[Gmecm] What does VSS do exactly?

[Charles McDowell]

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] 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.
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