Vehicle Controls
This domain covers the controls unique to heavy vehicles: multi-range gearboxes, retarders and engine brakes, differential locks, air-assisted steering and the gauge set that tells you the vehicle is safe to move.
Transmission and auxiliary braking
- Range and splitter changes are preselected before the move; do not split under full load or mid-corner.
- The retarder or engine brake is your main speed control on long descents. Switch it off on wet, oily or icy surfaces, because braking only the drive wheels can start a jackknife.
- The differential lock is for low-speed, straight-line use on loose ground only. Leaving it engaged on tarmac damages the driveline and ruins steering.
- Do not dry-steer a stationary vehicle with air-assisted steering; the tyres and steering gear take the load.
Gauges and warnings
- Air pressure gauges, coolant temperature, oil pressure, tachometer, fuel, and the warning lamp panel.
- Any red lamp, or an air gauge that will not build, means stop and investigate - not "continue to the workshop".
Worked example
On a long mountain descent with a full load, the air gauge sits steady, but the service brakes begin to feel weak after repeated applications. Reasoning: this is brake fade from heat, not an air leak. Select a lower gear before the next bend, bring the retarder in, and use short firm brake applications with time for the drums to recover. If fade continues, stop in a safe place and let the brakes cool; never pour water on hot drums.
Traps that cost marks
Coasting downhill in neutral to save fuel; riding the clutch at traffic lights; engaging the differential lock to "get better grip" on tarmac.