A converter dolly has one or two axles and its own fifth wheel. The rear semitrailer rests on that fifth wheel while the dolly hitches to the trailer ahead, which is how a double is put together.
Triples come apart from the back forward. First pull the dolly out from under the third trailer and unhitch that dolly, then uncouple the rest of the rig the same way you would a set of doubles.
Check the second trailer with a pull test against its kingpin, which shows whether the jaws of the dolly's fifth wheel have locked. Then verify by sight that there is no space between the upper and lower fifth wheel and that the jaws are shut around the kingpin.
Many converter dollies are built without spring brakes, so once uncoupled they may have nothing holding them in place. Before you leave a dolly, set its spring brakes if it has them or chock its wheels so it cannot roll away.
If the dolly is still under the rear semitrailer when you unlock the pintle hook, the tow bar can snap upward. That can injure you and makes the dolly very hard to hook up again.
Chocks wedged against the second trailer's wheels keep it from rolling while you back the converter dolly under it. Neither a helper nor a chain can reliably hold a heavy trailer in place.
An A-dolly hooks to the trailer ahead with a single drawbar, while B-dollies and C-dollies use a double drawbar attached at two points, which gives a steadier connection.
The manual describes the procedures for the more common combinations only, because there are too many types in use to cover them all. For any other setup, learn the correct method from the manufacturer or the owner.
Before the dolly's air lines are disconnected, the shut-offs at the back of the front trailer must be closed. If they were left open, air would rush out of the disconnected hoses and bleed down the brake system.
The dolly comes off last: you lower its landing gear, remove the safety chains, set its brakes or chock its wheels, and release the pintle hook. Only then do you slowly pull the tractor and first trailer away from it, which completes the job.
A second trailer can be held still either by its own spring or emergency brakes or by chocks placed at its wheels, so you may use whichever of these you have. If you have any doubt about the trailer's brakes, chock the wheels as well.
With the rig parked and braked, you lower the second semitrailer's landing gear enough to take some weight off the dolly, before closing the air shut-offs and pulling the dolly out. Releasing the pintle hook and lowering the dolly's own landing gear come later, when you uncouple the dolly itself.
Every wheel in a combination vehicle must have brakes, including the wheels on converter dollies. A dolly therefore carries its own air brakes and its own air tank, which is filled by the tractor's compressor through the air lines.
Uncoupling starts with stopping the whole rig on firm, level ground with every unit lined up straight, then setting the parking brakes so nothing can move. A straight, level position keeps the units aligned so the dolly can later be pulled straight out from under the trailer.
The trailer should sit just below the center of the dolly's fifth wheel, so the fifth wheel lifts it a little as the dolly slides under; if the trailer is too high, it may not couple correctly.
A converter dolly is a unit with one or two axles and a fifth wheel that lets a semitrailer be coupled to the back of a tractor-trailer, making a double or triple combination. The name can fool you, since it has no connection to converting electrical power.
The idea is to shift some of the trailer's weight onto the landing gear, not to lift the trailer completely off the dolly. With that load removed, the dolly's fifth wheel can be released and the dolly pulled clear more easily.
Charging the trailer's air tank through the emergency line and then disconnecting that line makes the trailer's emergency brakes come on and keep it from rolling. If you have doubts about those brakes, chock the wheels as well.
Air for the second trailer has to pass through the back of the first one, so the first trailer's rear valves must be open. The valves at the back of the last trailer stay closed so the air is held in the system instead of escaping.
With triples you start at the back: join the third trailer to the second using the doubles method, then couple the tractor to the first trailer and connect that unit to the second-and-third pair.
While the dolly still carries the trailer's weight, releasing the pintle hook can make the tow bar fly up suddenly, which can injure you and makes recoupling very hard. Pull the dolly out from under the trailer first and only then unhook it.
A converter dolly is a short coupling unit with one or two axles topped by a fifth wheel, much like the back of a tractor, so a semitrailer's kingpin can lock onto it. Its drawbar eye hooks to the pintle hook of the trailer ahead.
Set the trailer brakes and crank the landing gear up only until its pads leave the ground. Keeping it slightly raised protects the gear from damage if the trailer moves while you test the hookup.
Lowering the gear far enough to take some of the load off the dolly keeps the trailer supported while making the dolly easier to release and roll out. The gear should never stay up, and there is no need to jack the trailer entirely off the dolly.
Charging the trailer's tank fills it with air, and when you then disconnect the emergency line, that stored air sets the emergency brakes on the trailer, provided the slack adjusters are correctly adjusted. This keeps the trailer from moving while you finish the coupling; a generator would be no use, since these brakes run on air rather than electricity.
When the dolly is coupled correctly, the two fifth wheel surfaces sit tight against each other with no space, and the locking jaws are closed on the kingpin. If you can see daylight between them, the coupling is faulty and must be redone before you move.
Letting the air out of the dolly's tank through its petcock releases the dolly brakes. If the dolly has spring brakes, you use its parking brake control instead.
After the converter dolly is backed under the rear trailer, crank that trailer's landing gear up just enough to clear the ground, so the gear will not be damaged if the trailer moves during the tug test. You raise it all the way only after the coupling has been checked and the lines are connected.
Every question in this set has its own page with the correct answer and an explanation.
