Section 6
COMBINATION VEHICLES
This Section Covers
- Driving Combinations
- Combination Vehicle Air Brakes
- Antilock Brake Systems
- Coupling and Uncoupling
- Inspecting Combinations
This section provides information needed to pass
the tests for combination vehicles (tractor-trailer,
doubles, triples, straight truck with trailer). The
information is only to give you the minimum
knowledge needed for driving common combination
vehicles. You should also study Section 7 if you
need to pass the test for doubles and triples.
6.1 – Driving Combination Vehicles
Safely
Combination vehicles are usually heavier, longer,
and require more driving skill than single
commercial vehicles. This means that drivers of
combination vehicles need more knowledge and
skill than drivers of single vehicles. In this section,
we talk about some important safety factors that
apply specifically to combination vehicles.
6.1.1 – Rollover Risks
More than half of truck driver deaths in crashes are
the result of truck rollovers. When more cargo is
piled up in a truck, the “center of gravity” moves
higher up from the road. The truck becomes easier
to turn over. Fully loaded rigs are ten times more
likely to roll over in a crash than empty rigs.
The following two things will help you prevent
rollover–keep the cargo as close to the ground as
possible, and drive slowly around turns. Keeping
cargo low is even more important in combination
vehicles than in straight trucks. Also, keep the load
centered on your rig. If the load is to one side so it
makes a trailer lean, a rollover is more likely. Make
sure your cargo is centered and spread out as much
as possible. (Cargo distribution is covered in
Section 3 of this manual.)
Rollovers happen when you turn too fast. Drive
slowly around corners, on ramps, and off ramps.
Avoid quick lane changes, especially when fully
loaded.
6.1.2 – Steer Gently
Trucks with trailers have a dangerous “crack-thewhip” effect. When you make a quick lane change,
the crack-the-whip effect can turn the trailer over.
There are many accidents where only the trailer has
overturned.
“Rearward amplification” causes the crack-the-whip
effect. Figure 6.1 shows eight types of combination
vehicles and the rearward amplification each has in
a quick lane change. Rigs with the least crack-thewhip effect are shown at the top and those with the
most, at the bottom. Rearward amplification of 2.0
in the chart means that the rear trailer is twice as
likely to turn over as the tractor. You can see that
triples have a rearward amplification of 3.5. This
means you can roll the last trailer of triples 3.5 times
as easily as a five-axle tractor.
Steer gently and smoothly when you are pulling
trailers. If you make a sudden movement with your
steering wheel, your trailer could tip over. Follow far
enough behind other vehicles (at least 1 second for
each 10 feet of your vehicle length, plus another
second if going over 40 mph). Look far enough
down the road to avoid being surprised and having
to make a sudden lane change. At night, drive
slowly enough to see obstacles with your headlights
before it is too late to change lanes or stop gently.
Slow down to a safe speed before going into a turn.
6.1.3 – Brake Early
Control your speed whether fully loaded or empty.
Large combination vehicles take longer to stop
when they are empty than when they are fully
loaded. When lightly loaded, the very stiff
suspension springs and strong brakes give poor
traction and make it very easy to lock up the wheels.
Your trailer can swing out and strike other vehicles.
Your tractor can jackknife very quickly. You also
must be very careful about driving “bobtail” tractors
(tractors without semitrailers). Tests have shown
that bobtails can be very hard to stop smoothly. It
takes them longer to stop than a tractor-semitrailer
loaded to maximum gross weight.
In any combination rig, allow lots of following
distance and look far ahead, so you can brake early.
Don’t be caught by surprise and have to make a
“panic” stop.
Commercial Driver’s License Manual – 2005 CDL Testing System
Section 6 – Combination Vehicles Page 6-2
Version: July 2017
Figure 6.1
6.1.4 – Railroad-highway Crossings
Railroad-highway crossings can also cause
problems, particularly when pulling trailers with low
underneath clearance.
These trailers can get stuck on raised crossings:
Low slung units (lowboy, car carrier, moving van,
possum-belly livestock trailer).
Single-axle tractor pulling a long trailer with its
landing gear set to accommodate a tandem-axle
tractor.
If for any reason you get stuck on the tracks, get out
of the vehicle and away from the tracks. Check
signposts or signal housing at the crossing for
emergency notification information. Call 911 or
other emergency number. Give the location of the
crossing using all identifiable landmarks, especially
the DOT number, if posted.
6.1.5 – Prevent Trailer Skids
When the wheels of a trailer lock up, the trailer will
tend to swing around. This is more likely to happen
when the trailer is empty or lightly loaded. This type
of jackknife is often called a “trailer jackknife.” See
Figure 6.2.
