Combination Vehicle Knowledge
Combination Vehicle Knowledge
The Kansas CDL Combination Vehicle knowledge domain tests how a Class A driver inspects, couples, uncouples, and controls a tractor-trailer. Kansas uses federal CDL knowledge-test content, so expect questions on fifth wheels, kingpins, glad hands, landing gear, trailer air supply, and the tractor protection valve, plus turning, backing, and braking.
Core rules and concepts
- Coupling system: The fifth wheel locks onto the trailer kingpin. After backing under, perform a tug test and visually confirm the jaws are closed around the kingpin and the release handle is latched.
- Air lines: Connect the emergency (supply) line and service line to the correct glad hands; open the shut-off valves. The tractor protection valve keeps air from escaping if the trailer separates or pressure drops.
- Trailer hand valve: The red trailer hand valve (trolley valve) applies trailer brakes for a test only; never use it for parking.
- Leak test: With the engine off and at least 100 psi, release the brakes and watch the gauge for one minute. For a combination, loss should not exceed 3 psi. Then apply the brakes; loss should not exceed 4 psi in one minute.
- Turning: The trailer's rear wheels track inside the tractor's path (off-tracking). Swing wide before a turn and watch the trailer in the mirrors.
Worked example
A gauge reads 100 psi. With brakes released, it reads 97.2 psi after one minute: 2.8 psi loss, so that part passes. With the service brakes applied, it reads 96.4 psi after one minute: 3.6 psi loss, which is under the 4 psi limit, so it also passes. If either loss were higher, the driver would not dispatch until the leak was fixed.
Traps that cost marks
Do not confuse the trailer hand valve with the parking brake. Do not leave landing gear down after coupling or raise it before the tug test. Do not assume the fifth-wheel click means the jaws are locked. Do not forget to check that glad-hand seals are present and couplers are locked.