Tractor and trailer vehicle technology, brakes and coupling
What this domain tests
This domain tests the tractor-trailer as a technical unit: service brake, parking brake, trailer braking system, coupling, safety connections, lights and daily checks. The exam wants you to know what must be connected and tested before the combination moves.
Braking systems
- The tractor needs a service brake that works on all wheels and a parking brake that holds the combination on a slope.
- A trailer above the relevant mass threshold must have its own service brake. Small trailers may use an overrun brake, which applies when the trailer pushes into the tractor during braking.
- Larger trailers often use hydraulic or pneumatic brakes linked to the tractor. The brake line must be connected, leak-free and charged before driving.
- A breakaway cable or safety chain is a secondary safety device. If the coupling separates, it should apply the trailer brake or keep the trailer attached.
Coupling and daily checks
Check that the hitch, ball or drawbar is the correct type and that the coupling lock is fully engaged. Confirm the safety chain or breakaway cable is attached to a suitable point, not to a weak part. Plug in the electrical cable and test brake lights, indicators, tail lights and number-plate light. Check tyre condition and pressure on both tractor and trailer. Lower the jockey wheel and secure it. Test the brakes gently before joining traffic.
Worked example
You hitch a trailer with an overrun brake and a breakaway cable. The coupling clicks but you cannot see the locking indicator. The trailer lights work. Reasoning: a click is not proof of a locked coupling. You must check the lock visually or by lifting the release handle, fit the breakaway cable to the tractor, and test that the trailer brake applies when the overrun coupling is compressed. If the lock is doubtful, do not drive; an uncoupled trailer can break away and the cable is only a backup.
Traps that cost marks
- Driving on the tractor's brakes alone when the trailer brake line is not connected.
- Assuming a clicking hitch is definitely locked.
- Attaching the breakaway cable to a weak or moving part.
- Forgetting to test trailer lights after coupling.
- Ignoring unequal tyre pressures or damaged trailer tyres.