speedo wiring

[z.machine]

I have a transmission out of an 89 240sx with the electric speedo sender. There is one green and one black wire coming out of the speedo sender.

In the wiring diagram for my autometer speedometer it says one of those wires goes to ground, and the other one sends the signal to the speedo.

Anyone know which one (black or green) carries the signal and which one goes to ground?

Thanks

[Soravia]

Black is usually always ground, isn't it?