The procedure for stopping a trailer skid is:
Recognize the Skid. The earliest and best way to
recognize that the trailer has started to skid is by
seeing it in your mirrors. Any time you apply the
brakes hard, check the mirrors to make sure the
trailer is staying where it should be. Once the trailer
swings out of your lane, it’s very difficult to prevent
a jackknife. - (From R.D. Ervin, R.L. Nisconger, C.C. MacAdam,
and P.S. Fancher, “Influence of size and weight
variables on the stability and control properties of
heavy trucks, “University of Michigan Transportation
Research Institute, 1983).
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Section 6 – Combination Vehicles Page 6-3
Version: July 2017
Figure 6.2
Stop Using the Brake. Release the brakes to get
traction back. Do not use the trailer hand brake (if
you have one) to “straighten out the rig.” This is the
wrong thing to do since the brakes on the trailer
wheels caused the skid in the first place. Once the
trailer wheels grip the road again, the trailer will start
to follow the tractor and straighten out.
6.1.6 – Turn Wide
When a vehicle goes around a corner, the rear
wheels follow a different path than the front wheels.
This is called off-tracking or “cheating.” Figure 6.3
shows how off-tracking causes the path followed by
a tractor to be wider than the rig itself. Longer
vehicles will off-track more. The rear wheels of the
powered unit (truck or tractor) will off-track some,
and the rear wheels of the trailer will off-track even
more. If there is more than one trailer, the rear
wheels of the last trailer will off-track the most. Steer
the front end wide enough around a corner so the
rear end does not run over the curb, pedestrians,
etc. However, keep the rear of your vehicle close to
the curb. This will stop other drivers from passing
you on the right. If you cannot complete your turn
without entering another traffic lane, turn wide as
you complete the turn. This is better than swinging
wide to the left before starting the turn because it
will keep other drivers from passing you on the right.
See Figure 6.4.
Figure 6.3
Figure 6.4
6.1.7 – Backing with a Trailer.
Backing with a Trailer. When backing a car,
straight truck, or bus, you turn the top of the steering
wheel in the direction you want to go. When backing
a trailer, you turn the steering wheel in the opposite
direction. Once the trailer starts to turn, you must
turn the wheel the other way to follow the trailer.
Whenever you back up with a trailer, try to position
your vehicle so you can back in a straight line. If you
must back on a curved path, back to the driver’s side
so you can see. See Figure 6.5.
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Section 6 – Combination Vehicles Page 6-4
Version: July 2017
Figure 6.5
Look at Your Path. Look at your line of travel
before you begin. Get out and walk around the
vehicle. Check your clearance to the sides and
overhead, in and near the path your vehicle.
Use Mirrors on Both Sides. Check the outside
mirrors on both sides frequently. Get out of the
vehicle and re-inspect your path if you are unsure.
Back Slowly. This will let you make corrections
before you get too far off course.
Correct Drift Immediately. As soon as you see the
trailer getting off the proper path, correct it by turning
the top of the steering wheel in the direction of the
drift.
Pull Forward. When backing a trailer, make pullups to re-position your vehicle as needed.
Subsection 6.1
Test Your Knowledge
- What two things are important to prevent
rollover? - When you turn suddenly while pulling
doubles, which trailer is most likely to turn
over? - Why should you not use the trailer hand
brake to straighten out a jackknifing trailer? - What is off-tracking?
- When you back a trailer, you should
position your vehicle so you can back in a
curved path to the driver’s side. True or
False? - What type of trailers can get stuck on
railroad-highway crossings?
These questions may be on your test. If you can’t
answer them all, re-read subsection 6.1.
6.2 – Combination Vehicle Air Brakes
You should study Section 5: Air Brakes before
reading this. In combination vehicles the braking
system has parts to control the trailer brakes, in
addition to the parts described in Section 5. These
parts are described below.
6.2.1 – Trailer Hand Valve
The trailer hand valve (also called the trolley valve
or Johnson bar) works the trailer brakes. The trailer
hand valve should be used only to test the trailer
brakes. Do not use it in driving because of the
danger of making the trailer skid. The foot brake
sends air to all of the brakes on the vehicle
(including the trailer(s)). There is much less danger
of causing a skid or jackknife when using just the
foot brake.
Never use the hand valve for parking because all
the air might leak out unlocking the brakes (in
trailers that don’t have spring brakes). Always use
the parking brakes when parking. If the trailer does
not have spring brakes, use wheel chocks to keep
the trailer from moving.
