Antilock Braking Systems (ABS)
What this domain covers
ABS questions test what the system does and does not do. ABS prevents wheel lock-up during hard braking, helping you retain steering control, but it does not repeal physics or automatically shorten every stop.
Core rules
- ABS modulates brake pressure at individual wheels. It does not replace normal braking; you still apply firm, steady pressure.
- The dash ABS lamp should go out after the self-test. If it stays on, ABS is disabled but base brakes remain. The vehicle may be driven to repair, but with caution.
- Trailers have their own ABS lamp, often amber and mounted on the trailer side. A trailer ABS lamp that stays on means trailer ABS is not working.
- Do not pump the brakes in an ABS-equipped vehicle. Hold firm pressure and steer.
- On loose gravel or deep snow, ABS may lengthen stopping distance because a locked wheel can dig in. The trade-off is steerability.
Worked example
You are driving a loaded combination at 45 mph on wet pavement. A car stops suddenly. You brake firmly and hold; the pedal pulses and the ABS lamp flickers. You steer around the car. That is correct: maintain pressure and steer. Releasing and pumping the brakes would cancel the ABS cycle and reduce control.
Traps that cost marks
Pumping the brakes; thinking ABS always stops shorter; ignoring a trailer ABS lamp; assuming a steady ABS lamp means no brakes; and testing ABS by slamming brakes in traffic. Also, do not confuse ABS with automatic traction control.