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Coupling and uncoupling a tractor-semitrailer, step by step

Combination Vehicles8 minManual section 6.4.1–6.4.2

Bad coupling is one of the most dangerous mistakes a driver can make: a trailer that is not truly locked can drop off on the road. Learn the 16 coupling steps and 10 uncoupling steps, and the reason behind each one. Rigs differ, so also learn the details of the truck you will drive.

Coupling steps 1–6: inspect, position, back under

Step 1 – Inspect the fifth wheel. Look for damaged or missing parts. Check that the mounting to the tractor is secure and the frame has no cracks. Make sure the fifth wheel plate is greased — without grease, friction between tractor and trailer can cause steering problems. Check that the fifth wheel is in position for coupling:

  • Wheel tilted down toward the rear of the tractor.
  • Jaws open.
  • Safety unlocking handle in the automatic lock position.
  • With a sliding fifth wheel, be sure the slide is locked.
  • Make sure the trailer kingpin is not bent or broken.

Step 2 – Inspect the area and chock the wheels. Make sure the area around the rig is clear. Be sure the trailer wheels are chocked or the spring brakes are on. Check that any cargo is secured so it will not move when the tractor hits the trailer.

Step 3 – Position the tractor directly in front of the trailer. Never back under at an angle — you could push the trailer sideways and break the landing gear. Check position by looking down both sides of the trailer in the outside mirrors.

Step 4 – Back slowly until the fifth wheel just touches the trailer. Do not hit the trailer.

Step 5 – Secure the tractor. Put on the parking brake and put the transmission in neutral.

Step 6 – Check trailer height. The trailer should sit a little low, so the tractor lifts it slightly as you back under. Raise or lower it as needed. Too low: the tractor may hit and damage the trailer nose. Too high: it may not couple correctly (the kingpin can ride over the jaws). Also check that the kingpin and fifth wheel are aligned.

Coupling steps 7–10: air lines, air, lock brakes, back under

Step 7 – Connect the air lines. Check the glad hand seals. Connect the tractor emergency line to the trailer emergency glad hand, and the tractor service line to the trailer service glad hand. Support the lines so they will not be crushed or caught while you back under.

Step 8 – Supply air to the trailer. In the cab, push in the air supply knob (or move the lever from "emergency" to "normal"). Wait for normal air pressure. Check for crossed air lines. Turn the engine off so you can hear the brakes. Set and release the trailer brakes and listen: you should hear the brakes move when set and air escape when released. Watch the gauge for signs of major air loss. Once you know the trailer brakes work, restart the engine and let pressure come back to normal.

Step 9 – Lock the trailer brakes. Pull out the air supply knob (or move the lever from "normal" to "emergency").

Step 10 – Back under the trailer. Use the lowest reverse gear. Back slowly so you do not hit the kingpin too hard. Stop when the kingpin locks into the fifth wheel.

Coupling steps 11–16: test, inspect, connect, raise

Step 11 – Check the connection. Raise the landing gear slightly off the ground. Pull the tractor gently forward with the trailer brakes still locked to make sure the trailer is locked on (the tug test).

Step 12 – Secure the vehicle. Transmission in neutral, parking brakes on. Shut off the engine and take the key with you so nobody moves the truck while you are under it.

Step 13 – Inspect the coupling. Use a flashlight if needed.

  • There must be no space between the upper and lower fifth wheel. If there is a gap, something is wrong — the kingpin may be sitting on top of the closed jaws, and the trailer would come loose very easily.
  • Go under the trailer and look into the back of the fifth wheel. The locking jaws must be closed around the shank of the kingpin.
  • The locking lever is in the "lock" position.
  • The safety latch is over the locking lever. (On some fifth wheels you must set it by hand.)
  • If the coupling is not right, do not drive — get it fixed.

Step 14 – Connect the electrical cord and check the lines. Plug in the electrical cord and fasten the safety catch. Check the air and electrical lines for damage and make sure they will not hit moving parts.

