What Is the Purpose of the ABS Brake Sensor in a '99 Ford F150?

The F-150 first arrived in Ford's lineup of full-sized pickups in 1975, but Ford intended it to be just a gap filler between the smaller F-100 and the heavy-duty F-250. Sales of the F-150 increased at such a rate that Ford dropped the F-100 in 1984, and the F-150 took its place as the entry-level pickup. The 1999 F-150 came standard with a four-wheel anti-lock brake system to help prevent wheel lockup when braking. This system uses a small electromagnetic sensor -- ABS sensor -- to help keep tabs on wheel speed.

Function of the ABS Sensor

The 1999 Ford F-150's front ABS sensor reads the rotation of the wheel, by way of a rotating disc with notches on it -- the ABS sensor ring -- on the rear of the front rotor. The rear ABS sensor gets the same reading from an ABS sensor ring, but the ring is placed inside the rear differential instead of on the rear of a rotor.

The ABS sensor reads the rotation of the ABS sensor ring as magnetic pulses -- the peaks have a strong magnetic pull and the valleys have a weak pull -- and sends the magnetic pulse to the anti-lock brake system control module. The control module then converts the magnetic pulse to wheel speed.

How the ABS System Works

The 1999 F-150's ABS control module continuously monitors the pulse from the ABS sensor. When the control module senses that a wheel is about to lock up, it activates and deactivates the hydraulic control unit. This activation and deactivation causes the hydraulic pressure in the braking system to vary at a high rate, preventing the wheel from locking up.

Replacing an ABS Sensor

The front ABS sensor bolts to the rear of the F-150's hub -- one bolt secures the sensor on a two-wheel-drive truck and two bolts secure it on a four-wheel-drive truck. Unplug the ABS sensor from the truck's wiring harness and pull the ABS sensor's wiring from the clip on the brake hose. Remove the bolt(s) from the sensor and pull the sensor from the hub. Reverse the steps to install a new sensor, tightening the bolt(s) to 5 to 7 foot-pounds.

Ford mounted the rear ABS sensor in the top of the rear differential. Unplug the ABS sensor wiring harness from the F-150's wiring harness. Remove the bolt securing the sensor to the differential and pull the sensor from the differential. Reverse the installation process to install a new sensor and tighten the bolt to 25 to 30 foot-pounds.

False ABS Sensor Failure Code

Since the F-150 is a rugged truck, you may drive it in less-than-ideal terrain. This can cause dirt, mud and other debris to cover the sensor and interfere with the sensor's ability to read the pulse of the ABS sensor ring, resulting in a false ABS sensor failure code. To rectify this problem, remove the sensor, clean it off with water and a mild detergent and reinstall it.