CDL test guide · 50 states
CDL Combination Vehicles Test Guide
Tractor-trailers and trucks pulling a heavy trailer. Taken for Class A.
What every manual says
Driving a tractor-trailer safely, its trailer air brakes, coupling and uncoupling, and the extra checks a combination needs.
Steer Gently
Trucks with trailers have a dangerous crack-the-whip effect: a quick lane change can turn the trailer over, and many crashes involve only the trailer overturning, so avoid quick lane changes, especially when fully loaded, and steer gently.
Brake Early
Large combinations take longer to stop empty than fully loaded: when lightly loaded, stiff springs and strong brakes give poor traction and lock the wheels easily, so the trailer can swing out or the tractor jackknife.
Prevent Trailer Skids
To stop a trailer skid, stop using the brake and release it so the wheels get traction back; don’t use the trailer hand brake to "straighten out the rig", since the trailer brakes caused the skid. Once the trailer wheels grip again, the trailer follows the tractor and straightens out.
Trailer Hand Valve
The trailer hand valve (trolley valve or Johnson bar) works the trailer brakes.
Trailer Air Supply Control
On newer vehicles the trailer air supply control is a red eight-sided knob controlling the tractor protection valve: push it in to supply the trailer with air, pull it out to shut the air off and apply the trailer emergency brakes.
Trailer Air-lines
Every combination has two air lines between each unit: the service line (control or signal line) carries air controlled by the foot brake or trailer hand brake, its pressure changing with how hard you brake, and connects to relay valves that apply the trailer brakes faster.
Trailer Air-lines
The emergency line (supply line) supplies air to the trailer tanks and controls the emergency brakes: losing pressure in it (a trailer breaking loose and tearing the hose, or a broken hose or tube) applies the trailer emergency brakes and closes the tractor protection valve (the knob pops out).
Hose Couplers (Glad Hands)
Glad hands couple the service and emergency lines from the tractor to the trailer, with rubber seals that keep air in.
Hose Couplers (Glad Hands)
If you cross the air lines, supply air goes to the service line instead of charging the trailer tanks, so there is no air to release the trailer spring brakes.
Hose Couplers (Glad Hands)
Some vehicles have "dead end" or dummy couplers to attach the hoses to when not in use, keeping water and dirt out; use them when the lines aren’t connected to a trailer, or lock the glad hands together if possible, since keeping the air supply clean is very important.
Trailer Service, Parking and Emergency Brakes
Newer trailers have spring brakes, but converter dollies and trailers built before 1975 don’t need them.
Coupling Tractor-Semitrailers
Coupling steps 2–3: make sure the area is clear, chock the trailer wheels or have its spring brakes on, check that any cargo is secured against movement.
Coupling Tractor-Semitrailers
Coupling steps 7–9: connect the tractor emergency line to the trailer emergency glad hand and the service line to the service glad hand (checking seals), supporting the lines so they won’t be crushed.
Coupling Tractor-Semitrailers
Steps 14–16: plug in the electrical cord and fasten its safety catch, check the air and electrical lines for damage and that they won’t hit moving parts.
Uncoupling Tractor-Semitrailers
Lower the landing gear until it makes firm contact; if the trailer is loaded, turn the crank a few extra turns in low gear to lift some weight off the tractor (not off the fifth wheel), which makes unlatching and the next coupling easier.
Uncoupling Tractor-Semitrailers
To unlock the fifth wheel, raise the release handle lock and pull the release handle to "open", keeping legs and feet clear of the rear tractor wheels in case the vehicle moves.
6.4.3 Coupling a Pintle Hook
Before using a pintle hook, check for worn, damaged or missing parts and that the mount is secure; a loose pintle hook could separate from the vehicle, and a latch not correctly locked can let the trailer separate, which could cause death or serious injury.
Coupling a Drawbar
When hooking up a trailer by a ball or gooseneck hitch, attach the safety chains (all trailers are required by law to have them), connect the trailer light wiring and check all the lights including the brake lights, and lower and stow the trailer jacks.
Additional Things to Check during a Walk-around Inspection
With a sliding fifth wheel, check that the slide is undamaged, properly greased, without air leaks if air powered, all locking pins present and locked, and that it is not so far forward that the tractor frame hits the landing gear or the cab hits the trailer in turns.
Every state's manual: Combination Vehicles
Each links its combination vehicles practice test.
- AlabamaSection 6 · p. 85–98
- AlaskaSection 6 · p. 88–101
- ArizonaSection 6 · p. 88–98
- ArkansasSection 6 · p. 82–95
- CaliforniaSection 6 · p. 115–131
- ColoradoSection 6 · p. 71–83
- ConnecticutSection 6 · p. 83–96
- DelawareSection 6 · p. 89–102
- FloridaSection 6 · p. 89–102
- GeorgiaSection 6 · p. 154–180
- HawaiiSection 6 · p. 91–104
- IdahoSection 6 · p. 84–95
- IllinoisSection 6 · p. 81–91
- IndianaSection 6 · p. 83–96
- IowaSection 6 · p. 87–100
- KansasSection 6 · p. 82–95
- KentuckySection 6 · p. 92–105
- LouisianaSection 6 · p. 102–116
- MaineSection 6 · p. 85–98
- MarylandSection 6 · p. 172–199
- MassachusettsSection 6 · p. 108–121
- MichiganSection 6 · p. 95–107
- MinnesotaSection 6 · p. 129–142
- MississippiSection 6 · p. 82–95
- MissouriSection 6 · p. 79–92
- MontanaSection 6 · p. 90–103
- NebraskaSection 6 · p. 94–107
- NevadaSection 6 · p. 90–103
- New HampshireSection 6 · p. 82–95
- New JerseySection 6 · p. 82–95
- New MexicoSection 6 · p. 85–98
- New YorkSection 6 · p. 101–115
- North CarolinaSection 6 · p. 87–97
- North DakotaSection 6 · p. 87–96
- OhioSection 6 · p. 79–90
- OklahomaSection 6 · p. 82–95
- OregonSection 6 · p. 85–98
- PennsylvaniaSection 6 · p. 107–122
- Rhode IslandSection 6 · p. 80–90
- South CarolinaSection 6 · p. 97–110
- South DakotaSection 6 · p. 105–118
- TennesseeSection 6 · p. 87–100
- TexasSection 6 · p. 91–104
- UtahSection 6 · p. 87–100
- VermontSection 6 · p. 95–108
- VirginiaSection 6 · p. 82–95
- WashingtonSection 6 · p. 89–102
- West VirginiaSection 6 · p. 120–133
- WisconsinSection 6 · p. 120–138
- WyomingSection 6 · p. 81–92