Step 15 – Raise the landing gear. Start in low gear range (if equipped); once the gear is free of weight, switch to high gear range. Raise it all the way up — never drive with it part way up, because it can catch on railroad tracks or other things. Secure the crank handle. With the full trailer weight on the tractor, check: enough clearance between the rear of the tractor frame and the landing gear (a sharp turn must not hit it), and between the tops of the tractor tires and the trailer nose.

Step 16 – Remove the wheel chocks and store them in a safe place.

Uncoupling a tractor-semitrailer (6.4.2)

Step 1 – Position the rig. The parking surface must be able to support the trailer's weight. Line the tractor up with the trailer — pulling out at an angle can damage the landing gear.

Step 2 – Ease pressure on the locking jaws. Shut off the trailer air supply to lock the trailer brakes. Back up gently to take pressure off the jaws (this makes the release handle easier to pull). Put on the parking brakes while the tractor is pushing against the kingpin, so the rig is held with pressure off the jaws.

Step 3 – Chock the trailer wheels if the trailer has no spring brakes or you are not sure. Air can leak out of the trailer tank and release its emergency brakes; without chocks the trailer could roll.

Step 4 – Lower the landing gear. Empty trailer: lower it until it makes firm contact with the ground. Loaded trailer: after firm contact, turn the crank in low gear a few extra turns to lift some weight off the tractor — but do not lift the trailer off the fifth wheel. This makes it easier to unlatch now and to couple next time.

Step 5 – Disconnect the air lines and electrical cable. Put the glad hands on the dummy couplers at the back of the cab (or couple them together). Hang the electrical cable with the plug down so moisture cannot get in. Support the lines so they are not damaged while you drive the tractor.

Step 6 – Unlock the fifth wheel. Raise the release handle lock and pull the release handle to "open". Keep your legs and feet clear of the rear tractor wheels in case the vehicle moves.

Step 7 – Pull the tractor partly clear. Pull forward until the fifth wheel comes out from under the trailer, then stop with the tractor frame still under the trailer. If the landing gear collapses or sinks, the frame keeps the trailer from falling to the ground.

Step 8 – Secure the tractor. Parking brake on, transmission in neutral.

Step 9 – Inspect the trailer supports. Make sure the ground is supporting the trailer and the landing gear is not damaged.

Step 10 – Pull clear. Release the parking brakes, check the area, and drive forward until the tractor is clear.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 6.4.1–6.4.2

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

Preparing your questions…

Review questions with answers

  1. Before coupling, the fifth wheel should be:
    1. Tilted down toward the rear of the tractor with the jaws open
    2. Tilted up with the jaws closed
    3. Level with the jaws closed
    4. Removed from the tractor
    Show answer
    Tilted down toward the rear of the tractor with the jaws open

    For coupling: tilted down toward the rear, jaws open, safety unlocking handle in the automatic lock position.

  2. What can happen if the fifth wheel plate is not kept greased?
    1. The trailer will be too low
    2. The ABS will fail
    3. Friction can cause steering problems
    4. The air lines will cross
    Show answer
    Friction can cause steering problems

    Without lubrication, friction between tractor and trailer can cause steering problems.

  3. Why should you never back under a trailer at an angle?
    1. It wastes fuel
    2. You might push the trailer sideways and break the landing gear
    3. It sets off the ABS lamp
    4. The glad hands will not reach
    Show answer
    You might push the trailer sideways and break the landing gear

    Backing at an angle can push the trailer sideways and break the landing gear.

  4. Before backing under, how high should the trailer be?
    1. High enough that the fifth wheel misses it
    2. Exactly level with the top of the fifth wheel jaws
    3. As high as the landing gear allows
    4. Low enough that the tractor raises it slightly when backing under
    Show answer
    Low enough that the tractor raises it slightly when backing under

    The trailer should be slightly low so the tractor lifts it a little when backing under.

