Principles of Operation

Special Tools:

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 anti-lock brake 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 anti-lock brake 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 (12 mph), 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 anti-lock brake control module must be reconfigured upon replacement.

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.

Anti-Lock Brake Sensor

NOTE: Any time an anti-lock brake sensor is removed, thoroughly clean the mounting surfaces. On front anti-lock brake sensors, apply High Temperature 4x4 Front Axle and Wheel Bearing Grease E8TZ-19590A or equivalent meeting Ford specification ESA-M1C198-A.

The anti-lock brake system uses three variable-reluctance sensors to determine vehicle speed. The anti-lock brake sensors operate on magnetic induction principle. As the teeth on the anti-lock brake sensor indicator rotate past the stationary sensor, a signal proportional to the speed of the rotation is generated and sent to the anti-lock brake control module through a twisted cable and shielded wiring harness.

G-Switch
The G-switch is used on 4x4 vehicles only. When driven in the 4-wheel drive mode, all four wheels are mechanically linked and a situation could arise in which one wheel locks up and causes all four wheels to lock up and skid. The speed sensors would indicate the vehicle speed as zero. Without the G-switch, the anti-lock brake control module would have no data to compare and would react as if the vehicle were stopped when in fact it is moving.
The G-switch detects and signals the anti-lock brake control module that the vehicle is moving.