Antilock Braking Systems (ABS)
What this domain tests
This domain covers how ABS prevents wheel lockup on tractors and trailers, what the dash and trailer lamps mean, and how to brake and steer when ABS activates.
Core rules
- Wheel speed sensors feed the ECU, which pulses modulator valves to prevent lockup. ABS does not replace safe following distance or reduce stopping distance on all surfaces.
- ABS keeps wheels rolling so you can steer during hard braking. Brake normally and steer; do not pump the brake pedal.
- The tractor dash lamp should come on at start and go out. A trailer-mounted ABS lamp should also self-check.
- A steady ABS lamp means the system has a fault; normal brakes remain, but ABS may not work. Get it repaired.
- Do not release the brake when ABS pulses; hold firm pressure and steer.
Worked example
You start the tractor and the ABS lamp stays on after the self-check. You also notice the trailer ABS lamp remains lit. The vehicle still has conventional brakes, but wheel-lock protection may be disabled. The correct action is to note the fault, perform your pre-trip brake checks, and have the system serviced before relying on ABS. The reasoning: ABS is an add-on to the base brake system, so a lamp does not mean no brakes, but it does mean the anti-lock function is degraded.
Traps that cost marks
- Pumping the brake pedal during an ABS event.
- Releasing brake pressure when you feel pulsing.
- Assuming ABS always shortens stopping distance.
- Ignoring a trailer ABS lamp because the tractor lamp is off.