When a wheel starts to lock, rotational speed is decreased. The reduction of speed is detected by the wheel speed sensor. This information is interpreted by the electronic brake control module or EBCM. Being an electrical device, the EBCM cannot control the hydraulic brakes, without help.
Moving from electronics to hydraulics
The hydraulic-modulator allows an electric current to control hydraulic force. Most units include a pump and several solenoid-operated valves. A solenoid is a magnetic-coil controlled by the ABS computer. When power is applied to the solenoid, magnetic force opens and closes the valves in the modulator, controlling the flow of brake fluid.
The brake master-cylinder supplies fluid pressure when the pedal is applied. This pressure flows through the hydraulic modulator. Normally, at least one valve is used per wheel, in the modulator. Under normal conditions, a spring keeps the valves open and fluid passes right through.
Non-anti lock operation
Most of the time, the valves in the hydraulic modulator remain open. In this state, the unit does not affect brake operation. This normal-open condition is a safety feature of modern ABS. An electrical failure results in conventional brake operation, without the Anti-lock function. When such a failure occurs, the ABS light will illuminate to warn the driver. Fortunately, this does not mean brake failure. Modern vehicles have a full braking system as well as ABS.
When a brake lock-up of a wheel is detected, a solenoid closes the valve, between the master cylinder and the wheel. Simultaneously, a second stage in the valve releases pressure in the line to the wheel. This pressure may be routed to an accumulator or returned to the fluid reservoir.
Reduced pressure at the wheel causes speed to increase, which ends the lockup. A rotation speed equal in the wheels signals the valve to open again, restoring pressure to the wheel. The pump may also be used to supply additional pressure, under certain circumstances. This helps to prevent a brake pedal drop during an ABS-assisted stop. All wheels rotating at the same speed, signals the system to return to the open state.
What ABS can and cannot do
Anti-lock brakes can help keep the wheels from sliding. This will improve steering in an evasive maneuver. When tires lose traction, they will slide in the direction of motion, even when turned. Unfortunately, ABS brakes do not make a vehicle stop better or quicker.
No braking system can compensate for poor tire traction. Brakes stop the wheels but traction with the road will stop the vehicle. Worn tires cannot be compensated for with any type of braking system.
Anti-lock brake maintenance
Brake fluid absorbs moisture over time. Contaminated brake fluid will attack the metal in the hydraulic modulator. Most are constructed of aluminum, which is a highly reactive material. Many brake modulator failures result from internal corrosion, which is preventable. By properly replacing the brake fluid, most corrosion can be prevented.
When servicing the brakes, cleanliness is imperative. Pushing caliper pistons in, when replacing brake pads, can back-flush debris into the modulator. The brake fluid should always be replaced, before attempting brake service.
Love them or hate them, ABS brakes are here to stay. Understanding their operation, and a little care and maintenance can prevent a great deal of expense.