[TSM]

Well its prolly not a magnet in this case, just the sensor sits a couple of mm away from the gear teeth or a special cut pulse rotor and as they pass the sensor it generates enough change in the magnetic field in the sensor for it to be picked up. (or somthin along thoes lines)

To test it off the bike you need to apply voltage to a certain two wires and then mesure the voltage on the third wire and ground, usualy i very quickly move it backward/forwards across a peice of metal like a screwdriver.

how are the connection blocks, corrosion, check all the way to the clocks.

the SV ones tend to fail when people put the front wheel on badly and brake the speedo rotor.