Anti-lock Braking System
An anti-lock braking system prevents sustained wheel lock during hard braking so the tires can retain useful steering and braking force.
Sensors used
ABS primarily uses wheel-speed sensors to detect when one or more wheels are approaching lock. Brake-pressure information and vehicle-motion estimates can support control, but external road-perception sensors such as cameras, radar and lidar are not required for the core anti-lock function.
Wheel-speed, yaw and inertial measurements are covered in Positioning and Vehicle Motion. Their installation, fault detection and post-repair integrity follow the same system principles described in Calibration, Cleaning and Sensor Health.
What ABS controls
Each wheel-speed sensor reports how quickly its wheel is rotating. When the controller detects that a wheel is approaching excessive slip, hydraulic valves and a pump reduce, hold and restore brake pressure at that wheel. The cycle can repeat many times during one stop.
The driver may feel rapid pedal pulsation and hear the pump or valves. That feedback normally indicates that the system is working, not that the brakes have failed.
Why wheel lock matters
A locked tire slides rather than rolls. On most paved surfaces, controlled slip provides more usable braking and lateral grip than a sustained skid. By preventing lock, ABS helps the driver steer around an obstacle while braking.
ABS cannot create grip. Stopping distance still depends on speed, tires, surface, slope, load and driver reaction. On loose gravel or deep snow, a locked wheel can build a wedge of material ahead of it, so ABS may not produce the shortest possible straight-line stop even though it generally preserves control.
How the driver should brake
In a modern passenger vehicle with functioning ABS:
- press the brake pedal firmly;
- maintain pressure;
- steer toward the available escape path if necessary;
- do not pump the pedal.
Pumping can interrupt the controller's faster pressure modulation and increase stopping distance.
Foundation for other systems
ABS hardware is part of a larger brake-control platform. Electronic stability control can brake individual wheels to correct yaw, traction control can reduce wheel spin during acceleration, and automatic emergency braking can request deceleration without a pedal input.
These functions share sensors and actuators but solve different problems. A fault that disables one function may leave basic hydraulic braking available while also illuminating several related warning lamps.
EV considerations
An EV normally blends regenerative and friction braking. During emergency braking, low battery temperature, a full battery or traction limits can reduce available regeneration, and the friction brakes provide the remainder. ABS coordinates wheel slip regardless of where the requested deceleration originated.
Drivers should treat an illuminated ABS warning as a safety fault and follow the vehicle manual. Normal brakes may still operate, but anti-lock and connected stability functions can be unavailable.