Vehicle Controls and Correct Use
What the exam tests
This domain checks that you know the controls specific to heavy vehicles and when to use them: retarders, exhaust brakes, differential locks, power take-off, ABS, suspension controls and the gearbox. Questions often ask what to do on a long descent or a slippery surface.
Core rules
- Set the seat, mirrors and steering wheel before moving; check that gauges and warning lamps are normal.
- Use a retarder or exhaust brake on long descents to reduce brake heat; select a gear that holds the speed without continual braking.
- Engage a differential lock only when needed, at low speed, and disengage it once traction returns.
- ABS prevents wheel lock but does not shorten stopping distance on all surfaces; keep steering and brake pressure applied in an emergency.
- Use power take-off only when the vehicle is stationary and the equipment is safe to operate.
Worked example
A loaded truck starts a long mountain descent. The driver selects the same low gear used for climbing, sets the retarder to a moderate setting and uses the service brake only briefly. The reasoning: engine braking and the retarder dissipate energy without heating the wheel brakes; continuous service braking can cause fade. If speed rises, the driver stops, lets the brakes cool and selects a lower gear.
Traps
Do not coast in neutral to save fuel; you lose engine braking and control. Do not leave a differential lock engaged on dry pavement; it can damage the transmission and worsen steering. Do not use a retarder as a substitute for a lower gear on a steep descent.