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When you come to a stop normally, the brakes don't pulse when you stop. Since the car can only know its speed by the rotation of the wheels, how can it distinguish between the car is stopped normally and the car wheel's stopped rotating because they locked up?

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up vote 5 down vote accepted

You are correct that the car can't use the speedometer output for this. Instead there are sensors on the brakes that measure how quickly the wheel rotation is decelerating. If this is faster than the brakes can actually slow the car down, then it means the breaks are locking up and it is only slowing the wheels.

For more information and images, check out this link:

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Your link says: ""Speed Sensors The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The speed sensors, which are located at each wheel, or in some cases in the differential, provide this information."" The signal of this speed (better rotation) sensor is differentiated for evaluation. – Georg Mar 22 '11 at 9:38
Not all systems are smart enough to figure how fast you should be able to stop. The systems on my cars only appear to detect and activate on major differences in rotation speed between the various wheels. For example, if I mash the brakes at high speed, all 4 wheels will lockup at the same time and the ABS never activates. However, one wheel on slippery surface and another on dry and the ABS activates. – Brian Knoblauch Mar 22 '11 at 13:09

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