Inspection of Doubles/Triples Vehicles and Converter Dollies
Domain 2 is the pre-trip inspection written for a multi-trailer rig. Beyond the standard combination checklist, the exam expects you to inspect each extra connection, lead trailer to dolly and dolly to rear trailer, plus the converter gear itself.
What the dolly adds to the walk-around
- Pintle hook and lunette eye: no cracks, elongation, or excess wear; latch closed with the safety pin in place.
- Safety chains attached, crossed, and short enough to carry the load if the pintle fails. A dangling chain is a defect.
- Dolly frame and welds, fifth-wheel mounting bolts, grease, jaw condition, and locking latch.
- Dolly tires and wheels: inflation, cuts, loose lug nuts; dolly lights and reflectors.
- Relay valve, spring-brake chamber, and air lines on the dolly free of chafing and leaks.
Air and electrical checks beyond the tractor
- Glad hands sealed and locked; red emergency line and blue service line not crossed.
- Shut-off valves open at every trailer except the last, which must be closed.
- Electrical cord plugged, supported, not dragging. Use a helper to confirm stop, turn, and marker lights on the dolly and the rear trailer.
- Confirm the last trailer's brakes apply and release. A trailer that never receives air will not roll when you release the parking valve.
Brake test
Charge to governor cut-out, shut the engine off, and watch the gauge. A combination allows a slightly larger leak than a single vehicle, roughly 3 psi per minute with brakes released and 4 psi per minute with them applied, because there is far more line and more chambers. Then test the tractor protection valve: fan the brakes down and confirm it pops out around 20-45 psi, protecting the tractor's air supply.
Worked example
A driver hooks a second trailer, releases the parking valve, and the rear trailer's wheels stay locked and drag. Both glad hands are connected. Air reaches that trailer only through the dolly, so a shut-off valve left closed at the rear of the lead trailer, or on the dolly itself, blocks the emergency line and the rear trailer's spring brakes never release. Fix: open every valve except the last one, then re-check release before moving.
Traps that cost marks
- Inspecting one trailer and signing off the whole set.
- Assuming rear lights work because the first trailer's lights work.
- Forgetting that the dolly has its own lights, tires, and brake chamber.
- Skipping the leak test because the low-air warning did not sound.