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Section 6 – Combination Vehicles Page 6-5
Version: July 2017
6.2.2 – Tractor Protection Valve
The tractor protection valve keeps air in the tractor
or truck brake system should the trailer break away
or develop a bad leak. The tractor protection valve
is controlled by the “trailer air supply” control valve
in the cab. The control valve allows you to open and
shut the tractor protection valve. The tractor
protection valve will close automatically if air
pressure is low (in the range of 20 to 45 psi). When
the tractor protection valve closes, it stops any air
from going out of the tractor. It also lets the air out
of the trailer emergency line. This causes the trailer
emergency brakes to come on, with possible loss of
control. (Emergency brakes are covered later.)
6.2.3 – Trailer Air Supply Control
The trailer air supply control on newer vehicles is a
red eight-sided knob, which you use to control the
tractor protection valve. You push it in to supply the
trailer with air, and pull it out to shut the air off and
put on the trailer emergency brakes. The valve will
pop out (thus closing the tractor protection valve)
when the air pressure drops into the range of 20 to
45 psi. Tractor protection valve controls or
“emergency” valves on older vehicles may not
operate automatically. There may be a lever rather
than a knob. The “normal” position is used for
pulling a trailer. The “emergency” position is used to
shut the air off and put on the trailer emergency
brakes.
6.2.4 – Trailer Air-lines
Every combination vehicle has two air-lines, the
service line and the emergency line. They run
between each vehicle (tractor to trailer, trailer to
dolly, dolly to second trailer, etc.)
Service Air-line. The service line (also called the
control line or signal line) carries air, which is
controlled by the foot brake or the trailer hand brake.
Depending on how hard you press the foot brake or
hand valve, the pressure in the service line will
similarly change. The service line is connected to
relay valves. These valves allow the trailer brakes
to be applied more quickly than would otherwise be
possible.
Emergency Air-line. The emergency line (also
called the supply line) has two purposes. First, it
supplies air to the trailer air tanks. Second, the
emergency line controls the emergency brakes on
combination vehicles. Loss of air pressure in the
emergency line causes the trailer emergency
brakes to come on. The pressure loss could be
caused by a trailer breaking loose, thus tearing
apart the emergency air hose. Or it could be caused
by a hose, metal tubing, or other part breaking,
letting the air out. When the emergency line loses
pressure, it also causes the tractor protection valve
to close (the air supply knob will pop out).
Emergency lines are often coded with the color red
(red hose, red couplers, or other parts) to keep from
getting them mixed up with the blue service line.
6.2.5 – Hose Couplers (Glad Hands)
Glad hands are coupling devices used to connect
the service and emergency air-lines from the truck
or tractor to the trailer. The couplers have a rubber
seal, which prevents air from escaping. Clean the
couplers and rubber seals before a connection is
made. When connecting the glad hands, press the
two seals together with the couplers at a 90 degree
angle to each other. A turn of the glad hand attached
to the hose will join and lock the couplers.
When coupling, make sure to couple the proper glad
hands together. To help avoid mistakes, colors are
sometimes used. Blue is used for the service lines
and red for the emergency (supply) lines.
Sometimes, metal tags are attached to the lines with
the words “service” and “emergency” stamped on
them. See Figure 6.6
Figure 6.6
Commercial Driver’s License Manual – 2005 CDL Testing System
Section 6 – Combination Vehicles Page 6-6
Version: July 2017
If you do cross the air-lines, supply air will be sent
to the service line instead of going to charge the
trailer air tanks. Air will not be available to release
the trailer spring brakes (parking brakes). If the
spring brakes don’t release when you push the
trailer air supply control, check the air-line
connections.
Older trailers do not have spring brakes. If the air
supply in the trailer air tank has leaked away there
will be no emergency brakes, and the trailer wheels
will turn freely. If you crossed the air-lines, you could
drive away but you wouldn’t have trailer brakes. This
would be very dangerous. Always test the trailer
brakes before driving with the hand valve or by
pulling the air supply (tractor protection valve)
control. Pull gently against them in a low gear to
make sure the brakes work.
Some vehicles have “dead end” or dummy couplers
to which the hoses may be attached when they are
not in use. This will prevent water and dirt from
getting into the coupler and the air-lines. Use the
dummy couplers when the air-lines are not
connected to a trailer. If there are no dummy
couplers, the glad hands can sometimes be locked
together (depending on the couplings). It is very
important to keep the air supply clean.
6.2.6 – Trailer Air Tanks
Each trailer and converter dolly has one or more air
tanks. They are filled by the emergency (supply) line
from the tractor. They provide the air pressure used
to operate trailer brakes. Air pressure is sent from
the air tanks to the brakes by relay valves.
