Combination Vehicles Practice Questions

Explanation: The tractor protection valve keeps air in the tractor if the trailer breaks away or leaks badly. It closes automatically when pressure falls to between 20 and 45 psi, depending on the make.

Explanation: Never park using the trailer hand valve. All the air might leak away and the brakes release. Use the parking brakes.

Explanation: Set the trailer height so the trailer will be lifted slightly as the tractor backs under it. Too high and it may not couple correctly; too low and the tractor may strike the trailer nose.

Explanation: Use a flashlight to check that there is no space between the upper and lower fifth wheel, and that the locking jaws are closed around the shank of the kingpin, not its head.

Explanation: The tug test: with the trailer brakes locked, pull gently forward in a low gear to make sure the trailer is locked to the tractor.

Explanation: In a turn, the rear wheels of a combination follow a shorter path than the front wheels. Longer vehicles off-track more, so allow room at corners.

Explanation: Trailer skids usually come from locked trailer wheels. Release the brakes to let the wheels grip again, and do not use the trailer hand brake to straighten the rig.

Explanation: With crossed lines, supply air goes to the service line. A trailer with spring brakes would stay braked, but an older trailer without spring brakes could be driven away with no trailer brakes.

Explanation: Raise the landing gear all the way and secure the crank handle in its bracket. Partly raised gear can catch on railroad tracks or bumps.

Explanation: Lower the landing gear until it makes firm contact with the ground. Don't lift too much weight off the tractor, which makes it harder to recouple.

Explanation: Keep the cargo low and centered, and go slowly around turns and ramps. Posted speeds can be too fast for loaded trucks.

Explanation: Sudden steering movements are amplified at the rear trailer and can roll it over. Steer gently, and look far enough ahead to avoid sudden moves.

Explanation: If the emergency (supply) line breaks or the trailer breaks away, the loss of pressure puts the trailer emergency brakes on.

Explanation: Connect the air lines, supply air to charge the trailer, then pull out the air supply knob so the trailer brakes are locked before you back under it.

Explanation: Set the tractor parking brake, supply air to the trailers, open the emergency line shut-off at the back of the last trailer: you should hear air escaping. Then close it and check the service line the same way.

Explanation: With the trailer air supply knob pulled out, the trailer emergency brakes should be on. Pull forward gently; the trailer should not move.

Explanation: Don't use the trailer hand valve while driving: braking only the trailer can make it skid. Use it to test the trailer brakes, not for normal stops or parking.

Explanation: Dummy couplers or a dead-end coupler keep dirt and water out of the lines when they are not connected to a trailer.

Explanation: Shut-off valves let air pass through to the next trailer, so they are open on all trailers except the last, where they are closed.

Explanation: Put the heaviest trailer first, right behind the tractor. A lighter trailer at the rear is less likely to whip and roll over.

Explanation: Check the slide is locked and positioned so the tractor frame won't hit the landing gear and the cab won't hit the trailer during turns.

Explanation: Trailers built on or after March 1, 1998 must have a yellow ABS malfunction lamp on the left side, near the rear corner.

Explanation: Poor lubrication causes friction between the fifth wheel and trailer that can make steering difficult.

Explanation: Bobtail tractors have little weight on the drive axles and can take longer to stop than a loaded tractor-trailer because of reduced traction.

Explanation: Park on firm, level ground that can support the trailer's weight on its landing gear, then chock and secure the vehicle.

Explanation: Put the tractor directly in front of the trailer, never back under at an angle, which could push the trailer sideways and break the landing gear.

Explanation: As pressure falls to 20–45 psi, the tractor protection valve should close, popping the supply knob from 'normal' to 'emergency' and protecting the tractor's air if the trailer leaks.

Explanation: Turning wide as you complete the turn is better than swinging wide to the left before starting, because it keeps drivers from passing you on the right.

Explanation: Backing a car or straight truck, you turn the top of the wheel the way you want to go. With a trailer, you turn the wheel in the opposite direction, then turn it back to follow the trailer.

Explanation: Shut the engine off so you can hear the brakes. Apply and release the trailer brakes and listen for them to apply and for air to escape on release.

Explanation: Trailer-only ABS makes the trailer less likely to swing out. If you lose steering control or start a tractor jackknife, let up on the brakes until you regain control.

Explanation: Use the trailer hand valve only to test the trailer brakes. Using it while driving can make the trailer skid.

Explanation: Trailers without spring brakes have emergency brakes that work from stored air. They hold only as long as there is air in the tank, so always chock the wheels when parking them.

Explanation: Trailers have yellow ABS malfunction lamps on the left side, either on the front or rear corner.

Explanation: Check signposts or the signal housing for emergency notification information, and give the crossing location using all identifiable landmarks, especially the DOT number if posted.

Explanation: For older units it may be hard to tell. Look under the vehicle for the ECU and wheel speed sensor wires coming from the back of the brakes.

Explanation: A single-axle tractor pulling a long trailer whose landing gear is set for a tandem-axle tractor has reduced clearance and can hang up on raised crossings.

Explanation: Sudden lane changes can tip a trailer. At night, drive slowly enough to see obstacles in time to change lanes or stop gently.

Explanation: Piling up cargo raises the center of gravity. Fully loaded rigs are ten times more likely to roll over in a crash than empty rigs.

Explanation: The foot brake sends air to all brakes, including the trailers. There is much less danger of a skid or jackknife using just the foot brake.

Explanation: Water and oil buildup can keep the brakes from working right. Drain each tank every day; even with automatic drains, open the drains to make sure.

Explanation: Large combination vehicles take longer to stop when empty. Stiff springs and strong brakes give poor traction and may cause bouncing, making wheel lockup very easy.

