Anti-lock brake systems, or ABS, are available on a variety of BMW vehicles and provide a degree of safety that does not always come standard on other cars. The ABS speed sensor is a device on each wheel that measures the speed at which the wheel is turning. If the wheel is not turning as fast as the other wheels, then the computer adjusts braking to keep all wheels turning and slowing at the same speed. Sometimes these sensors will go bad and need replacement. Learning to replace these sensors will save you a large amount of money in the event that one of them goes bad over the course of your BMW's life. Replacing an ABS sensor is fairly easy and can be accomplished by any home mechanic.
With the car in park, pop the hood and disconnect the battery cables with the appropriately sized socket wrench.
Jack one side of the car up and use the lug nut wrench to remove the lugnuts on one wheel. Pull the wheel off, reach in behind the wheel column, and locate a small black box. Use the socket wrenches to remove the mounting screws on the edges of the box and slide it around to look at the underside of the box.
Pull the tabs on either side of the small round speed sensor and disconnect the wiring harness underneath. Pull the ABS sensor free and then place the new sensor.
Reattach the box and mounting screws on the back of the column and then replace the wheel. Repeat Steps 1 through 3 on the remaining wheels that have bad ABS sensors and then take the car for a test drive to make sure the sensors are working.