The pressure in the service line tells how much
pressure the relay valves should send to the trailer
brakes. The pressure in the service line is controlled
by the brake pedal (and the trailer hand brake).
It is important that you don’t let water and oil build
up in the air tanks. If you do, the brakes may not
work correctly. Each tank has a drain valve on it and
you should drain each tank every day. If your tanks
have automatic drains, they will keep most moisture
out. But you should still open the drains to make
sure.
6.2.7 – Shut-off Valves
Shut-off valves (also called cut-out cocks) are used
in the service and supply air-lines at the back of
trailers used to tow other trailers. These valves
permit closing the air-lines off when another trailer
is not being towed. You must check that all shut-off
valves are in the open position except the ones at
the back of the last trailer, which must be closed.
6.2.8 – Trailer Service, Parking and Emergency
Brakes
Newer trailers have spring brakes just like trucks
and truck tractors. However, converter dollies and
trailers built before 1975 are not required to have
spring brakes. Those that do not have spring brakes
have emergency brakes, which work from the air
stored in the trailer air tank. The emergency brakes
come on whenever air pressure in the emergency
line is lost. These trailers have no parking brake.
The emergency brakes come on whenever the air
supply knob is pulled out or the trailer is
disconnected. A major leak in the emergency line
will cause the tractor protection valve to close and
the trailer emergency brakes to come on. But the
brakes will hold only as long as there is air pressure
in the trailer air tank. Eventually, the air will leak
away and then there will be no brakes. Therefore, it
is very important for safety that you use wheel
chocks when you park trailers without spring
brakes.
You may not notice a major leak in the service line
until you try to put the brakes on. Then, the air loss
from the leak will lower the air tank pressure quickly.
If it goes low enough, the trailer emergency brakes
will come on.
Subsection 6.2
Test Your Knowledge - Why should you not use the trailer hand
valve while driving? - Describe what the trailer air supply control
does. - Describe what the service line is for.
- What is the emergency air-line for?
- Why should you use chocks when parking
a trailer without spring brakes? - Where are shut-off valves?
These questions may be on your test. If you can’t
answer them all, re-read subsection 6.2.
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Section 6 – Combination Vehicles Page 6-7
Version: July 2017
6.3 – Antilock Brake Systems
6.3.1 – Trailers Required to Have ABS
All trailers and converter dollies built on or after
March 1, 1998, are required to have ABS. However,
many trailers and converter dollies built before this
date have been voluntarily equipped with ABS.
Trailers will have yellow ABS malfunction lamps on
the left side, either on the front or rear corner. See
Figure 6.7. Dollies manufactured on or after March
1, 1998, are required to have a lamp on the left side.
In the case of vehicles manufactured before the
required date, it may be difficult to tell if the unit is
equipped with ABS. Look under the vehicle for the
ECU and wheel speed sensor wires coming from
the back of the brakes.
Figure 6.7
6.3.2 – Braking with ABS
ABS is an addition to your normal brakes. It does
not decrease or increase your normal braking
capability. ABS only activates when wheels are
about to lock up.
ABS does not necessarily shorten your stopping
distance, but it does help you keep the vehicle under
control during hard braking.
ABS helps you avoid wheel lock up. The computer
senses impending lockup, reduces the braking
pressure to a safe level, and you maintain control.
Having ABS on only the trailer, or even on only one
axle, still gives you more control over the vehicle
during braking.
When only the trailer has ABS, the trailer is less
likely to swing out, but if you lose steering control or
start a tractor jackknife, let up on the brakes (if you
can safely do so) until you gain control.
When you drive a tractor-trailer combination with
ABS, you should brake as you always have. In other
words:
Use only the braking force necessary to stop safely
and stay in control.
Brake the same way, regardless of whether you
have ABS on the tractor, the trailer, or both.
As you slow down, monitor your tractor and trailer
and back off the brakes (if it is safe to do so) to
stay in control.
Remember, if your ABS malfunctions, you still have
regular brakes. Drive normally, but get the system
serviced soon.
ABS won’t allow you to drive faster, follow more
closely, or drive less carefully.
6.4 – Coupling and Uncoupling
Knowing how to couple and uncouple correctly is
basic to safe operation of combination vehicles.
Wrong coupling and uncoupling can be very
dangerous. General coupling and uncoupling steps
are listed below. There are differences between
different rigs, so learn the details of coupling and
uncoupling the truck(s) you will operate.
6.4.1 – Coupling Tractor-Semitrailers
Step 1. Inspect Fifth Wheel
Check for damaged/missing parts.
Check to see that mounting to tractor is secure, no
cracks in frame, etc.
