Antilock Brakes / Traction Control Systems: Description and Operation
Description
Warning: Brake fluid contains polyglycol ethers and polyglycols. Avoid contact with eyes. Wash hands thoroughly after handling. If brake fluid contacts eyes flush eyes with running water for 15 minutes. Get medical attention of irritation persists. If take internally, drink water and induce vomiting. Get medical attention immediately.
The taurus/Sable has a 4-wheel anti-lock brake system (ABS) as an option. The system is standard on Taurus SHO. The system prevents wheel lockup by automatically modulating the brake pressure during an emergency stop. By not locking the wheels, the driver can maintain steering control, and stop the vehicle in the shortest possible distance under most conditions.
The brake force required to engage the ABS function may vary with the road surface conditions. A dry surface requires a higher force, while a slippery surface requires much less force.
During the ABS operation, the driver will sense a pulsation in the brake pedal, accompanied by a slight up and down movement in the pedal height and clicking sound. The pedal effort and pedal feel during normal brakeing are similar to that of a conventional power brake system.
The ABS consists of the following major components:
- Vacuum Booster and Master Cylinder Assembly.
- Hydraulic Control Unit (HCU).
- ABS Module.
- Wheel Sensors.
- Pedal Travel Switch.
Operation
When the brakes are applied, fluid is forced from the master cylinder outlet ports to the hydraulic control unit (HCU) inlet ports. This pressure is transmitted through four normally open solenoid valves contained inside the HCU, then through the outlet ports of the HCU to each wheel. The primary (rear) circuit of the master cylinder feeds the right front and left rear brakes. The secondary (front) circuit of the master cylinder feeds the left front and right rear brakes. If the ABS module senses that a wheel is about to lock, based on wheel speed sensor data, it pulses the normally open solenoid valve closed, for that circuit. The prebents any more fluid from entering that circuit. The ABS module then looks at the sensor signal from the affected wheel again. If that wheel is still decelerating, it opens the normally closed solenoid valve for that circuit. This dumps any pressure that is trapped between the normally open valve and the brake back to the reservoir. Once the affected wheel comes back up to speed, the ABS module returns the valves to their normal condition allowing fluid flow to the affected brake.
The ABS module monitors the electro-mechanical components of the system. Malfunction of the anti-lock brake system will cause the ABS module to shut off or inhibit the system. However, normal power assisted braking remains. Malfunctions are indicated by one or two warning indicators inside the vehicle.
Loss of hydraulic fluid in the HCU reservoir will disale the anti-lock system.
The 4-wheel anti-lock brake system is self monitoring. When the ignition switch is placed in the RUN position, the ABS module will perform a preliminary self check on the anti-lock electrical system indicated by a three to four second illumination of the amber ANTI-LOCK indicator in the instrument cluster. During vehicle operation including normal and anti-locking braking, the ABS module monitors all electrical anti-lock functions and some hydraulic operations.
In most malfunctions of the anti-lock brake system, the amber ANTI-LOCK BRAKE and/or red BRAKE indicator(s) will be illuminated. The sequence of illumination for these warning indicators combined with the symptoms, can determine the appropriate diagnostic test to perform. However, most malfunctions are recorded as a coded number us the ABS module memory and assist in pinpointing the component needing service.