[Gmecm] What does VSS do exactly?

[Jay Vessels]

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] 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.