Coupling and Uncoupling a Tractor-Trailer

Coupling means connecting a tractor to a semi-trailer. It looks simple, but a bad coupling can drop a trailer in the yard or lose it on the road. A trailer that falls can crush anyone near it and wreck the tractor. Your job is to prove the connection is good and never to assume it is.

This work is part of the CDL skills test for many drivers and a daily task for most tractor-trailer drivers. It takes practice with a qualified trainer. Vehicles differ, so follow your carrier's procedure and the maker's instructions where they add to these steps.

What You Will Be Able to Do

• Name the parts of the fifth wheel and trailer that make the connection.
• Couple a tractor to a semi-trailer in the right order.
• Run a tug test and explain which brakes are set.
• Inspect a coupling for a gap and for jaws closed on the kingpin shank.
• Uncouple a trailer so it stays standing and the tractor stays safe.

The Parts That Make the Connection

Coupling Step by Step

Step 1: Inspect the Fifth Wheel

Look for damaged or missing parts and secure mounting to the frame. Confirm the fifth wheel plate is greased as required, because a dry plate adds friction and can cause steering problems. The jaws must be open, and the release handle must be in the automatic lock position. On a sliding fifth wheel, confirm it is locked in place. Check that the fifth wheel is tilted down toward the rear of the tractor.

Step 2: Inspect the Area and Secure the Trailer

Look around the trailer and the path you will back through. Check for people, debris and soft ground. Make sure the trailer's wheels are chocked or its spring brakes are on. Check that the cargo will not shift when the trailer is lifted.

Step 3: Position the Tractor

Line the tractor up directly in front of the trailer. Do not back under it at an angle, because that can push the trailer sideways and break the landing gear. Use your mirrors and look at both sides.

Step 4: Back Until You Just Touch

Back slowly until the fifth wheel just touches the trailer. Do not hit it. Set the tractor's parking brake and put the transmission in neutral.

Step 5: Check Trailer Height

The trailer should sit slightly lower than the center of the fifth wheel, so that the tractor raises it a little when you back under. Use the landing gear crank to adjust the height. If the trailer is too high, it may not couple correctly and the kingpin may ride over the jaws. If it is too low, the tractor can hit and damage the nose of the trailer.

Warning: A kingpin that rides on top of the closed jaws is called a high hook. It can feel coupled and then let the trailer drop. This is why you look with a flashlight every time.

Step 6: Connect the Air Lines

Connect the emergency line (red) and the service line (blue) to the trailer's glad hands. Check that the seals are good and the connections are tight. Support the lines so they will not be crushed while you back under.

Step 7: Supply Air and Check the Trailer Brakes

1. Get in the cab and push in the trailer air supply knob.
2. Let the pressure return to normal.
3. Check that the trailer's brakes are working and the lines are not crossed.
4. Pull the trailer air supply knob out to apply the trailer's spring brakes.

Step 8: Back Under the Trailer

With the trailer brakes locked, use your lowest reverse gear and back slowly. Do not slam the kingpin into the jaws. Stop when you feel the kingpin lock into the fifth wheel.

Step 9: Do the Tug Test

The tug test proves the jaws have hold of the kingpin.

1. Raise the landing gear slightly off the ground.
2. Keep the trailer's brakes locked and release the tractor's parking brake.
3. Put the tractor in a low forward gear.
4. Pull forward gently against the trailer's locked brakes.

The trailer should not separate from the tractor. Do this more than once if you are in doubt. The test works because the trailer's brakes are holding it back while the tractor pulls.

Step 10: Secure the Tractor

Put the transmission in neutral and set the parking brake. Shut off the engine and take the key with you so nobody moves the vehicle while you are under it.

Step 11: Inspect the Coupling Up Close

Use a flashlight. Look, do not guess.

• No gap: There must be no space between the upper and lower fifth wheel. A gap means something is wrong, and the kingpin may be riding on top of the jaws.
• Jaws on the shank: Get under the trailer and look at the back of the fifth wheel. The jaws must be closed around the shank of the kingpin, not its head.
• Locking lever: The lever must be in the locked position.
• Safety latch: It must be over the lever. On some fifth wheels you set it by hand.

