The 4-Wheel Anti-lock Brake System (ABS) used on 1993-96 Bronco models is an electronically operated, all wheel brake control system. Major components include the master cylinder, vacuum power brake booster, ABS Control Module, Hydraulic Control Unit (HCU) and various control sensors and switches.
The brake system is a three channel design. The front brakes are controlled individually and the rear brakes in tandem.
The system is designed to retard wheel lockup during periods of high wheel slip when braking. Retarding wheel lockup is accomplished by modulating fluid pressure to the wheel brake units.
Power Brake Booster
The diaphragm-type power brake booster is self contained, and is mounted on the engine compartment side of the dash panel. The booster uses engine intake manifold vacuum and atmospheric pressure for its power. It is serviced as an assembly only, except for the power brake booster check valve.Master Cylinder
The brake master cylinder is a tandem master cylinder. The primary (rear) circuit feeds the right side front and left side rear brakes. The secondary (front) circuit feeds the left side front and right side rear brakes. The master cylinder is serviced as a complete assembly.Brake Pressure Control Valve
The brake pressure control valve regulates pressure to the rear brakes to create a balance between the front and rear brakes.Brake Master Cylinder and Brake Fluid Level Switch
The master cylinder reservoir is a clear, translucent plastic container with three main chambers:
Hydraulic Control Unit (HCU)
See Figures 1 and 2
The Hydraulic Control Unit (HCU) is located outside of the engine compartment on a bracket which attaches to the left frame rail. It consists of the following:
During normal braking, fluid from the master cylinder enters the HCU through two inlet ports located at the rear of the HCU. The fluid then passes through four normally open inlet valve, one to each wheel. The the anti-lock brake control module senses that a wheel is about to lock, the control module closes the appropriate inlet valve. This prevents any more fluid from entering the affected brake.
The anti-lock brake control module then looks at that wheel again. If it is still decelerating, then anti-lock brake control module opens the normally closed outlet valve which decreased the pressure trapped in the line. If equipped, the Traction Assist (TA) valve body contains two isolation valves for traction assist function: one for the primary circuit and one for the secondary circuit. The isolation valves close during traction assist operation to prevent front brake operation.
The following components are serviced separately:
Anti-Lock Brake Control Module
The anti-lock brake control module is located on the anti-lock brake hydraulic control bracket mounting bracket located outside of the engine compartment attached to the outer frame rail.
The module is an on-board diagnostic non-repairable unit consisting of two microprocessors and the necessary circuitry for their operation. These microprocessors are programmed identically. The anti-lock brake control module monitors system operation during normal driving operation as wheel as during anti-lock braking and traction assist cycling, if equipped.
Under normal driving conditions, the microprocessors produce 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.
If brake pedal travel exceeds a preset dimension determined by the ABS sensor switch setting, the control module will send a signal to the pump to turn on and provide high pressure to the brake system. When the pump motor starts to run, a gradual rise in pedal height will be noticed. This rise will continue until the anti-lock brake pedal sensor switch closes and the pump motor will shut off until the pedal travel exceeds the sensor switch setting again. During normal braking, the brake pedal feel will be identical to a standard brake system.Anti-Lock Brake Sensors
The anti-lock brake system uses three variable reluctance brake anti-lock sensors to determine the rotational speed of each of the two front wheels and the rear wheels as a pair. The sensors operate on the magnetic induction principle. As the teeth on the sensor rotate past the sensor indicator, a signal proportional to the speed of the rotation is generated and sent to the anti-lock brake control module through a coaxial cable and shielded wiring harness.
The front brake anti-lock sensors are attached to the front wheel spindles, and the front brake anti-lock sensor indicators are pressed onto the wheel hub assembly. The rear brake anti-lock sensors are attached to the rear axle housing.
The front and rear anti-lock sensors are not adjustable. The rear brake anti-lock sensors and sensor indicators are serviced separately.Stop Light Switch
The anti-lock brake system uses a brake pedal sensor switch which monitors brake pedal travel and sends this information to the anti-lock brake control module through the wire harness. The anti-lock brake pedal sensor switch adjustment is critical to pedal feel during ABS cycling.