 ppm outputs are available, as well as a 128000 ppm signal used for
RWAL.  The ecm originally used the 2000 ppm output.  I believe you'll need to
use the cruise output for 4kppm.

> Am I all wet 
I dunno.  Seem to be on track about most things.


> or just wrong. 
You're right about this.

> The
> DRAC is a pulse amount number changer to the PCM. I guess what I'm wondering
> is if there is enough change available in the DRAC to make this combination
> work? Without the vss the idle is off and won't pass state inspection.

Sounds like there's two questions here.  Will the DRAC provide a valid speed
signal for the pcm, and will the drac provide a speed signal that will allow me
to pass emissions.  I think the answer to both is yes.  

> Many
> thanks for any observations on this. 
Mark
> 

Not all trucks use a DRAC.  If you have found one in your truck, you can use a
frequency counting DVOM or a scope to check the outputs and confirm them with
the drac2 doc.  As yet, no one has confirmed those pinouts.  They are a
combination of different service manuals and may not be completely accurate. 
Needless to say, if you check them please forward the results to the list. 
Also, watch voltage drop caused by connecting both the pcm and the cruise to the
same output.  You may end up switching over to LT1 style cruise control.  And
check that cruise output voltage is not significantly higher than the voltage
the pcm expects to see.  What is "significantly higher"?  I dunno.  But if there
is a difference and you smoke the pcm, you can let us know what not to do. <g>

If there is no DRAC on the truck, it's probably because the 88 model year used
an instrument cluster based divisor, accessible through the back of the
speedometer.  If the truck uses a DRAC but the divide ratio is set to 1, the
same reason may apply.  My memory's foggy here, so get pinouts for the 88 to see
where the cruise control signal originates. (Don't use the pinout that is
included with the diagnosis for code 24.  It's incorrect.  And make sure you get
1988 specific pinouts, they're different.)  Basically, if the speedo was correct
before the swap, you *should* be able to get what you need without too much
work.

gl.  
Shannen

----- End of forwarded message from owner-diy_efi at diy-efi.org -----
----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org