kev, it may 'work' with wires either way round, but the 'wrong' way will give an ignition timing offset error - normally the sensor signal will rise as the flywheel tooth approaches, and fall as it centres on the sensor, the oposite will happen with the wires connected the wrong way. As the ECU will detect the falling edge of the sensor signal, this will give a timing error equivalent to approx half a flyweel tooth, approx 5 degrees retard from memory..