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1997 Ford Taurus. I had the classic problem indicating a intermittent failure in the VSS sensor. The speedometer didn't work and and transmission didn't shift correctly or sometimes remained in low gear when the car got hot. I looked around on the passenger side output shaft and couldn't find the sensor. I then moved to the drivers side and looked in under the exhaust manifold and there was a sensor way back in there. To verify this sensor was bad, I disconnected the wiring while the engine was cold and everything was working. The speedometer and transmission stopped working. I installed a new sensor and the speedometer and transmission worked again. My question is, did I change the VSS(Vehicle Speed Sensor) or the TSS(Transmission Shaft Speed)? Can either of these cause the same problem? I sort of vaguely remember when I had this same problem 100K miles ago with this same car, the mechanic had a ratchet stuck down on the passenger's side of the engine compartment where most articles say the VSS is located.

The Vehicle Speed Sensor is located at the rear center of the engine compartment, mounted on transaxle (see diagrams below).
Depending on the transmission of the vehicle AX4N or AX4S, the VSS can mount differently

I have included the VSS diagrams for both transmission below.

Please let me know if you have any questions.
Thank you.

I replace the speed sensor in my 1997 Taurus back in 2011. The details are a little fuzzy, but I have attached an article I wrote that has pictures and details. I have uploaded the article in .PDF format so you should be able to open and view it. I have additional pictures if that would be helpful.

Appreciate the input.