Anti-lock brakes (ABS) are one of the biggest safety advances in braking. In a hard stop - especially on wet or slick roads - ABS keeps the wheels from locking up, so the tires keep gripping and you keep steering. Here is how the system does it.
Why locked wheels are bad
A locked, skidding tire actually stops the car less effectively than one that is still rolling at the edge of grip - and worse, a locked front tire cannot steer. Before ABS, a panic stop could send you sliding straight ahead with no control. ABS prevents that.
The parts that make it work
- Wheel speed sensors at each wheel constantly measure how fast that wheel is spinning.
- The ABS module (controller) watches those signals. If it sees a wheel decelerating far faster than the others - about to lock - it acts.
- The hydraulic unit and pump rapidly reduce and reapply brake pressure to that wheel, many times per second.
What you feel
When ABS activates in a hard stop, you feel a rapid pulsing in the brake pedal and may hear a buzzing or grinding from the pump. That is normal - it is the system pumping the brakes far faster than any human could. The correct technique is to press firmly and hold; let ABS do the modulating. Do not pump the pedal yourself.
When the ABS light comes on
If the ABS warning light is on, the system has a fault and has switched off - your normal brakes still work, but without anti-lock. Common causes are a dirty or failed wheel speed sensor. See our guide on the ABS warning light. Note that ABS also often requires a special bleed procedure - see brake bleeding basics. Keeping the basic brakes healthy with good pads helps the whole system perform.