[Gmecm] What does VSS do exactly?

[Jay Vessels]

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] On Behalf
> Of WopOnTour
> Sent: Saturday, June 25, 2005 1:53 PM
> To: gmecm at diy-efi.org
> Subject: Re: [Gmecm] What does VSS do exactly?
> 
> Charles
> Depending on your box, VSS is used to trigger an IAC "reset" when a
> combination of TP and VSS are above predetermined limits. (typically high
> enough so the reset routine is not intrusive) At that time the ECM will
> usually command 255 "in" counts and then back the pintle out to a
> predetermined decel "ready" position (usually 60-140). It typically only
> does this once per IGN cycle. This is necc as the IAC doesn't really have
> any physical feedback circuit, so the ECM bases it's count strategy on this
> reset position. On newer PFI units this reset occurs ay IGN off instead. So
> not having a VSS may affect idle  performance since the ECM can't know
> actual pintle position and can get quite a bit "out-of-sync" from the
> predicted position posted up in the data stream and displayed on a scan
> tool.
> HTH
> Regards
> WopOnTour
> 
> ----- Original Message -----
> From: "Charles McDowell" <charles at taildragger.info>
> To: "'For discussion of GM EFI hardware and software'" <gmecm at diy-efi.org>
> Sent: Saturday, June 25, 2005 11:43 AM
> Subject: [Gmecm] What does VSS do exactly?
> 
> 
> 
>>I converted my Blazer to TBI using a pre-made kit with no provision for 
>>VSS.
>>Everything seems fine, but I'm reading that I could have problems?  Can
>>anyone say for sure what purpose the VSS serves?  I have all the emissions
>>stuff disabled, so I don't care about that.  I've read some comments
>>regarding IAC, but nobody can explain quite what that's all about.
>>
>>Thanks.