AK CDL Combination cheat sheet

A combination vehicle is a tractor (power unit) towing one or more trailers; the pivot point between tractor and trailer is the fifth wheel coupling.

The tractor's fifth wheel is a horseshoe-shaped plate that locks onto the trailer's kingpin; the kingpin is a vertical steel pin mounted under the trailer's nose.

Trailer air lines are color-coded: the service line (blue) carries brake-application pressure; the emergency/supply line (red) supplies air and triggers trailer brakes if it loses pressure.

The electrical cord (usually green) supplies trailer lights; the glad hands connect the air lines and use a rubber seal to prevent leaks.

The landing gear (support legs) holds an uncoupled trailer level; crank it fully up before towing and fully down before uncoupling.

Trailer light check: running lights, left turn, right turn, four-way flashers, brake lights, and clearance/marker lights must all function before driving.

The trailer's spring brakes set automatically when air pressure is lost; a trailer with no air supply cannot be moved safely.

The coupling jaws must be closed around the kingpin, and the safety latch (release handle) must be secured before the tractor pulls the trailer.

Inspect coupling: fifth wheel jaws locked, release handle secured, kingpin seated, and no gap between tractor frame and trailer nose.

Check that the trailer apron rests flat on the fifth wheel and that the locking jaws fully surround the kingpin shank.

Test the trailer air lines and electrical cord for chafing, cracks, and secure glad-hand connections; lines should have slack but not drag.

Verify the landing gear is fully raised, the crank handle is secured, and no part of the gear extends below the trailer frame.

Check trailer tires for tread depth, inflation, sidewall damage, and proper dual spacing; look for missing lug nuts and cracked rims.

Inspect mud flaps, splash guards, and rear impact guards for secure mounting and no damage.

Confirm the trailer's rear doors or cargo securement and that the load is blocked and braced so it cannot shift.

Check the tractor's fifth wheel mounting bolts and the frame for cracks or loose fasteners.

Coupling sequence: set trailer brakes, back tractor squarely under trailer, raise fifth wheel to contact the trailer apron, then push until the kingpin locks.

After coupling, do a pull test: with trailer brakes set, gently pull forward in low gear to confirm the kingpin is locked and the tractor cannot separate.

Visual check after coupling: the release handle should be in the locked position and the jaws should be closed around the kingpin.

Connect the emergency (red) line first, then the service (blue) line, then the electrical cord; confirm glad-hand seals are seated.

Uncoupling sequence: set trailer brakes, lower landing gear to firm ground, chock wheels if needed, disconnect lines and cord, then release the fifth wheel.

After pulling out from under a trailer, lower the tractor's fifth wheel to prevent it from catching on the trailer apron.

Never couple or uncouple on a slope without chocking the trailer wheels; the trailer can roll and the kingpin can miss the fifth wheel.

Keep hands and body clear of the space between tractor and trailer during coupling; the gap closes as the tractor backs under the trailer.

Static leak test: with full pressure and engine off, release the parking brake and watch the gauge; pressure loss should not exceed 3 psi in one minute for a single vehicle.

For a combination vehicle, the allowable static leak rate is 4 psi per minute with the parking brake released and service brakes applied.

Applied-brake leak test: charge the system, apply and hold the brake pedal, and confirm loss does not exceed 4 psi per minute on a combination.

Low-air warning: the warning buzzer/light must activate before pressure drops below 60 psi.

Spring brake test: with the system charged, release the parking brake, then fan off pressure; the tractor and trailer spring brakes should set around 40-45 psi.

Governor cut-out should be near 120-125 psi and cut-in near 100 psi; the system should build from 85 to 100 psi within about 45 seconds.

Check that the trailer spring brakes apply automatically if the emergency line is disconnected (tug test on the trailer).

Drain air tanks daily to remove moisture; check that the safety valve and drain cocks operate and do not leak.

A combination vehicle requires wider turns than a single unit; the trailer's rear wheels track inside the tractor's path.

Swing wide on right turns to keep the trailer's rear wheels from striking the curb or a pedestrian; watch the trailer's right side mirror.

Braking distance increases with weight and speed; a loaded combination needs more stopping distance than a light one.

Use the stab-braking technique on slippery surfaces: a firm controlled application to set the brakes, then release if wheels lock.

If the trailer begins to jackknife, release the brakes and steer to pull the tractor and trailer back into line.

If the tractor's drive wheels skid, ease off the throttle and steer in the direction of the skid to regain traction.

Use engine braking and lower gears on downgrades; avoid continuous service brake application that can overheat and fade.

Maintain a following distance of at least one second per 10 feet of vehicle length plus one second for speeds over 40 mph.