If anything is wrong, do not drive until it has been fixed.

Step 12: Connect the Electrical Cord

Plug in the pigtail and fasten its safety catch. Check the air and electrical lines for damage. Make sure they will not hit moving parts, and that they have enough slack for turns.

Step 13: Raise the Landing Gear

Use the low speed gear to start and then the high speed gear, if the crank has one. Raise the gear all the way. Never drive with the landing gear only part way up, because it can catch on railroad tracks or other things. Secure the crank handle.

Check that the tractor frame clears the landing gear in a turn, and that the tractor will not strike the trailer.

Step 14: Remove the Chocks

Remove the trailer's wheel chocks and store them. Then do your light and brake checks before you move.

Uncoupling Step by Step

Step 1: Choose the Spot

Pick firm, level ground that can hold the landing gear. Hot asphalt and soft dirt can let the legs sink. Line the tractor up with the trailer, because pulling out at an angle can damage the landing gear.

Step 2: Ease the Pressure on the Locks

Set the trailer's brakes by pulling the supply knob. Ease pressure off the fifth wheel locks by backing up gently. Then put on the tractor's parking brake while the engine pushes the tractor toward the kingpin.

Step 3: Secure the Trailer

If the trailer does not have spring brakes or you are on a grade, chock its wheels.

Step 4: Lower the Landing Gear

Lower the gear until it makes firm contact with the ground. If the trailer is empty, lower it until the gear just takes the weight. Do not raise the trailer off the fifth wheel. That makes it harder to unlatch now and harder to couple the next time. Secure the crank handle.

Step 5: Disconnect the Lines

Disconnect the air lines and the electrical cord. Hang them on the tractor's hangers or plug them into the dummy couplers, so that they stay clean and do not drag.

Step 6: Release the Fifth Wheel

Raise the release handle to the open position. Keep your legs and feet clear of the tractor's drive tires. A sudden movement of the tractor could cause serious injury.

Step 7: Pull Partly Clear

Pull forward until the fifth wheel clears the trailer's apron, but stop with the tractor frame still under the trailer. This way, if the landing gear collapses or sinks, the trailer lands on the frame and not on the ground.

Step 8: Check the Trailer

Set the tractor's parking brake and put the transmission in neutral. Get out and look. The landing gear must be holding the trailer, with no sinking and no damage. If it is not holding, couple back up and find a better spot.

Step 9: Pull Clear

Release the parking brake and slowly pull forward until the tractor is clear of the trailer.

Sliding the Fifth Wheel or Tandems

Sliding fifth wheels and sliding trailer tandems let you move weight between axles. Check that the locking pins are fully seated after every move. Tug against the set brakes to confirm the slider has locked. A slider that is not locked can run to the end of its rails during a hard stop or a start.

Common Mistakes

Check Your Understanding

You have backed under the trailer and heard the jaws lock. Which brakes are set for the tug test?

The trailer's brakes are locked and the tractor's parking brake is released. You pull forward gently in a low gear, and the trailer's brakes hold it back. If the tractor's brakes were set, the tractor could not pull at all.

With your flashlight you see a thin line of daylight between the fifth wheel and the apron. What does it mean?

There should be no gap. The kingpin may be riding on top of the jaws. Do not drive. Uncouple, fix the problem and couple again, then inspect it again.

You are uncoupling on a hot afternoon on an asphalt lot. What should you look out for?

Soft asphalt can let the landing gear sink. Choose firm ground or use a support under the legs if your carrier allows it. Stop with the tractor frame still under the trailer and confirm it is holding before you pull clear.

The trailer sits higher than the fifth wheel. What do you do before you back under?

Lower the trailer with the landing gear until it sits slightly below the center of the fifth wheel, so the tractor lifts it a little as you back under.

Key Takeaways

• Line up straight and back slowly. Never hit the kingpin hard.
• Set the trailer slightly lower than the fifth wheel.
• The tug test means trailer brakes locked, tractor pulling gently forward.
• Inspect with a flashlight: no gap, jaws closed on the shank, lever locked, latch in place.
• Raise the landing gear fully before you drive.
• When uncoupling, stop with the tractor frame under the trailer and check the legs before you pull clear.