Combination Vehicles Test Study Guide for the CDL

Coupling and Uncoupling

Learning how to couple and uncouple a combination vehicle is essential for safe driving. Procedures vary from rig to rig, but there are common steps to be followed.

Coupling

Coupling is a lengthy process with a total of 15 steps. The first 6 steps for coupling are these:

1. Inspect fifth-wheel.

Check for damaged/missing parts.
Properly mounted to frame, no cracks.
Plate greased. (An improperly greased plate could cause steering problems, due to friction between tractor and trailer.)
Fifth-wheel properly located: plate tilted down toward trailer, jaws open, unlocking handle (if equipped) in automatic (ready to lock) position, kingpin (on trailer) not bent or broken.

2. Inspect area and chock wheels.

Area around vehicle clear
Wheels chocked (if trailer is not equipped with spring brakes)
Check that cargo is secure.

3. Position tractor.

Put the tractor directly in front of trailer.
Do not back under the trailer at an angle. You could push the trailer sideways and/or bend the landing gear.

4. Back slowly.

Back until the fifth-wheel touches trailer.
Do not back under trailer at this point. You need to get out and inspect fifth-wheel and kingpin alignment, before sliding under the trailer.

5. Secure tractor.

Set the parking brake.
Place transmission in Neutral (or, Park—if equipped with an automatic transmission).

6. Check trailer height.

Get out of the tractor, and look at the fifth-wheel and the nose of the trailer.
Trailer should be lifted slightly when the tractor slides under the nose of the trailer.
Be certain that the rear of the fifth-wheel will not strike the nose (front) of the trailer.
Check that the kingpin will enter the open end of the fifth-wheel.
If the trailer is too high, the kingpin could latch improperly. The jaws could attempt to latch onto the head of the kingpin. Hint: This might be on the written DMV test.
If the trailer is much too high, the kingpin could pass right over the plate of the fifth-wheel. The nose of the trailer could strike the back of the tractor cab. Hint: This might be on the written DMV test.

Retrieved from: https://www.dmv.ca.gov/portal/dmv/detail/pubs/cdl_htm/sec6

The final 10 steps for coupling are these:

7. Connect air lines to trailer.

Check glad hand gaskets. (They should be clean and free of cracks.)
Connect “Service” to “Service” and “Emergency” to “Emergency.”
Support airlines so they will not be pinched or crushed during coupling.
Do not connect electrical lines at this point.

8. Supply air to the trailer.

Charge trailer air tanks (Press the trailer air supply control—red knob).
Let the trailer air tanks normalize.
Press and release trailer brakes, and listen for normal brake operation sounds.
Watch the gauges for signs of massive air leak.
Start the engine.
Watch the gauges until air pressure is normal.

9. Lock trailer brakes.

Set trailer emergency/spring brakes (Pull out the trailer air supply control—red knob).

10. Back under trailer.

Use lowest reverse gear.
Back under the trailer slowly.
Stop when kingpin is locked (you can hear it “click”).

11. Check connection for security.

Raise landing gear slowly.
Pull tractor forward slightly (while trailer brakes are still applied).
Check for movement. (Tractor should “buck” slightly. Trailer should not move.)

12. Secure truck.

Put transmission in neutral (or park, if equipped with an automatic transmission).
Set parking brakes.
Shut off engine and take key with you.

13. Inspect coupling. (Get out and look. Use a flashlight.)

Make sure there is no space or gap between fifth-wheel and bottom of trailer.
Make sure the kingpin is properly located in fifth-wheel.
Make sure fifth-wheel jaws are fully clamped onto shaft of the kingpin (not onto the head of the kingpin).
Make sure that locking lever (if equipped) is locked.
Inspecting involves getting out of the tractor, climbing under the trailer, and looking into the jaws of the fifth-wheel. Use a flashlight if necessary. Hint: This might be on the written DMV test.
There should be no space between the fifth-wheel plate and the underside of the trailer nose. (Hint: This might be on the written DMV test.)

14. Connect the electrical cord and check air lines.

Connect the electrical cord to the nose of the trailer. (The electrical cord is usually green.)
Check electrical cord and air lines for signs of wear.
Support electrical cords and airlines, so nothing will not be pinched or crushed during driving (including turning).

15. Raise front trailer supports (landing gear).

Raise the landing gear fully. (Do not operate trailer with landing gear only partially raised. Partially raised landing gear can catch on railroad tracks or uneven terrain.) (Hint: This might be on the written DMV test.)
Secure the crank handle.
Check that landing gear or nose of trailer will not touch tractor during driving/turning.
Check that tractor tires will not touch trailer

16. Remove and store trailer wheel chocks.

Remove the chocks (if used).

Uncoupling

Here are the steps for uncoupling:

1. Position rig.

Pick an appropriate landing spot for the trailer.
Make certain that the ground will support the weight of the trailer.
LIne up the tractor with the trailer. (Pulling out at an angle can damage the landing gear.)

2. Ease pressure on locking jaws.

Lock trailer brakes. (Pull out the trailer air supply control—red knob.)
Release pressure on fifth-wheel jaws by backing gently. (Tractor should “buck” slightly. Trailer should not move.)
Set parking brakes, while tractor is pressing back into trailer.

3. Chock trailer wheels.

Chock wheels (if trailer is not equipped with spring brakes).

4. Lower the landing gear.

If the trailer is empty, lower the gear until it firmly touches the ground.
If the trailer is loaded, lower the gear until it firmly touches the ground—then give the crank handle a few extra turns. (These extra turns will reduce pressure on the fifth-wheel and the jaws—making uncoupling easier—and also making it easier for the next driver to pick up the trailer.)

5. Disconnect air lines and electrical cable.

Disconnect air lines and place glad hands in the dummy coupler.
Disconnect electrical cord and place electrical connector in the dummy coupler.
Make certain that air lines and electrical cords are supported and will not be damaged while driving.
Note: The glad hands are designed to come apart in an emergency—but they will “snap back” and slam into the cab of the tractor. This could damage the tractor. The electrical cord will be damaged (or ruined) by pulling it apart without disconnecting it. Disconnect the air lines and electrical cord properly, and store them neatly.

6. Unlock the fifth wheel.

Unlock handle lock (if equipped).
Pull the release handle.
Warning: During this step, be extra careful to keep your body clear of the fifth-wheel and watch for shifting equipment. Expect the equipment to move, possibly in an unexpected direction. Once the release handle is pulled, the tractor or trailer could shift. Do not place your body in harm’s way. Be particularly careful when uncoupling in tight spaces (or between trailers). Leave yourself an “out.”

7. Pull tractor somewhat clear of the trailer.

Pull fifth-wheel out from underneath trailer.
Stop with the frame under the trailer, in case the landing gear collapses.

8. Secure tractor.

Set parking brake.
Put transmission in Neutral (or Park, if equipped with an automatic transmission).

9. Inspect trailer supports.

Check landing gear.
Check that ground is supporting trailer.

10. Pull tractor clear of the trailer.

Release the parking brakes.
Pull forward and clear the trailer.

Combination Vehicle Care

Always follow the standard seven-step inspection procedure when inspecting a combination vehicle. During the walk-around inspection, you should also look at the coupling system areas (the lower and upper fifth wheel, air and electric lines to trailer, and the sliding fifth wheel), as well as the landing gear.

During the air brake systems inspection, the trailer air brakes must also be tested. This involves checking air flow to all trailers, testing the tractor protection valve, testing the trailer emergency brakes, and testing the trailer service brakes. Combination vehicles have additional parts to control the trailer brakes. These parts include the trailer air lines, trailer air supply control, hose couplers (glad hands), tractor protection valve, trailer air tanks, the trailer hand valve, shut-off valves, and the trailer service, parking, and emergency brakes. It is necessary to familiarize oneself with all of these parts before operating a combination vehicle.

Bonus Facts

You should know how to:

Test trailer service brakes.
Test trailer emergency brakes.
Test the tractor protection valve.
Test that air flows to all trailers (shut-off valve test).

Directions for performing these tests are in this study guide and/or in our study guide for the Air Brakes Test.

Converter dollies and trailers built before 1975 are not required to have spring brakes like newer trailers; they have emergency brakes instead.

Typically, the permits and vehicle registrations are the most common item overlooked on the pre-trip inspection. Be sure you have the trailer registration, as well as the tractor registration before leaving on a trip. The trailer registration is typically found in a water-tight container on the nose of the trailer.