Well...your getting into an area that at least for me is kinda hard to get documentation on.....but I'll take a stab at it.....
The Vehicle Speed Sensor (VSS) and Turbine Shaft Speed (TSS) sensor are analog inputs that are checked for
rationality. If the engine rpm is above the torque converter stall speed and engine load is high, it can be inferred
that the vehicle must be moving. If there is insufficient output from the VSS sensor, a malfunction is indicated.
If there is insufficient output from the TSS sensor, a malfunction is indicated.
VSS is located on the final drive carrier assembly.
The TSS is on the other end...mounted on the external case where oil pump is located.
Quote:
??Am I correct in supposing that if the TSS indicates that the turbine is spinning, and if the transmission is in any setting other than Park or Neutral, that the car ought to be in motion?? It seems to me, then, that a VSS failure should always be immediately detectable.? If the transmission is not in P or N, if the TSS indicates movement, and the VSS does not, then there's a failure.
I think the key is torque converter stall speed. By comparing the engine speed, stall speed and turbine speed the program detects faults.......
When a failure is detected then, knowing Ford, manual control if you will of shift solenoids, fluid pressures, etc are not well handled (by micro code).....just a laymen's opinion mind you 
Bob
