Anti-Lock Brakes

Principles of Operation
The anti-lock brake system (ABS) operates as follows:
^ When the brakes are applied, fluid is forced from the brake master cylinder outlet ports to the hydraulic control unit (HCU) inlet ports. This pressure is transmitted through three normally open solenoid valves contained inside the HCU through the outlet ports of the HCU to each wheel.
^ If the anti-lock brake control module senses a wheel is about to lock, based on wheel speed sensor data, it closes the normally open solenoid valve for that circuit. This prevents any more fluid from entering that circuit.
^ The anti-lock brake control module then looks at the wheel speed sensor signal from the affected wheel(s) again.
^ If that wheel(s) is still decelerating, it opens the closed solenoid valve for that circuit to further reduce hydraulic pressure trapped in the line.
^ Once the affected wheel comes back up to speed, the anti-lock brake control module returns the solenoid valves to their normal condition allowing fluid flow to the affected brake.
^ The anti-lock brake control module monitors the electromechanical components of the system.
^ A malfunction in the anti-lock brake system will cause the anti-lock brake control module to shut off or inhibit the system. However, normal power-assisted braking remains.
^ Malfunctions are indicated by the yellow ABS warning indicator in the instrument cluster.
^ The anti-lock brake system is self-monitoring. When the ignition switch is turned to the RUN position, the anti lock brake control module will carry out a preliminary self-check on the anti-lock electrical system indicated by a three second illumination of the yellow ABS warning indicator in the instrument cluster.
^ During vehicle operation, including normal and anti-lock braking, the anti-lock brake control module monitors all electrical anti-lock functions and some hydraulic operations.
^ Each time the vehicle is driven, as soon as vehicle speed reaches approximately 20 km/h (12mph), the anti lock brake control module turns on the pump motor for approximately one-half second. At this time, a mechanical noise may be heard. This is a normal function of the self-check by the anti-lock brake control module.
^ Pedal pulsation coupled with noise while braking on loose gravel, bumps, wet or snowy roads is normal and indicates correct functioning of the anti-lock brake system.

Anti-Lock Brake Control Module

NOTE: The 4x2 service replacement module cannot be interchanged with the 4x4 module. The service replacement 4x4 module is interchangeable with the 4x2 module.

The anti-lock brake control module is an on-board, diagnostic, non-repairable unit consisting of two microprocessors and the necessary circuitry for their operation. The anti-lock brake control module monitors system operation during normal driving as well as during anti-lock braking.

Anti-lock brake module operation is as follows:
^ Under normal driving conditions, the microprocessor produces short test pulses to the solenoid valves that check the electrical system without any mechanical reaction.
^ Impending wheel lock conditions trigger signals from the anti-lock brake control module that open and close the appropriate solenoid valves. This results in moderate pulsations in the brake pedal.

During normal braking, the brake pedal feel will be identical to a standard brake system.

Most faults which occur to the anti-lock brake system will be stored as a diagnostic trouble code (DTC) in the keep alive memory of the anti-lock brake control module. The DTCs can be retrieved by following the on-board diagnostic procedures.

Wheel Speed Sensor

NOTE: Any time a wheel speed sensor is removed, thoroughly clean the mounting surfaces. On front wheel speed sensors, apply High Temperature 4x4 Front Axle and Wheel Bearing Grease E8TZ-1 9590A or equivalent meeting Ford specification ESA-M1C198-A.

The anti-lock brake system uses three "active" sensors and three sensor rings to detect the speed of each wheel. The teeth on the sensor ring rotate past the stationary sensor at wheel speed. As the teeth pass the sensor, a digital input signal is generated. The control module uses the input to compute the speed of each wheel.

Electronic Brake Distribution (EBD)
The electronic brake distribution (EBD) controls rear brake pressure and acts as an electronic proportioning valve. It is controlled by the anti-lock brake control module. When EBD is disabled, the red brake warning indicator will illuminate.