[Mike Noc]

To be honest I've never checked to see if the outputs reset to one voltage or the other from ignition on, or if you can reverse the polarity and they will still work.
In all the ABS faultfinding I have done I have had a couple of magnetic reluctors fail due to rust on the rear back plate, one sensor had lifted away due again to rust on the rear brake plate where it mounted, which increased the air gap slightly, and two sensors that were switching at double the voltage they should have and so confusing the system. They came fitted to the car after the previous owner had suffered the ABS light coming on and we're cheapies from Ebay. Also had a couple of front sensors fail and one front reluctor.
All the faults were easy enough to spot with a voltmeter, some chalk and a flying lead I made up from an old ABS plug and socket and some bell wire to save pinning through the ABS sensor cables.