[Gmecm] What does VSS do exactly?

[WopOnTour]

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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