Inspecting Combination Vehicles
What this domain tests
This domain tests the pre-trip and on-road inspection of a combination vehicle: tractor, trailer, coupling, air brakes, lights, tires, wheels, suspension, cargo and securement. The exam wants a systematic walk-around, not a quick glance at the tractor.
Core rules
- Start with a complete walk-around. Check for leaks, damaged air lines, cracked hoses, loose glad hands, chafed electrical cords, and missing or broken safety latches.
- Inspect the coupling: fifth-wheel jaws closed around the kingpin, locking lever secured, no gap between the fifth wheel and trailer apron, and landing gear fully raised.
- Check tires and wheels: front tread at least 4/32 inch, other tires at least 2/32 inch, no cuts, bulges, exposed cords, missing lug nuts, cracked rims or oil leaks.
- Test the air brake system: leakage, low-pressure warning, spring-brake application, and trailer brake hold. Check that the ABS lamps self-check and then go out.
- Check lights: headlights, turn signals, four-way flashers, brake lights, clearance lights, trailer marker lights and license plate light. Use a helper or reflection if needed.
- Inspect cargo securement and weight distribution before moving; recheck tiedowns, straps and load shifts after the first miles and during stops.
Worked example
During the walk-around, a driver finds the trailer emergency air line chafed through the outer layer near the glad hand. It is not leaking yet. The correct decision is to replace or repair the line before dispatch. Tape or a temporary wrap does not restore the line's strength; a failure could trigger the tractor protection valve, apply trailer spring brakes or cause a breakaway loss of control.
Traps
Inspecting only the tractor, skipping the trailer brake test, ignoring a trailer ABS lamp, missing a fifth-wheel gap, and checking lights without confirming trailer circuits are common. Do not sign a pre-trip report with a known defect that affects safe operation.