Antilock Brakes / Traction Control Systems: Description and Operation
Anti-Lock Control - (4WABS)The anti-lock control system consists of the following components:
^ Hydraulic Control Unit (HCU)
^ anti-lock brake control module
^ rear anti-lock brake sensor
^ rear anti-lock brake sensor indicator
^ front anti-lock brake sensor
^ front anti-lock brake sensor indicator
^ yellow ABS warning indicator
^ brake pressure switch
Principles of Operation
The ABS operates as follows:
^ When the brakes are applied, brake fluid is forced from the master cylinder outlet ports to the HCU inlet ports. This pressure is transmitted through three normally open solenoid valves and three normally closed valves (2 valves per circuit, 1 normally open and 1 normally closed) and then through the outlet ports of the HCU to each wheel.
^ If the anti-lock brake control module senses a wheel(s) is decelerating at a rate greater than the vehicle, based on anti-lock brake sensor data, it closes the normally open solenoid valve for the circuit(s). This prevents any additional fluid from entering that circuit(s).
^ The anti-lock brake control module then looks at the anti-lock brake sensor data again from the affected wheel(s).
^ If that wheel(s) is still decelerating at a greater rate than the entire vehicle, the normally closed solenoid valve is then opened to relieve pressure in the affected circuit(s).
^ Once the affected wheel(s) returns to the corrected speed, the anti-lock brake control module returns the valves to their normal operating conditions, allowing fluid to once again flow through the affected circuit(s).
^ 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 a 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 to approximately 32 km/h (20 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 vehicle's anti-lock brake control system.
Hydraulic Control Unit
The HCU consists of the following components:
^ brake pressure control valve block
^ pump motor
New brake pressure control valve block and pump motor are installed as an assembly.
Anti-Lock Brake Control Module
The anti-lock brake control module is mounted to the HCU.
It 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 that occur in 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-19590-A 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.