pics!!! and are you taking orders on? Duane
Does the Ford probe have its own signal conditioner? IE you hooked 12 volts up to it, and it puts out the wave signal when the crank is turning, at a lower voltage.I don't know if that VDO probe will drive the VW tach, as I suspect the VDO speedometer sends a couple volts to the VDO probe, and the probe switches it on and off.The Ford probe may be the better choice for driving the VW tach with out extra electronic stuff. (technically speaking)
If you have a sensor that takes voltage and sends back a switched signal then that is a "hall effect" switch....this sensor(the ford speedo sensor) and all "inductive" sensors produce current with no needed voltage input only a bit a metal passed through the magnetic field will "move" electrons in the windings around the magnet....in this application there is NO external voltage going to the probe, that would probably prove destructive to the probe itself IE complete meltdown or at best a non functioning part.....Not sure what VDO probe you refer to but if it is an inductive probe it should work
Square wave:8 volts AC: