There is a sensor called a VSS ( Vehicle Speed Sensor ) If the VSS goes out the spedo will go dead, and the trans will stop shifting. The spedo, and the trans both use the VSS to operate. Without its signal neither one will work normally. As the name implies the VSS tells the spedo, and the transmission computer how fast the vehicle is going. Without it these systems are blind so to speak, and cant function.
Thanks for the info....next question: is this a job for a qualified mechanic, or can a handy do it-yourselfer handle this? Does the car's computer need to be reset once this part is replaced?
Odds are the computer will instantly recognize the new sensor. The sensor screws into the the transmission near the rear. I don't know your skill level, or the tools you have. On a scale of 1 to 10 this job is about a 2, and it is only a 2 because you have to jack up the car and do it from underneath the car. If you decide to do it buy the part so you will know what it looks like, and it will help you identify the part on the transmission.
Dr. Hamman, you are the man! Thanks so much for the info...according to everything I am reading on-line, you are on the money with this.....I hope it works!