  5. During coupling, what can go wrong if the trailer sits too high?
    1. It may not couple correctly
    2. The tractor will damage the trailer nose
    3. The air lines will cross
    4. The ABS lamp will come on
    Show answer
    It may not couple correctly

    Too high and it may not couple correctly; too low and the tractor may hit the trailer nose.

  6. If the trailer is too low when you back under it, what can happen?
    1. The kingpin will bend upward
    2. The landing gear will raise itself
    3. The tractor may strike and damage the trailer nose
    4. The brakes will release
    Show answer
    The tractor may strike and damage the trailer nose

    A trailer that is too low can be hit in the nose by the tractor.

  7. During coupling, why do you shut off the engine after supplying air to the trailer?
    1. To save fuel
    2. So you can hear the trailer brakes apply and release
    3. To reset the ABS
    4. So the trailer air tanks can drain
    Show answer
    So you can hear the trailer brakes apply and release

    Shut the engine off so you can hear the brakes move when applied and air escape when released.

  8. Just before you back under the trailer (step 9), what should you do with the trailer brakes?
    1. Release them with the hand valve
    2. Disconnect the emergency line
    3. Leave them as they are
    4. Lock them by pulling out the air supply knob
    Show answer
    Lock them by pulling out the air supply knob

    Lock the trailer brakes by pulling out the air supply knob (or moving the lever to "emergency").

  9. Which gear should you use when backing the tractor under the trailer?
    1. The lowest reverse gear
    2. The highest reverse gear
    3. Neutral, and let it roll
    4. Any gear; it does not matter
    Show answer
    The lowest reverse gear

    Use the lowest reverse gear and back slowly to avoid hitting the kingpin too hard.

  10. How do you check that the trailer is locked onto the tractor (step 11)?
    1. Look at the ABS lamp
    2. Push the trailer air supply knob in
    3. Raise the landing gear slightly and pull forward gently with the trailer brakes locked
    4. Back up hard against the kingpin
    Show answer
    Raise the landing gear slightly and pull forward gently with the trailer brakes locked

    With landing gear slightly off the ground, pull gently forward against the locked trailer brakes.

  11. Before you go under the trailer to inspect the coupling, why should you take the key with you?
    1. To lock the trailer doors
    2. So no one else moves the truck while you are under it
    3. To turn off the ABS
    4. Because the law requires it at all stops
    Show answer
    So no one else moves the truck while you are under it

    Shut off the engine and take the key so someone else won't move the truck while you are underneath.

  12. After coupling, how much space should there be between the upper and lower fifth wheel?
    1. About 1 inch
    2. About 1/2 inch
    3. Enough to see light
    4. None
    Show answer
    None

    There must be no space. A gap may mean the kingpin is on top of the closed jaws.

  13. If there is a gap between the upper and lower fifth wheel after coupling, what may be wrong?
    1. The kingpin may be on top of the closed jaws
    2. The air lines are crossed
    3. The trailer brakes are released
    4. The sliding fifth wheel is locked
    Show answer
    The kingpin may be on top of the closed jaws

    A gap means something is wrong; the kingpin may be riding on the closed jaws and the trailer would come loose easily.

  14. True or False: When checking the coupling, you should look into the rear of the fifth wheel to make sure the jaws have closed around the kingpin.
    1. True
    2. False
    Show answer
    True

    True. Go under the trailer and look into the back of the fifth wheel to see the jaws closed around the kingpin shank.

  15. When inspecting the coupling, the locking jaws should be closed around the:
    1. Head of the kingpin
    2. Glide plate
    3. Shank of the kingpin
    4. Landing gear
    Show answer
    Shank of the kingpin

    The jaws must close around the shank, not the head, of the kingpin.

  16. True or False: To drive, you only need to raise the landing gear until it just lifts off the pavement.
    1. True
    2. False
    Show answer
    False

    False. Raise it all the way up; partly raised gear can catch on railroad tracks or other things.

