VSS Circut

[VileZambonie]

You are right, we can disagree...  So let’s go over a few things here:

2)The TPS is not an electronically controlled output device, it's an input not an output actuator like IAC or EGR so are you saying the TPS is controlled by speed and engine load? The TPS isn’t “controlled” it is again an input that relays information to the ECM based on driver demand ie how far the throttle plate is open or closed.
c)The EGR in this truck is controlled by the ECM and uses information from the following sensors to regulate the solenoid:
•   Coolant Temperature
•   Throttle Position (TPS)
•   P/N switch
•   Distributor (RPM Signal)
The EGR vacuum control has an EVRV solenoid that uses "pulse width modulation". This means the ECM turns the solenoid "ON" and "OFF" many times a second and varies the amount of "ON" time ("pulse width") to vary the amount of EGR.
d)IAC Idle Air Control controls idle speed (Idle speed… It means when the throttle plate is closed, it doesn’t mean the speed at which your truck is rolling down the road).
Engine idle speed is computer controlled by signals to the Idle Air Control (IAC) valve mounted on the throttle body. The IAC valve regulates idle speed by moving a pintle valve in and out, varying air flow through a passage bypassing the throttle valve. The more air that is allowed to bypass the throttle valve, the faster the idle speed. The ECM sends voltage pulses to the IAC motor windings causing the IAC motor shaft to move a given distance for each pulse, (referred to as IAC counts). The number of IAC counts can be read on a Scan tool display. 0 counts corresponds to fully closed passage, while 140 counts or more (depending on application) corresponds to full flow. The ECM monitors idle speed and increases the IAC counts if the idle speed is too slow. If the idle speed is too fast, IAC counts will be decreased.
3)Your engine load sensors here are MAP & TPS based on RPM. (Speed Density) – Air fuel ratio in this truck is targeted and adjusted by the ECM utilizing Fuel Integrator and Block Learn. Corrections to the A/F ratio are based on the primary sensors in both open and closed loop. I will list them for you:
RPM – from the distributor
TPS – Throttle Position Sensor
MAP – Manifold Absolute Pressure
ECT - Engine Coolant Temp
O2 – Oxygen Sensor (closed loop only)
Other sensors like Knock, VSS etc are not considered primary load sensors.
Nevertheless If the VSS circuit faults, the ECM will set DTC 24 and will ignore the VSS signal. So even if it did use the VSS to control the A/F ratio here how would it trick the ECM and allow the truck to run lean as you suggested?

[topp]

2-The ECM does control the IAC, and EGR and Fuel mixture based on it's program, and it USES the SPEED of the vehicle (and RPM, and MAP)to adjust the A/F ratio as you drive....If it did not, when you let off the throttle it would stall(which is why stock carbs have that little plunger- to raise idle for A/C and prevent stall when throttle blades shut rapidly) and when you were under load and accelerating heavily it would bog.  That is just the fact, and is STATED in MANY CHEVY SERVICE MANUALS regarding TBI.  FACT.  It is also listed in the Snap-On Mitchells Shop Software.  It also controls the converter lock up.  Fact.  Not assumptions or guesses...  It also contributes (hence is USED and NEEDED) to adjust the EGR under the same conditions.
IAC changes postition as you drive (based on RPM, SPEED(VSS), and Load...

what does the IAC do when you go on the throttle?  It is SIGNALED by the ECM from the VSS and RPM that you are not at idle and are MOVING above 2MPH and it CLOSES.  If it does not, you are Leaning out the mixture (and according to GM, the ECM will run in LEAN mode at Highway speeds as a default, and the VSS CORRECTS THAT by SIGNALING the ECM that you are moving at over 2MPH and RICHENS the mixture slightly.
4-MAP and RPM are not the only thing that the ECM needs, and yes, if VSS fails, the ECM ignores it and goes into default mode.  Yet, if you did NOT need it, you would have no code and it would not be there to begin with.

[VileZambonie]

The original poster was given information to which I believe needed clarification. There were no personal attacks on my behalf here and if it was taken that way by topp I offer my apologies. You are correct in that there is no point in discussing this anymore as there is plenty of information here for the original poster to determine if in fact losing his vss signal would make his truck run lean. To anyone who wants to use the information posted here and do their own fact finding please go right ahead and choose to draw your own conclusions.