[Gmecm] What does VSS do exactly?

[Rick McLeod]

Correct me if I'm wrong:

It may be different on trucks, but on the mid-late
80's Camaros there is a opto-buffer mounted to the
back of the instrument cluster, a yellow box, wires to
the opto sensor on the speedo, and an edge connector
that the power/output lines connect to. It creates a
ecm compatible 2K signal of square wave type.

--- Jay Vessels <jay at vessels-clan.com> wrote:

> Hi there!
> 
> The vacuum/electronic cruise system on the mid-80s
> trucks was the servo 
> underhood, the VSS+buffer, the turn signal stalk,
> and a small black box 
> which was the standalone controller.
> 
> The underhood wiring did not have the VSS signal to
> it; rather, it was 
> just wiring for two vacuum solenoids, and for some
> systems, a feedback 
> signal (moving magnet/wire coil).
> 
> I've never seen the VSS buffer mounted to anything
> (seems like it just 
> hangs in space) so I'd do just what you mentioned --
> I'd remove the 
> speedometer from the cluster, and follow the ribbon
> cable.  Chances are 
> you didn't remove the controller, so if you find
> that (usually around 
> the steering column) you'll find where the post-VSS
> buffer wiring goes. 
>   That harness already has power and ground, so it's
> a simple matter of 
> running a wire from your ECM to the VSS output wire
> that's already there.
> 
> You may be closer to having a VSS than you thought!
> 
> Jay Vessels
> 1982 Chevrolet S-10 Sport, 2.8V6 TBI
> 1984 Chevrolet S-10 Blazer Sport, 2.8V6 carb. (for
> now)
> 
> 
> Charles McDowell wrote:
> > Well, I removed the cruise, but yes, I think I had
> the first kind you
> > described (no CCC though).  The speedo cable goes
> into the gauge in the
> > dash.  The cruise looked like a vaccum thingy
> controlled by wires.  So, you
> > are saying that my speedo is already doing the job
> for me probably?  In
> > principle, I could trace the wires from the speedo
> to the buffer and get VSS
> > from there.  Is that right?  Any guess where they
> typically mount these
> > things?
> > 
> > Would the VSS signal go all the way out to the
> cruise stuff under the hood?
> > If so, since I removed the cruise, it would just
> be hanging open at a
> > connector and life would be incredibly easy...
> except for finding the
> > connection in the EFI harness.
> > 
> > Thanks for the help.
> > 
> > -----Original Message-----
> > From: gmecm-bounces at diy-efi.org
> [mailto:gmecm-bounces at diy-efi.org] On Behalf
> > Of Jay Vessels
> > Sent: Sunday, June 26, 2005 10:05 PM
> > To: gmecm at diy-efi.org
> > Subject: Re: [Gmecm] What does VSS do exactly?
> > 
> > Hi there!
> > 
> > A few questions:
> > 
> > 1)  Do you have factory cruise control?  If it's
> the vacuum/electronic
> > system (i.e. there is a vacuum servo cabled to the
> throttle that has about 4
> > or 5 wires to it, as opposed to the
> vacuum/mechanical system that had the
> > speedo cable routed to a module under the hood)
> then you already have a VSS.
> > 1983 was a weird year for that, since at least
> some vehicles got the
> > electronic version.
> > 
> > 2)  Did it have the computer-controlled carburetor
> on it?  Some of the
> > California trucks did, and they would also have a
> VSS already.
> > 
> > If 1 or 2 is true, then the VSS is already there. 
> Find the VSS buffer
> > (small box, ribbon cable to the sensor, card-edge
> connector to the truck),
> > and the pin that's not 12V or ground is the VSS
> signal.  Hook it to the
> > ECM's VSS input pin and you're done.
> > 
> > Barring that, 3)  Do you have the stock
> speedometer?  If so, VSS is simple.
> > The factory speedometer has a rectangular hole in
> the back of it with a
> > small hole to one side.  The VSS
> phototransistor/LED pair fit that hole and
> > is attached with a single screw.  Any 1981-1989
> > (approximately) GM car with EFI or CCC will have a
> VSS in the back of the
> > speedometer (some FWD and later-80s cars exempted,
> just look for a
> > speedometer cable).  Grab it.  It will be a small
> black module attached to
> > the speedometer with a ribbon cable to a small
> box.  That box has a
> > card-edge connector with three pins.  Get the
> connector, too.  The connector
> > is ground, +12V, and the VSS output.  Connect the
> power supply to whatever's
> > handy, and run the VSS output to the ECM's VSS
> input pin.
> > 
> > Jay Vessels
> > 1982 Chevrolet S-10 Sport, 2.8V6 TBI
> > 1984 Chevrolet S-10 Blazer Sport, 2.8V6 carb. (for
> now)
> > 
> > Charles McDowell wrote:
> > 
> >>You're probably right.  Can you (or anyone else)
> point me in the right 
> >>direction for VSS stuff for an 83 K-5 Blazer with
> a 350?
> >>
> >>I think the most pain will be that I bought a
> pre-made harness with no 
> >>VSS, so I'll have to dig into that to make the
> connections.
> >>
> >>Thanks. 
> >>
> >>-----Original Message-----
> >>From: gmecm-bounces at diy-efi.org
> [mailto:gmecm-bounces at diy-efi.org] On 
> >>Behalf Of Jay Vessels
> >>Sent: Sunday, June 26, 2005 3:34 PM
> >>To: gmecm at diy-efi.org
> >>Subject: Re: [Gmecm] What does VSS do exactly?
> >>
> >>Hi there!
> >>
> >>VSS does several things, and the newer the ECM the
> more things it's tied
> > 
> > to.
> > 
> >>The older 1227747 vintage ECMs do several things
> with VSS, most 
> >>notably parking the IAC at speed, and I believe
> stall saver (i.e. 
> >>sudden throttle close, such as letting off the
> throttle and pushing in 
> >>the clutch at speed when coming to a stop).
> >>
> >>Newer ones such as the $A1 code in the '730
> (1991-1994 3.1V6 MPI) use 
> >>VSS to determine much more, such as throttle
> follower (manual shift 
> >>assist), stall saver, idle speed adjustment, spark
> advance, etc. since 
> >>that code changes fuel and spark strategy based on
> whether the vehicle is
> > 
> > moving.
> > 
> >>Considering how simple it is to hook up (stock
> parts abound as do 
> >>aftermarket ones) and the benefits in drivability
> (which is a big reason
> >>  to go EFI in the first place), why not connect
> it?
> >>
> >>Jay Vessels
> >>1982 Chevrolet S-10 Sport, 2.8V6 TBI
> >>1984 Chevrolet S-10 Blazer Sport, 2.8V6 carb. (for
> now)
> >>
> >>Charles McDowell wrote:
> >>
> >>
> >>>Thanks for the reply.  I always thought the IAC
> "parked" itself when 
> >>>the car was shut off.  Do I understand correctly
> that I will probably 
> >>>not have a problem if my computer is the type
> that operates that way 
> >>>instead of recalibrating above a certain speed?
> Is there any good way 
> >>>to tell which I have?
> >>>
> >>>-----Original Message-----
> >>>From: gmecm-bounces at diy-efi.org
> [mailto:gmecm-bounces at diy-efi.org] 
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