  17. How should you raise the landing gear?
    1. In high gear range from the start
    2. With the tractor moving forward
    3. Only half way
    4. Start in low gear range, then switch to high once free of weight
    Show answer
    Start in low gear range, then switch to high once free of weight

    Begin in low gear range (if equipped) and switch to high gear range once the gear is free of weight.

  18. After the landing gear is up and the trailer weight is on the tractor, what clearance must you check?
    1. Between the glad hands and the cab
    2. Between the trailer roof and bridges
    3. Between the rear of the tractor frame and the landing gear
    4. Between the mud flaps and the ground
    Show answer
    Between the rear of the tractor frame and the landing gear

    Check clearance between the tractor frame and landing gear (for sharp turns) and between tractor tires and the trailer nose.

  19. When uncoupling, why should the tractor be lined up with the trailer?
    1. Pulling out at an angle can damage the landing gear
    2. It keeps the ABS on
    3. It makes the air lines easier to cross
    4. It is needed to test the service brakes
    Show answer
    Pulling out at an angle can damage the landing gear

    Pulling out at an angle can damage the landing gear.

  20. When uncoupling, why do you back up gently before pulling the release handle?
    1. To lower the landing gear
    2. To ease pressure on the fifth wheel locking jaws
    3. To charge the trailer tanks
    4. To test the ABS
    Show answer
    To ease pressure on the fifth wheel locking jaws

    Backing gently eases pressure on the locking jaws, making the release handle easier to pull.

  21. When uncoupling a LOADED trailer, how far should you lower the landing gear?
    1. Only until it is near the ground
    2. Until it lifts the trailer completely off the fifth wheel
    3. Until firm contact, then a few extra turns in low gear
    4. Do not lower it; the fifth wheel holds the trailer
    Show answer
    Until firm contact, then a few extra turns in low gear

    After firm contact, a few extra turns lift some weight off the tractor, but do not lift the trailer off the fifth wheel.

  22. After disconnecting the electrical cable from the trailer, how should you hang it?
    1. Plug up, so it drains
    2. Coiled on the fifth wheel
    3. Tied to the landing gear
    4. With the plug down to keep moisture out
    Show answer
    With the plug down to keep moisture out

    Hang the electrical cable with the plug down to prevent moisture from entering.

  23. When you first pull the tractor out from under the trailer, where should you stop?
    1. When the tractor is fully clear
    2. With the tractor frame still under the trailer
    3. As soon as the jaws open
    4. After 10 feet
    Show answer
    With the tractor frame still under the trailer

    Stop with the frame under the trailer so it won't fall if the landing gear collapses or sinks.

  24. When pulling the fifth wheel release handle to the open position, what safety rule applies?
    1. Keep your legs and feet clear of the rear tractor wheels
    2. Keep the engine running in gear
    3. Stand between the tractor and trailer
    4. Hold the glad hands
    Show answer
    Keep your legs and feet clear of the rear tractor wheels

    Keep legs and feet clear of the rear tractor wheels in case the vehicle moves.

  25. When uncoupling, you should chock the trailer wheels if:
    1. The trailer is loaded
    2. It is raining
    3. You are on a level surface
    4. The trailer has no spring brakes or you are not sure
    Show answer
    The trailer has no spring brakes or you are not sure

    Chock if there are no spring brakes or you're unsure; the tank air could leak and release the emergency brakes.

  26. When you inspect the coupling from under the trailer, where should the fifth wheel locking lever and safety latch be?
    1. Lever in "open," latch hanging free
    2. Lever in "open," latch removed
    3. Lever halfway, latch beside it
    4. Lever in "lock," safety latch over the lever
    Show answer
    Lever in "lock," safety latch over the lever

    The locking lever must be in the "lock" position with the safety latch in place over it. On some fifth wheels you must set the latch by hand.