tonkin4

I've got a '92 Hardbody and the inst. cluster without a tach.
I got another cluster with the tach but when I installed it everything except the tach worked.
So I don't know if the tach is bad or if it's something with the truck.
It has a 4 wire harness for the tach 2 white and 2 black. I tested them for power and I only get voltage from one of the white wires.
I'm guessing that the black wires are grounds, one white for power and the other white a signal from the ignition coil?
If I'm right then I know I can hook the wire up directly to the coil, I've got the coil with the power translator and it looks like I'll have to splice into a wire and I have no idea which wire.
I have a haynes repair manual, but the wire diagrams are lousy for my vehicle. It only shows 3 wires in the diagram?
If anyone has any info that would help me I would greatly appreciate it.
Thanks