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Could the speed sensor be activated when pulled out of its mount, if I pass a piece of metal near it will the speedometer register it? Does it work using some other data or does it only register gearbox (flywheel?) rotations and flux in magnetic field by teeth getting closer-further from the magnet?
Also would it work if hooked up to a signal generator?
Yes. Most shaft speed sensors are reluctance sensors that trigger when a piece of ferrous metal passes by them. The sensor will register each time the tab of metal on the shaft passes by.
If you manually do it I think it would be hard to register on the speedo but if you used something to spin a metallic item by it, say chucked in an electric drill, you may be able to get it to register.
A reluctance sensor does not require a magnet to pass by as a Hall Effect sensor does. It's not practical to magnetize parts of the transmission so they use a reluctance sensor instead.
You could also simulate the signal from the sensor with some kind of a circuit drive by a signal generator but you would need to determine the characteristics of the signal and duplicate that.
