Converter Dollies and Air Line Connections
Converter Dollies and Air Line Connections
This domain tests how a converter dolly turns a semitrailer into a full trailer and how its air and electrical connections are made and verified. Expect questions on line colors, safety chains, and the crossed-line check.
Dolly parts and hookup
- A converter dolly carries a fifth wheel for the rear trailer's kingpin and a drawbar with a lunette eye that locks into the pintle hook on the rear of the lead trailer.
- The dolly also has safety chains, two air lines, and an electrical cord.
- Check the pintle hook latch is closed, the safety chains are attached, glad-hand seals are present, and the lines are supported so they cannot rub or drag on the dolly frame.
Air line colors and tests
- Red is the emergency or supply line. Blue is the service line.
- The dolly's spring brakes apply when the emergency line is disconnected. That is how you test breakaway protection.
- Crossed-line check: pull forward slowly and apply the trailer brakes. If the trailer does not hold, suspect crossed lines.
Worked example
After hookup, the dolly's brakes will not release when the tractor parking brake valve is pushed in. Reasoning: the red line was connected to the blue glad hand on the dolly, so the emergency circuit never received supply air and the spring brakes stayed set. The fix is to reconnect red-to-red and blue-to-blue, recharge the system, and retest before moving.
Traps
- Confusing the red and blue glad hands.
- Leaving the safety chains off because the pintle looks secure.
- Allowing the air lines to drag on the dolly frame until they chafe through.
- Skipping the breakaway test on the dolly.