Off-tracking is the difference between the path of the tractor's front wheels and the trailer's rear wheels; it increases with sharper turns and longer wheelbases.

In a 90-degree turn, a long combination can off-track by several feet; the rear of the trailer cuts the corner.

Trailer sway (fishtailing) is amplified when the trailer is lightly loaded or the load is concentrated at the rear.

Rearward amplification is the tendency of the last trailer in a multi-trailer combination to swing wider than the tractor during a sudden maneuver.

To correct sway, ease off the throttle gradually, hold the steering steady, and avoid abrupt brake or steering inputs.

Load the heaviest cargo over or ahead of the trailer axles to reduce sway and improve stability.

Check that the trailer's tires are inflated evenly; uneven inflation or mismatched duals increases sway.

Multi-trailer combinations (doubles/triples) have greater rearward amplification and require slower speeds in curves and lane changes.

In high winds, reduce speed and grip the wheel firmly; empty or lightly loaded trailers are most vulnerable to being blown over.

On ice or snow, reduce speed, increase following distance, and avoid sudden steering or braking; use gentle, smooth inputs.

Before descending a steep grade, select a gear low enough that engine braking controls speed without continuous service brake use.

Use the escape ramp if brakes fail on a downgrade; aim for the gravel arrester bed and keep the vehicle straight.

When stopping on a highway shoulder, turn on four-way flashers, place reflective triangles behind the vehicle, and set the parking brake.

Place warning triangles within 10 minutes of stopping: one at 10 feet behind, one at 100 feet behind, and one at 100 feet ahead on a two-way road.

At railroad crossings, stop, look, and listen; a combination vehicle needs more clearance time than a single unit, so never start across if the far side is blocked.

In fog or heavy rain, reduce speed, use low beams, and increase following distance; do not use high beams in fog.

The fifth wheel's locking jaws must fully close around the kingpin; a partially latched jaw allows the trailer to separate under load.

The trailer's emergency (red) line supplies air to the trailer's spring brakes; if it is disconnected, the trailer brakes set automatically.

The service (blue) line carries the driver's brake application signal to the trailer's relay valve.

A trailer's spring brakes are held off by air pressure; loss of air pressure causes them to apply.

The glad hand seals must be free of damage; a leaking seal causes slow pressure loss and weak trailer braking.

Landing gear must be fully raised before towing; a partially lowered leg can strike the road and damage the trailer.

A pre-trip inspection must confirm the fifth wheel is locked and the trailer apron is flat against the fifth wheel plate.

Trailer tires must have adequate tread depth and no cuts, bulges, or exposed cords.

Check that the trailer's rear impact guard (underride guard) is present and secure.

Confirm the electrical cord is plugged in and not chafed against the frame or fifth wheel.

Check that the trailer's air lines have enough slack to turn but do not drag on the ground.

Always set the trailer brakes before backing the tractor under the trailer during coupling.

A pull test confirms the kingpin is locked; if the tractor moves forward without resistance, the coupling is secure.

Lower the landing gear to firm ground before uncoupling; soft ground lets the trailer sink and the kingpin drop.

Chock the trailer wheels before uncoupling on any slope.

Connect the emergency line first and disconnect it last so the trailer stays braked during the procedure.

The low-air warning must sound before pressure falls below 60 psi.

The tractor and trailer spring brakes should set automatically when pressure drops to about 40-45 psi.

A combination vehicle's static leak rate should not exceed 4 psi per minute.

The air compressor should build from 85 to 100 psi in about 45 seconds at operating RPM.

Drain the air tanks daily to remove moisture that can freeze and block air lines.

Swing wide on right turns because the trailer's rear wheels track inside the tractor's path.

If the trailer starts to jackknife, release the brakes and steer to straighten the combination.

Use stab braking on slippery roads: apply firmly, release if wheels lock, and reapply.

Engine braking in a low gear controls speed on downgrades without overheating the service brakes.

Increase following distance as speed and weight increase; a loaded combination needs more stopping distance.

Off-tracking increases with sharper turns and longer wheelbases, so the trailer's rear wheels cut inside the turn.

Rearward amplification makes the last trailer swing widest in a sudden maneuver, especially in doubles and triples.

Place the heaviest cargo over or ahead of the trailer axles to reduce sway.

Ease off the throttle gradually and hold the steering steady to correct trailer sway.

Uneven tire inflation or mismatched duals increases trailer sway.

In high winds, reduce speed; empty or lightly loaded trailers are most likely to be blown over.

On ice and snow, use gentle, smooth steering and braking inputs and increase following distance.

Before a steep downgrade, select a gear low enough to control speed with engine braking.

If brakes fail on a downgrade, use the escape ramp and keep the vehicle straight.

At railroad crossings, never start across unless the far side has room for the entire combination.