2004 F150 ABS Issue

[RiskyGuy]

My supercab 2WD has about 100,000 miles on it. When I hit the brakes, the ABS kicks in at low speed (less than 10 mph) even though the tires are not slipping. Any ideas? Thanks!

[65comet]

Is the ABS light on? You didn't mention it, so I will assume no. A few things I've seen that could cause this. First a little discussion on how it works. There is a toothed gear that is usually mounted behind the brake rotor. As it goes around, a electric/magnetic sensor reads each tooth as it goes by and can tell the rotation and speed of rotation of each wheel that has a sensor. A4WD vehicles that are normally rear wheel drive have sensors on all four wheels, though the ones on the rear in most cases only engage the 4WD when slippage is encountered. So you are primarily concerned with the front gear rotor and sensor.
I have encountered the magnetic sensor being completely covered in metal filings from brakes worn down to the metal on the pads. This caused false readings of brakes being locked up.
I have encountered a cracked ring that has widened the gap between two of the teeth, also causing a bad reading because it thinks the wheel is stopping for that extra gap.
I haven't encountered it but have heard of teeth being broken off. Just like a gap from a crack, a gap from a missing tooth will cause a bad reading thinking the wheel has stopped for that moment.
A sensor wire worn and shifts position during stops from that momentum could also be an issue.
I've had sensor cases crack and in wet weather let in water, but that usually leads to an ABS light and the ABS won't work at all until they dry out.