There's been much discussion on this forum about the late 740 / 940 speedos. If you had two speedos that acted the same, the problem may lie with the pickup in the differential or the associated wiring. However, the Yasaki (140 MPH) speedos are almost sure to fail sooner or later. So the fact that you got a used one that didn't work is no surprise. The 700/900 FAQ describes how to test the differential sensor output and how to replace capacitors inside the speed itself. If you suspect a problem with the diff pickup, you can simulate its output by disconnecting the wires and then, with key ON, momentarily jumping the wires together in a rapid close-open-close-open action and seeing if it makes the speedo needle move.

Read this:

http://www.brickboard.com/FAQ/700-900/ElectricalSpeedometerRepair.htm