VSS

[rauscher at icst.com]

Mike Pilkenton wrote:

>Could someone tell me what functions the VSS signal might affect in the 92
>3.1L V6 Camaro ECM.  The motor/ECM came off a manual tranny car and my
>project car application won't have this signal available unless I install an
>aftermarket speed sensor.  Do I need this signal?
>
>Mike
>V6 Opel GT


Mike, there is a ton of information in the archives, to go the DIY EFI home
page and follow the links. Personally, I believe that the VSS is important,
there is also info on what the signal looks like and some ideas and also
off the shelf solutions to this.

http://efi332.eng.ohio-state.edu/diy_efi

I got the following from there:

the following text is from:
http://www.jagsthatrun.com/Pages/SpeedSensors_Speedometer.html

-------- quoted text from webpage ------------

Without a VSS, the Chevrolet Fuel Injected engines will not run correctly,
and they will not pass a smog test.

Basically, the VSS tells the ECM how fast the vehicle is going. Most
people think the VSS is only used for the lock-up torque converter. The
VSS is also used to control the EGR valve, the charcoal canister purge
valve, the electric cooling fans, idle speed, and air/fuel ratio. This is
all explained in the Chevrolet shop manuals. 

It must be emphasized that the VSS is used to control the idle speed when
the vehicle is moving. Without the VSS, a vehicle may have stalling
problems under certain conditions.  Needless to say, this is dangerous. 
Raising the minimum idle speed with the adjusting screw can eliminate
stalling, but the engine will still not run optimally without a VSS. 

A lot of people think that running "closed loop" is best for fuel mileage. 
Closed loop simply means that the oxygen sensor is being used to monitor
the fuel/air ratio. Some of the Chevrolet fuel-injected engines are
programmed to run lean under certain conditions (called "highway mode") to
improve fuel mileage during steady cruise conditions. Without a VSS, the
engine computer (ECM) will not get the correct signals to run the engine
for best fuel mileage. There are a lot of programs in the ECM that depend
on the VSS. For best operation, the VSS needs to be connected and
functioning.

-------- end quoted text from webpage ------------

Here is a list of things affected by the VSS sensor.  This list covers
three different engines, two of them 4 cylinder TBIs.  All three
applications are from the same era as the 1226867 2.0 TBI.

Code 24
TCC lockup (auto)
Upshift light (manual)
IAC position learning (above 30 MPH)
Cooling fan control (not used in these applications)
Determine if manual transmission is in gear (compare VSS vs RPM)
Disable some idle speed control if manual in gear (see previous item)
Disable idle learn if moving
Enable idle learn if moving with manual in neutral
Prevent DecelFuelCutOff if manual in neutral
Prevent DecelFuelCutOff at very slow speeds
Top speed limiter (not used in these applications)

The third application adds one more item:

Amount to open IAC and rate at which to close it upon throttle closing

Note - the idle speed control logic is a rats nest of complicated code. 
It is hard to figure what really happens.

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