What does the speed sencor acualy do and how do I know it it needs to be replaced

 

This is how the sensor works..

With the ignition switch in ON (II) or START (III),
battery voltage is supplied through fuse 1 and the
YEL wire to the vehicle speed sensor (VSS). The
sensor is grounded by the BLK wire to G101. The
speedometer and other control units in the circuit
supply 5 volts or more to the ORN wire. The vehicle
speed sensor (VSS) intermittently grounds the ORN
wire which generates a pulsed signal in it. The
number of pulses per minute increases/decreases
with the speed of the car.

As for when to replace it when its broken
like when your speedometer goes out.

 

doesnt "speed" sensor give you a clue?

its on the top back side of the tranny. auto and manual vss are the same, pull one from a junkyard. most likely the 90-97 vss are the same.