87 Firebird v6 Auto.

My Speedometer has started to jump up and down sometimes staying on zero.

i have the dash apart, the instrument cluster loose, Can pull it up to the stearing column.

looks like there should be three parts to it really? Speedometer, Speedometer cable, and the speed sensor.

I dont see a cable leading to the back of my speedometer. Just a sircuit board on back of instrument panel.
On my speed sensor there is a two wire connection. When I unplug this and drive around no check engine light and no speedometer reading.

I seen in one speed sensor replacement video, that the speed sensor had the cable go directly into it, along with the electrical wire.

My speed sensor only has the electrical wire and no cable going to it. Does this mean my speedometer is electrically driven and not mechanically driven, and there is no speedometer cable in my car?

 

Yes. You also should get a CE light with the VSS unplugged (at the transmission). Are you sure the CEL is working ?

 

IDK about yours, but my speedo cable is hella tight, I can't move the cluster without first disconnecting it. Are you sure it didn't just get disconnected when you were pulling it out?