GMECM Digest V1 #160 (VSS / DRAC)

[Shannen Durphey]

Jim Sloan wrote:
> 
> > 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?
> 
> According the the Syclone Service Manual, " The DRAC processes inputs from
> the VSS and output signal to the speedometer, ECM, and cruise control
> module. The DRAC takes the voltage pulses from the VSS and uses them to open
> and close four solid state output switches to ground at a rate proprotional
> to vehicle speed."
> 
> This makes it sound like you need the DRAC. Of course, the service manual
> has been wrong before.
> 
> Jim Sloan
> leroy at sunflower.com
> Syclone VIN# 1206
> http://www.sunflower.com/~leroy
> Lawrence, Kansas

You can use a dc signal directly to the ECM.  GM makes a real handy
speed signal generator that outputs 30 and  60hz dc signals to verify
ecm operation.
On trucks equipped with RWAL, the DRAC also provides a signal for that
module.
Shannen
Shannen