Explanation: For a loaded trailer, a few extra turns in low gear take some weight off the tractor. This makes it easier to unlatch the fifth wheel and to couple next time.

Explanation: Look into the back of the fifth wheel to make sure the jaws have closed around the shank of the kingpin, not the head.

Explanation: Older vehicles may have a lever. The "normal" position is for pulling a trailer, and the "emergency" position shuts off the air and applies the trailer emergency brakes.

Explanation: Any time you brake hard, check the mirrors to make sure the trailer is staying where it should be. Once it swings out of your lane, a jackknife is very difficult to prevent.

Explanation: First, the emergency line supplies air to the trailer air tanks. Second, it controls the emergency brakes on combination vehicles.

Explanation: Hang the electrical cable with the plug down to keep moisture out, and support the lines so they are not damaged while driving the tractor.

Explanation: Try to back in a straight line. If you must back on a curve, back toward the driver's side so you can see.

Explanation: Pull forward until the fifth wheel comes out from under the trailer, but stop with the tractor frame under the trailer. This keeps the trailer from falling if the landing gear collapses or sinks.

Explanation: If the lines are crossed, supply air goes to the service line instead of charging the trailer tanks, so there is no air to release the spring brakes. Check the air line connections.

Explanation: Move forward slowly and apply the trailer brakes with the hand control (trolley valve), if equipped. You should feel the brakes come on.

Explanation: Brake as you always have: use only the force needed, brake the same way whether ABS is on the tractor, the trailer or both, and back off if needed to stay in control.

Explanation: Back slowly until the fifth wheel just touches the trailer, without hitting it. Then secure the tractor and check trailer height before connecting air lines.

Explanation: The service line, also called the control line or signal line, carries air controlled by the foot brake or trailer hand brake.

Explanation: Air lines should be properly connected, free of leaks and damage, and secured with enough slack for turns.

Explanation: Steer the front end wide enough to clear the curb, but keep the rear close to the curb so other drivers cannot pass you on the right.

Explanation: Because of rearward amplification, the last trailer of triples can be rolled 3.5 times as easily as a five-axle tractor.

Explanation: A service line leak may go unnoticed until you brake. Then the leak quickly lowers tank pressure, and if it drops low enough the trailer emergency brakes come on.

Explanation: Correct drift immediately. As soon as the trailer gets off the proper path, turn the top of the steering wheel in the direction of the drift.

Explanation: Use the lowest reverse gear and back slowly to avoid hitting the kingpin too hard. Stop when the kingpin is locked into the fifth wheel.

Explanation: The manual states that more than half of truck driver deaths in crashes are the result of truck rollovers.

Explanation: Check that all locking pins are present and locked, and that the fifth wheel is not so far forward that the frame hits the landing gear or the cab hits the trailer in turns.

Explanation: When it closes, the tractor protection valve stops air leaving the tractor and lets air out of the trailer emergency line, so the trailer emergency brakes come on.

Explanation: The tractor protection valve keeps air in the tractor or truck brake system if the trailer breaks away or develops a bad leak.

Explanation: When trailer wheels lock up, the trailer tends to swing around. This is more likely when the trailer is empty or lightly loaded.

Explanation: The trailer air supply control valve in the cab lets you open and shut the tractor protection valve.

Explanation: Backing gently eases pressure on the locking jaws. Setting the parking brakes while the tractor pushes against the kingpin holds the rig with pressure off the jaws.

Explanation: Before coupling, the fifth wheel should be tilted down toward the end of the tractor, jaws open, and the safety unlocking handle in the automatic lock position.

Explanation: Following far enough back lets you avoid sudden steering. Use at least 1 second for each 10 feet of vehicle length, plus another second over 40 mph.

Explanation: Use the low gear range (if equipped) to begin raising the landing gear, then switch to high once it is free of weight. Raise it all the way and secure the crank handle.

Explanation: Rearward amplification describes the crack-the-whip effect. A value of 2.0 means the rear trailer is twice as likely to turn over as the tractor.

Explanation: The rubber seal on each coupler prevents air from escaping. Clean the couplers and rubber seals before making a connection.

Explanation: Newer trailers have spring brakes, but converter dollies and trailers built before 1975 are not required to have them.

Explanation: The trailer should be raised slightly by the tractor as you back under it. If it is too low, the tractor may strike and damage the trailer nose.

Explanation: Each trailer and converter dolly has one or more air tanks. They are filled by the emergency (supply) line from the tractor.

Explanation: The trailer hand valve, also called the trolley valve or Johnson bar, works the trailer brakes.

Explanation: Longer vehicles off-track more. The rear wheels of the trailer off-track more than the tractor's, and with more than one trailer, the last trailer's rear wheels off-track the most.

Explanation: ABS only activates when wheels are about to lock up. It does not necessarily shorten stopping distance but helps you keep the vehicle under control.

Explanation: Secure the vehicle: transmission in neutral, parking brakes on, engine off, and take the key so no one moves the truck while you are under it.

Explanation: Trailers with low underneath clearance can get stuck on raised crossings, including lowboys, car carriers, moving vans and possum-belly livestock trailers.

Explanation: Service line pressure changes with how hard you brake. It is connected to relay valves, which apply the trailer brakes faster than would otherwise be possible.

Explanation: Press the two seals together with the couplers at a 90 degree angle, then turn the glad hand attached to the hose to join and lock them.

Explanation: Put the tractor directly in front of the trailer. Backing at an angle might push the trailer sideways and break the landing gear.

Explanation: Air stored in a trailer without spring brakes can leak away and release the brakes. Use wheel chocks to keep it from moving.

Explanation: Check for enough clearance between the rear of the tractor frame and the landing gear so the tractor will not hit the landing gear in a sharp turn. Also check clearance between the tractor tires and the trailer nose.