Just might be something simple like, power to the guage . I believe two wires come in from the engine, than power and it should work.
OK, I just looked in the rotax heavy duty maint. manual and the tach runs with two wires. The bl/ge wire and the ws/ge wire. those go to the tach and I would think the third is power . Now your saying what are rotax's codes for the wire colors. I got those also. GEt this, the bl = blue ge=yellow and ws = white. Those wires come off the back of the engine where all those sensors are off the crank. I believe you should have had a wire harness with connector for install, just take that harness and run it right to the tach. Hope this helps.