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A recent VSS failure has left me wondering something about the logic by which such a failure is detected and dealt with.
  I now know that initially, when you start driving with a non-functioning VSS, the transmission simply won't shift out of 1st gear unless you manually put in in 2nd. At some point, however, the PCM detects that the VSS has failed; at that point, the O/D OFF light begins flashing, and the transmission when in Drive, will shift at least as far as third gear, but it does so very roughly. I know that at this point, it sets a P0500 code to indicate that the VSS has failed, and one of the Ford-specific data fields (as seen through an OBDâ??II scanner) changes to indicate as well that a VSS failure has been detected.
  Some times, it would detect the VSS failure almost immediately. Other times, it might take quite a bit of driving before the failure is detected.
  I am wondering why this is, and what logic is used to detect this failure.
  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.
  The only reason I can think of, then, why a VSS failure is not always detected immediately might be that the PCM won't report a failure until it knows what kind of failure has occurred, and that it must somehow figure out whether the car is moving despite the zero reading from the VSS (indicating the VSS itself is failed) or if the car is not moving (indicating some other more serious mechanical failure in the transmission itself).
  Any thoughts?