Be sure that the fifth wheel plate is greased as
required. Failure to keep the fifth wheel plate
lubricated could cause steering problems because
of friction between the tractor and trailer.
Check if fifth wheel is in proper position for
coupling:
Wheel tilted down toward rear of tractor.
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Section 6 – Combination Vehicles Page 6-8
Version: July 2017
Jaws open.
Safety unlocking handle in the automatic lock
position.
If you have a sliding fifth wheel, make sure it is
locked.
Make sure the trailer kingpin is not bent or broken.
Step 2. Inspect Area and Chock Wheels
Make sure area around the vehicle is clear.
Be sure trailer wheels are chocked or spring
brakes are on.
Check that cargo (if any) is secured against
movement due to tractor being coupled to the
trailer.
Step 3. Position Tractor
Put the tractor directly in front of the trailer. (Never
back under the trailer at an angle because you
might push the trailer sideways and break the
landing gear.)
Check position, using outside mirrors, by looking
down both sides of the trailer.
Step 4. Back Slowly
Back until fifth wheel just touches the trailer.
Don’t hit the trailer.
Step 5. Secure Tractor
Put on the parking brake.
Put transmission in neutral.
Step 6. Check Trailer Height
The trailer should be low enough that it is raised
slightly by the tractor when the tractor is backed
under it. Raise or lower the trailer as needed. (If
the trailer is too low, the tractor may strike and
damage the trailer nose; if the trailer is too high, it
may not couple correctly.)
Check that the kingpin and fifth wheel are aligned.
Step 7. Connect Air-lines to Trailer
Check glad hand seals and connect tractor
emergency air-line to trailer emergency glad hand.
Check glad hand seals and connect tractor service
air-line to trailer service glad hand.
Make sure air-lines are safely supported where
they won’t be crushed or caught while tractor is
backing under the trailer.
Step 8. Supply Air to Trailer
From cab, push in “air supply” knob or move
tractor protection valve control from the
“emergency” to the “normal” position to supply air
to the trailer brake system.
Wait until the air pressure is normal.
Check brake system for crossed air-lines.
Shut engine off so you can hear the brakes.
Apply and release trailer brakes and listen for sound
of trailer brakes being applied and released. You
should hear the brakes move when applied and air
escape when the brakes are released.
Check air brake system pressure gauge for signs of
major air loss.
When you are sure trailer brakes are working, start
engine.
Make sure air pressure is up to normal.
Step 9. Lock Trailer Brakes
Pull out the “air supply” knob or move the tractor
protection valve control from “normal” to
“emergency.”
Step 10. Back Under Trailer
Use lowest reverse gear.
Back tractor slowly under trailer to avoid hitting the
kingpin too hard.
Stop when the kingpin is locked into the fifth wheel.
Step 11. Check Connection for Security
Raise trailer landing gear slightly off ground.
Pull tractor gently forward while the trailer brakes
are still locked to check that the trailer is locked
onto the tractor.
Step 12. Secure Vehicle
Put transmission in neutral.
Put parking brakes on.
Shut off engine and take key with you so someone
else won’t move truck while you are under it.
Step 13. Inspect Coupling
Use a flashlight, if necessary.
Make sure there is no space between upper and
lower fifth wheel. If there is space, something is
wrong (kingpin may be on top of the closed fifth
wheel jaws, and trailer would come loose very
easily).
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Section 6 – Combination Vehicles Page 6-9
Version: July 2017
Go under trailer and look into the back of the fifth
wheel. Make sure the fifth wheel jaws have closed
around the shank of the kingpin.
Check that the locking lever is in the “lock”
position.
Check that the safety latch is in position over
locking lever. (On some fifth wheels the catch must
be put in place by hand.)
If the coupling isn’t right, don’t drive the coupled
unit; get it fixed.
Step 14. Connect the Electrical Cord and Check
Air-lines
Plug the electrical cord into the trailer and fasten
the safety catch.
Check both air-lines and electrical line for signs of
damage.
Make sure air and electrical lines will not hit any
moving parts of vehicle.
Step 15. Raise Front Trailer Supports (Landing
Gear)
Use low gear range (if so equipped) to begin
raising the landing gear. Once free of weight,
switch to the high gear range.
Raise the landing gear all the way up. (Never drive
with landing gear only part way up as it may catch
on railroad tracks or other things.)
After raising landing gear, secure the crank handle
safely.
When full weight of trailer is resting on tractor:
Check for enough clearance between rear of tractor
frame and landing gear. (When tractor turns
sharply, it must not hit landing gear.)
Check that there is enough clearance between the
top of the tractor tires and the nose of the trailer.
Step 16. Remove Trailer Wheel Chocks
Remove and store wheel chocks in a safe place.
6.4.2 – Uncoupling Tractor-Semitrailers
The following steps will help you to uncouple safely.
Step 1. Position Rig
Make sure surface of parking area can support
weight of trailer.
Have tractor lined up with the trailer. (Pulling out at
an angle can damage landing gear.)
Step 2. Ease Pressure on Locking Jaws
Shut off trailer air supply to lock trailer brakes.
Ease pressure on fifth wheel locking jaws by
backing up gently. (This will help you release the
fifth wheel locking lever.)
Put parking brakes on while tractor is pushing
against the kingpin. (This will hold rig with pressure
off the locking jaws.)
Step 3. Chock Trailer Wheels
Chock the trailer wheels if the trailer doesn’t have
spring brakes or if you’re not sure. (The air could
leak out of the trailer air tank, releasing its
emergency brakes. Without chocks, the trailer could
move.)
Step 4. Lower the Landing Gear
If trailer is empty, lower the landing gear until it
makes firm contact with the ground.
If trailer is loaded, after the landing gear makes firm
contact with the ground, turn crank in low gear a few
extra turns. This will lift some weight off the tractor.
(Do not lift trailer off the fifth wheel.) This will:
Make it easier to unlatch fifth wheel.
Make it easier to couple next time.
Step 5. Disconnect Air-lines and Electrical Cable
Disconnect air-lines from trailer. Connect air-line
glad hands to dummy couplers at back of cab or
couple them together.
Hang electrical cable with plug down to prevent
moisture from entering it.
Make sure lines are supported so they won’t be
damaged while driving the tractor.
Step 6. Unlock Fifth Wheel
Raise the release handle lock.
Pull the release handle to “open” position.
Keep legs and feet clear of the rear tractor wheels
to avoid serious injury in case the vehicle moves.
Step 7. Pull Tractor Partially Clear of Trailer
Pull tractor forward until fifth wheel comes out from
under the trailer.
Stop with tractor frame under trailer (prevents trailer
from falling to ground if landing gear should collapse
or sink).
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Section 6 – Combination Vehicles Page 6-10
Version: July 2017
Step 8. Secure Tractor
Apply parking brake.
Place transmission in neutral.
Step 9. Inspect Trailer Supports
Make sure ground is supporting trailer.
Make sure landing gear is not damaged.
Step 10. Pull Tractor Clear of Trailer
Release parking brakes.
Check the area and drive tractor forward until it
clears.
Subsections 6.3 and 6.4
Test Your Knowledge - What might happen if the trailer is too high when
you try to couple? - After coupling, how much space should be
between the upper and lower fifth wheel? - You should look into the back of the fifth wheel
to see if it is locked onto the kingpin. True or
False? - To drive you need to raise the landing gear only
until it just lifts off the pavement. True or False? - How do you know if your trailer is equipped with
antilock brakes?
These questions may be on your test. If you can’t
answer them all, re-read subsections 6.3 and 6.4.
6.4.3 Coupling a Pintle Hook
Step 1. Inspect Pintle Hook
Before operating, check for worn, damaged, or
missing parts, and make sure mount is secure.
If the pintle hook is not secured to the mounting
surface, the pintle hook could separate from the
vehicle which, if not avoided, could result in death
or serious injury.
Step 2. Unlock Lock Pin and Open Latch
Unlock and remove the tethered lock pin, if
applicable (Figure 16).
Lift the lock handle away from the vehicle until the
lock clears the lock seat on the hook body.
Open the latch by rotating the latch assembly up
toward the vehicle until the latch is in its most
upright position, then release the lock handle.
(Figure 17 and 18)
.
Step 3. Lower Drawbar into Place
Position the drawbar eye over the horn of the pintle
hook and lower it into place.
Step 4. Lock Pintle Hook
Push the latch closed. When correctly locked, the
lock handle will rotate and move up until it is flush
with the top of the latch (Figure 19).
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Section 6 – Combination Vehicles Page 6-11
Version: July 2017
Insert the tethered lock pin through the latch and
lock holes, and close the tethered wire lock pin, if
applicable (Figure 16).
Failure to correctly lock the latch can result in
separation of the trailer and vehicle which, if not
avoided, could result in death or serious injury.
6.4.4 – Uncoupling a Pintle Hook
Step 1. Park on Level Surface
Park the trailer on a firm level surface and block
trailer tires.
Step 2. Disconnect Electrical Connector,
Breakaway Brake Switch and Safety Chains
Disconnect electrical connector.
Disconnect breakaway brake switch lanyard.
Disconnect safety chains from tow vehicle.
Step 3. Unlock the Coupler
Unlock the coupler and open it.
Step 4. Check Ground Surface for Correct
Support
Before extending jack, make certain the ground
surface below the jack pad will support the tongue
load.
Step 5. Rotate Jack Handle
Rotate jack handle to extend the jack and transfer
the weight of the trailer tongue to the jack.
Step 6. Raise Trailer Coupler
Raise the trailer coupler above the tow vehicle
hitch.
Step 7. Drive Forward
Drive tow vehicle forward.
6.4.5 – Coupling a Drawbar
Step 1. Remove Safety Lock screw and Rotate
Safety Cover Bar
Remove safety lock screw (2), recover and keep
the relative self-locking nut (12) aside then also
loosen safety lock screw (2a) and turn out the
adjustment screw (3) by at least five turns.
Rotate the safety cover bar (4) outwards so that it
is completely open.
Step 2. Reverse Truck
Reverse truck very slowly until the ball cup
drawbar eye (6) is in position exactly above the
drawbar coupling ball (5).
Step 3. Lower the Drawbeam
Lower the drawbeam until the ball cup drawbar eye
(6) lies completely covering the drawbar coupling
ball (5).
Step 4. Rotate the Safety Cover Bar
Rotate the safety cover bar back inwards (4) and fit
in safety lock screw (2) together with its selflocking nut.
Tighten in both the lock screws (2, 2a) together
with their relative self-locking nuts (12, 12a) at a
350 to 400 Nm torque wrench setting.
Step 5. Adjust the Adjustment Screw
Adjust the adjustment screw (3) until a 0.3 – 0.5
mm vertical clearance between the guard disk (13)
and the ball cup (6) is reached. Lock setting with
counter nut (11).
Commercial Driver’s License Manual – 2005 CDL Testing System
Section 6 – Combination Vehicles Page 6-12
Version: July 2017
In the event that the safety cover bar (4) will not
perfectly lodge into its seating appropriately, travel
is strictly forbidden.
Step 6. Protect Coupling Ball and Anchor Edge
onto Ball
Protect the part that is still visible of the coupling
ball (5) with the rubber dust proof bellows
protection cover (7). Accurately anchor the edge
directly onto the ball itself.
Step 7. Lubricate the Drawbar Eye
Lubricate the inside of the drawbar eye ball cup (6)
directly through the grease nipple (18).
6.4.6 – Uncoupling a Drawbar
Step 1. Turn Trailer Brake On
Make sure that the trailer brake is on.
Step 2. Remove Cover and Loosen Screws
Remove the rubber dust proof bellows protection
cover (7).
Loosen the adjustment screw (3) and the counter
nut (11).
Loosen safety lock screw (2a) and remove safety
lock screw (2) together with its self-locking nut
(12).
Step 3. Rotate Safety Cover Bar and Lift Trailer
Drawbeam
Rotate the safety cover bar (4) outwards so that it
is completely open.
Lift the trailer drawbeam until the drawbar coupling
ball (5) is fully visible, then travel forward with the
trailer very slowly.
Rotate the safety cover bar inwards (4) until it
lodges back in its housing.
Step 4. Lock Safety Screw and Tighten SelfLocking Nut
Fit in safety lock screw (2) and tighten in selflocking nut (12).
6.4.7 – Coupling a Gooseneck Hitch
If you are hooking up a Gooseneck or a fifth wheel
hitch, the procedure is a little different from a
receiver and ball, but it is not more difficult.
Step 1. Open the Latch and Lubricate the
Gooseneck Ball
Open the clamp latch on the Gooseneck coupler.
Make sure that the Gooseneck ball is properly
lubricated.
Step 2. Position Coupler and Latch the Clamp
Position the trailer’s coupler directly over the ball
and lower the Gooseneck trailer into position and
latch the clamp.
Step 3. Attach Safety Chains
Attach your safety chains. Remember that all
trailers are required by law to have safety chains.
Step 4. Connect Trailer Light Wiring
Connect your trailer light wiring to your vehicle’s
connector.
Check all of your lights, including your brake lights.
Step 5. Lower and Stow the Trailer Jacks
Completely lower and stow the trailer jacks,
allowing the weight to settle onto the tow vehicle.
6.4.8 Uncoupling a Gooseneck Hitch
Step 1. Remove Safety Pin and Clip
Simply remove safety pin and clip.
Step 2. Rotate Handle and Raise Trailer Off Ball
Rotate handle to this position (Fig. B) and raise
trailer off ball. Coupler will return to load position
automatically.
Step 3. Install Safety Pin and Clip
Install safety pin and clip. (Fig. B)
Commercial Driver’s License Manual – 2005 CDL Testing System
Section 6 – Combination Vehicles Page 6-13
Version: July 2017
6.5 – Inspecting a Combination Vehicle
Use the seven-step inspection procedure described
in Section 2 to inspect your combination vehicle.
There are more things to inspect on a combination
vehicle than on a single vehicle. (For example, tires,
wheels, lights, reflectors, etc.) However, there are
also some new things to check. These are
discussed below.
6.5.1 – Additional Things to Check during a
Walk-around Inspection
Do these checks in addition to those already listed
in Section 2.
Coupling System Areas
Check fifth wheel (lower).
Securely mounted to frame.
No missing or damaged parts.
Enough grease.
No visible space between upper and lower fifth
wheel.
Locking jaws around the shank, not the head of
kingpin. See Figure 6.8.
Release arm properly seated and safety
latch/lock engaged.
Figure 6.8
Check fifth wheel (upper).
Glide plate securely mounted to trailer frame.
Kingpin not damaged.
Air and electric lines to trailer.
Electrical cord firmly plugged in and secured.
Air-lines properly connected to glad hands, no air
leaks, properly secured with enough slack for turns.
All lines free from damage.
Sliding fifth wheel.
Slide not damaged or parts missing.
Properly greased.
All locking pins present and locked in place.
If air powered–no air leaks.
Check that fifth wheel is not so far forward that
tractor frame will hit landing gear, or the cab hit the
trailer, during turns.
Landing Gear
Fully raised, no missing parts, not bent or
otherwise damaged.
Crank handle in place and secured.
If power operated, no air or hydraulic leaks.
Commercial Driver’s License Manual – 2005 CDL Testing System
Section 6 – Combination Vehicles Page 6-14
Version: July 2017
6.5.2 – Combination Vehicle Brake Check
Do these checks in addition to Section 5.3:
Inspecting Air Brake Systems.
The following section explains how to check air
brakes on combination vehicles. Check the brakes
on a double or triple trailer as you would any
combination vehicle.
Check That Air Flows to All Trailers. Use the
tractor parking brake and/or chock the wheels to
hold the vehicle. Wait for air pressure to reach
normal, then push in the red “trailer air supply” knob.
This will supply air to the emergency (supply) lines.
Use the trailer handbrake to provide air to the
service line. Go to the rear of the rig. Open the
emergency line shut-off valve at the rear of the last
trailer. You should hear air escaping, showing the
entire system is charged. Close the emergency line
valve. Open the service line valve to check that
service pressure goes through all the trailers (this
test assumes that the trailer handbrake or the
service brake pedal is on), and then close the valve.
If you do NOT hear air escaping from both lines,
check that the shut-off valves on the trailer(s) and
dolly(ies) are in the OPEN position. You MUST have
air all the way to the back for all the brakes to work.
Test Tractor Protection Valve. Charge the trailer
Air-brake system. (That is, build up normal airpressure and push the “air supply” knob in.) Shut
the engine off. Step on and off the brake pedal
several times to reduce the air pressure in the tanks.
The trailer air supply control (also called the tractor
protection valve control) should pop out (or go from
“normal” to “emergency” position) when the air
pressure falls into the pressure range specified by
the manufacturer. (Usually within the range of 20 to
45 psi.)
If the tractor protection valve doesn’t work right, an
air hose or trailer brake leak could drain all the air
from the tractor. This would cause the emergency
brakes to come on, with possible loss of control.
Test Trailer Emergency Brakes. Charge the trailer
air brake system and check that the trailer rolls
freely. Then stop and pull out the trailer air supply
control (also called tractor protection valve control
or trailer emergency valve), or place it in the
“emergency” position. Pull gently on the trailer with
the tractor to check that the trailer emergency
brakes are on.
Test Trailer Service Brakes. Check for normal air
pressure, release the parking brakes, move the
vehicle forward slowly, and apply trailer brakes with
the hand control (trolley valve), if so equipped. You
should feel the brakes come on. This tells you the
trailer brakes are connected and working. (The
trailer brakes should be tested with the hand valve
but controlled in normal operation with the foot
pedal, which applies air to the service brakes at all
wheels.)
Subsection 6.5
Test Your Knowledge
- Which shut-off valves should be open and
which closed? - How can you test that air flows to all trailers?
- How can you test the tractor protection valve?
- How can you test the trailer emergency brakes?
- How can you test the trailer service brakes?
These questions may be on your test. If you can’t
answer all of them, re-rea
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