Nissan Repair: Intermittent malfunctionning of the speedometer, 1992 nissan stanza, vehicle speed sensor

Question
I have a 1992 Nissan Stanza XE Automatic. My problem began with the speedometer malfunctionning intermittently. Every time it stops functionning the car loses power and will drop to 20 to 30 mph no matter what I do. Now the problem seems to have gotten worse. The transmission goes into what can be described as fail-safe mode. It remains in 2nd gear and will not shift. I am being told that this is the  engine control computer failing. What do you think?

Answer
Bruno,

The speedometer is driven by a square wave generator that is called a vehicle speed sensor.  The computer gets the input for vehicle speed from this sensor at the speedometer connection.  Basically, if the VSS goes bad it will make the computer freak out and the car will run like crap.  So, replace the VSS and see if this corrects the problem.  It is located on the transmission where the speedometer cable used to go.  The speedometer is electric and you should be fine after the replacement of the VSS.  Disconnect the battery for about five minutes to clear the codes.  Oh, the computer uses this information to shift the transmission too.  So, that is why it is locking in second.  This is a protection mode to keep from hurting your transmission.