[DirtyLarry]

Rfmaster,

What you said above about VSS sounds right for 1996 and later OBD II sequential multiport EFI systems but I can’t find anything in any of my older GM STG service manuals for TBI years that state the VSS has anything to do with Deceleration Enleanment or Deceleration Fuel Cut Off. It could be one of the functions that takes place behind the scenes that isn’t really called out in the service manual.

From what I understood, the main sensor to control fuel on OBD I such as TBI is the MAP as it constantly monitors engine load and adjusts fuel pulses accordingly. Once you get into OBD II on sequential multiport things get a lot more complicated and more inline with your description above.

The only function I can find for the VSS in TBI years is to provide a pulse signal to the DRAC for the speedometer, RWAL ABS module, and cruise control module. The VSS isn’t even mentioned anywhere in any of the drivability symptom flowcharts or anything other than inop speedometers, cruise, ABS, etc.

For what it is worth, my 8.1L running multiport EFI on a Delphi MEFI-4 controller (similar to a GMPP RamJet engines and GM Marine engines) doesn’t have any VSS inputs either to run, and run well even with a manual trans. The whole MEFI-4 system is an entirely weird deal anyway.

I know you are an engineer and know this stuff a lot better than any of us but are you sure about the VSS function on TBI?