The ABS system consists of the following major components.
POWER BRAKE BOOSTER
The power brake booster is a self-contained unit that is mounted on the engine compartment side of the dash panel and uses engine intake manifold vacuum and atmospheric pressure for its power. If it becomes damaged or stops functioning properly, it must be replaced as an assembly, except for the power brake booster check valve.
BRAKE MASTER CYLINDER
The brake master cylinder is a tandem master cylinder. The primary (rear) circuit feeds the right-hand front and left-hand rear brakes. The secondary (front) circuit feeds the left-hand front and right-hand rear brakes. The master cylinder is serviced as a complete assembly.
HYDRAULIC CONTROL UNIT
The anti-lock brake Hydraulic Control Unit (HCU) is located in the front of the engine compartment on the left-hand side of the vehicle. If consists of the brake pressure control valve block assembly, pump motor, and the master cylinder filler cap with fluid level indicator assembly.
During normal braking, fluid from the brake master cylinder enters the HCU reservoir through two inlet ports at the rear of the HCU. The fluid then passes through four normally open inlet valves, one to each wheel. If the ABS control module senses that a wheel is about to lock, the module activates the appropriate inlet valve which closes that valve. This prevents any more fluid from entering the affected brake. The ABS control module then opens the normally closed outlet valve which decreases the pressure trapped in the line.
The brake pressure control valve block, pump motor and HCU reservoir are serviced separately. Other than seals and gaskets, no internal parts can be serviced.
ANTI-LOCK BRAKE (ABS) CONTROL MODULE
The Anti-lock Brake System (ABS) control module is located on the front right-hand side, next to the windshield washer reservoir for all vehicles except the SHO. On the Taurus SHO, it is mounted on top of the front left brake anti-lock sensor.
It is an on-board diagnostic, non-serviceable unit consisting of two microprocessors and the necessary circuitry for their operation. The module monitors system operation during normal driving, as well as during anti-lock braking. Under normal driving conditions, the ABS control module 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 ABS 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 anti-lock brake pedal sensor switch setting, the ABS control module will send a signal to the pump motor to turn on and provide high pressure to the brake system. Each time the vehicle is driven, as soon as the speed reaches 42 mph (70 km/h), the ABS control module turns on the pump motor for about 1 / 2 second (a mechanical noise will be heard; this is normal). When the pump motor starts to run, a gradual rise in brake pedal height will be noticed. The rise will continue until the sensor switch closes, and the pump motor will shut off until the brake pedal travel again exceeds the anti-lock brake pedal sensor switch setting.
Most malfunctions to the anti-lock braking system will be stored as a Diagnostic Trouble Code (DTC) in the keep-alive memory of the ABS control module.
ANTI-LOCK BRAKE SENSOR
Four sets of variable-reluctance brake anti-lock sensors and sensor indicators which determine the rotational speed of each wheel are used in the ABS system. The sensors operate on magnetic induction principle. As the teeth on the ABS sensor indicators rotate past the sensors, a signal proportional to the speed of rotation is generated and sent to the ABS control module.
The front brake anti-lock sensors are attached to the front wheel spindles. The front brake anti-lock sensor indicators are pressed into the outer CV-joints. The rear brake anti-lock sensors are attached to the right and left-hand rear disc brake adapters. The rear brake anti-lock sensor indicators are pressed into the wheel hub assemblies.
BRAKE PEDAL TRAVEL SWITCH
The brake pedal travel switch monitors brake pedal travel, then sends this information to the ABS control module through the wire harness. The brake pedal sensor switch adjustment is critical to pedal feel during ABS cycling. The switch is mounted in a hole in the right-hand side of the brake pedal support bracket, and to a pin on the speed control dump valve adapter bracket.
The switch is normally closed. When brake pedal travel exceeds the switch setting during an anti-lock stop, the ABS control module senses that the switch is open and grounds the pump motor relay coil. This energizes the relay and turns the pump motor on. When the pump motor is running, the HCU reservoir is filled with high pressure brake fluid, and the brake pedal will be pushed up until the brake pedal travel switch closes.
When the switch closes, the pump motor is turned off; the brake pedal will drop some with each ABS control cycle until the switch opens again and the pump motor is turned on again. This minimizes pedal feedback during ABS cycling. If the switch is not adjusted properly or is not electrically connected, it will result in objectionable pedal feel during ABS stops.