Diff speed speed sensor voltages

[blitzracing]

Working on a car with a speedo that does not work. I can confirm there is no speed signal coming out of the sensor on the diff, as it has just two wires, red and black and there are no significant voltage on either wire as the wheels turn. The Steve Heath bible refers to a 12 volt switched signal from the transducer, and refers to a supply fuse- that we have checked. Im not however convinced that a 2 wire sensor would have 12 volts on it, but can any one confirm this or other wise? If there is 12 volts on it, it should certainly be visible on the loom with the transducer unplugged. If there is 12 volts- any ideas where the feed is?

Thanks
Mark