California CDL Combination Vehicles Test cheat sheet

For CDL purposes a combination vehicle is a tractor-trailer or truck-trailer unit whose power unit tows one or more cargo units; the class of the combination is set by the heaviest unit's GVWR or by the towed unit's GVWR.

Inspection order front to back: engine compartment, coupling system, light check with the helper signal, brake check (applied and released), then walk-around confirming clearance lights, reflective devices, and cargo securement.

Coupling inspection points: fifth wheel jaws closed around the kingpin, locking lever in the engaged position, no gap between apron and fifth wheel plate, safety latch secured, air and electrical lines free of chafing.

Trailer inspection points: landing gear fully raised, crank handle secured, mud flaps present, tires matched and properly inflated, rims free of cracks, lug nuts tight, and no missing or loose spacers.

Tire rules of thumb: tread depth at least 4/32 inch on front steer tires and 2/32 inch on all other tires; check with a gauge, not visually, because tread wear on duals must be measured at multiple points.

Basic control skills are scored on the driving test: straight-line backing, alley docking, parallel parking, and conventional or offset backing; a pull-up is allowed but counts as one of the limited correction moves.

When backing a combination vehicle, the trailer's rear responds opposite to the steering direction: turn the wheel left and the trailer's rear swings right, so correct early and use small inputs.

The pre-trip inspection is a pass/fail item on the CDL skills test; a missed critical defect such as a cracked brake drum or a leaking air line fails the inspection portion outright.

A combination vehicle has a longer stopping distance and wider off-tracking than a single unit; the rear wheels of the trailer follow a shorter path than the tractor's wheels in a turn.

Coupling sequence: inspect the fifth wheel and kingpin, back under the trailer until the jaws lock, tug-test with the tractor in a low gear, then connect the air lines, electrical cord, and trailer supply valve, and finally raise the landing gear.

Always perform the tug test before raising the landing gear: pull gently forward in low gear; if the tractor moves and the trailer does not, the coupling is secure.

Uncoupling sequence: park on firm level ground, apply the parking brake, chock the wheels if needed, lower the landing gear to the ground, disconnect the air and electrical lines, then release the fifth wheel lock and pull out slowly.

Before pulling out from under a trailer, verify the landing gear is on firm ground and the trailer is supported; a loaded trailer on soft ground can sink and drop.

Fifth wheel height error: too high and the tractor strikes the trailer nose; too low and the tractor fails to lift the trailer, so the kingpin can ride over the fifth wheel.

After coupling, check the gap between the tractor frame and the trailer apron; a visible gap indicates the kingpin is not locked in the jaws.

Release the tractor parking brake only after the trailer supply valve is open and the trailer air tanks have charged; a closed supply valve leaves the trailer spring brakes applied.

The trailer supply valve (red knob) must be pushed in to charge the trailer system; pulling it out applies the trailer emergency spring brakes and should be done only when parking or uncoupling.

Inspect the air lines after coupling: they should have enough slack to turn and back without kinking, rubbing, or contacting the catwalk.

Off-tracking: the rear wheels of a turning combination cut inside the tractor's path, so swing wide before a turn and check mirrors to keep the trailer clear of curbs and poles.

Swing-out: the rear of the trailer swings outward in the opposite direction of the turn, so watch the outside mirror before completing a tight turn.

Following distance: at least one second per 10 feet of vehicle length plus one second for speeds over 40 mph; a 60-foot combination at 55 mph needs about 7 seconds.

Speed on curves: enter at or below the posted advisory speed and do not brake hard in the curve; a combination rolls over at far lower speeds than a passenger car on the same curve.

Downgrades: select a gear low enough to hold a safe speed before starting down, use engine braking and the trailer hand valve sparingly, and never coast in neutral.

Right turns require a wider approach and a later start of the turn; left turns require the driver to stay close to the center line to keep the trailer from clipping the inside curb.

Lane changes: signal early, check both mirrors plus the blind spot, and confirm the trailer is fully in the new lane before canceling the signal.

Crosswinds and passing large vehicles: reduce speed, grip firmly, and expect the trailer to be pushed sideways when exiting a bridge or passing a truck.

Backing a combination requires a helper whenever possible; if backing alone, get out and check the path first, and stop immediately if visibility is lost.

The tractor protection valve closes automatically when trailer air pressure drops below about 45 psi, sealing the tractor's air system so the tractor can still brake after a trailer breakaway.

The trailer supply valve (red) and parking brake valve (yellow) are push-pull controls: pushing in charges and releases, pulling out applies the spring brakes.

A dual air brake system has two separate circuits; a loss in one circuit leaves the other to stop the vehicle, and the low-air warning must activate before pressure falls below 60 psi.

The low-air warning buzzer and light must come on before 60 psi, and the spring brakes must fully apply by about 20 to 45 psi as pressure bleeds down.

The tractor must have a trailer hand valve (spike) for testing trailer brakes; it applies only the trailer brakes and must not be used for normal stopping.

The brake check: with the system charged to governor cut-out, shut off the engine, release the parking brake, and watch the gauge; a loss of more than 3 psi in one minute indicates a leak.

Static leak test: with the brakes fully applied and the engine off, pressure loss should not exceed 4 psi per minute for a combination vehicle.

The supply pressure gauge shows air available for the brakes; the application gauge shows the pressure applied to the brake chambers, and the two should be compared during a brake test.

After a trailer breakaway, the trailer's spring brakes apply automatically through the emergency line, so always connect both the service and emergency (supply) lines before moving.