The magnet is in the sensor. The ring in the bearing is just thinner and thicker steel. The interaction makes a ripple in the magnetic field which changes the sensor output.

It takes a tiny amount of rust to throw these things off or a tiny misalignment. Are you sure you are working on the right sensor? also what errors are you getting and what do you see for wheel speed ? Do you have a scanner that reads wheel speed? If not, get one. It's silly to fly blind when repairing.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)