Massachusetts CDL Combination cheat sheet

A combination vehicle is a tractor towing one or more trailers; the tractor carries the fifth wheel (or pintle hook) and the trailer carries the kingpin (or lunette eye).

Massachusetts basic CDL weight thresholds: Class A covers combinations with a gross combination weight rating (GCWR) of 26,001 lb or more, provided the towed unit exceeds 10,000 lb; Class B covers a single vehicle of 26,001 lb or more, or such a vehicle towing 10,000 lb or less.

Tractor protection valve: closes automatically when trailer air lines separate, sealing the tractor's air supply so the tractor can still brake; the trailer emergency line charges the trailer reservoir and triggers trailer spring brakes when pressure drops.

Trailer hand valve (trolley valve) applies only the trailer brakes; the foot valve applies the tractor and trailer brakes together. The trailer hand valve is for testing, never for normal stopping.

Walk-around inspection order for a combination: coupling system, air lines and electrical cord, landing gear, trailer frame and doors, lights and reflectors, tires, brakes and suspension, then rear of trailer.

Fifth wheel jaws must completely close around the kingpin shank with no gap; the release handle must be secured and the safety latch engaged.

Air loss limits: with the engine off and full pressure, a combination may lose no more than 3 psi in one minute (single vehicle 2 psi) with the brakes released, and no more than 4 psi in one minute with the brakes applied.

Tire minimum tread depth: 4/32 inch on front steering tires, 2/32 inch on all other tires; recapped or regrooved tires are prohibited on the front wheels of a commercial vehicle.

Coupling sequence: inspect fifth wheel and kingpin, verify jaws open and release handle out, back under slowly, hear/feel the click, tug test twice (pull forward gently in low gear), then visually confirm jaws closed and pull the release handle to check it is locked.

Always chock the trailer wheels before coupling or uncoupling so the trailer cannot roll when the tractor is removed.

Raise the trailer to take pressure off the tractor before pulling the release handle during uncoupling; pulling the handle under load can damage the fifth wheel and cause injury.

After uncoupling, lower the landing gear until it touches the ground, then give it a few extra turns to lift the tractor frame slightly free of the trailer.

Disconnect the electrical cord and air lines before pulling the tractor out from under the trailer, and secure the lines so they are not dragged or crushed.

The tug test is the primary defense against a high-hook: if the tractor pulls out from under the trailer during the tug, the kingpin was not locked.

Air line color convention: the emergency (supply) line is red and the service line is blue; crossed connections leave the trailer without a functioning emergency brake.

Pintle hook coupling: the lunette eye must sit fully down in the hook with the latch closed and a safety pin or clip installed; always test with a gentle pull before driving.

Fifth wheel types: fixed, sliding (adjustable to shift weight between axles), and convertible; sliding fifth wheels must be locked with the locking pins fully engaged before driving.

The kingpin is the trailer's coupling point; its diameter and the fifth wheel jaw size must match, and the mounting plate must be free of cracks and loose bolts.

Pintle hook capacity is rated by the manufacturer; the hook, mounting, and lunette eye must all be rated for the load, and the safety chains must be crossed and rated for the trailer.

Safety chains on a pintle-hitched trailer: attach with enough slack to allow turning but short enough to catch the trailer if the hook fails; do not twist chains to shorten them.

Glad hands: seals must be in place and free of cracks; couplers lock when the two halves are pressed together and the locking rings turn. Listen for leaks after connection.

Trailer electrical cord: check that the plug is fully seated and the cord is supported so it does not rub or drag; test all lights (clearance, brake, turn, and ABS lamp if equipped) before moving.

Trailer spring brakes apply automatically when the emergency line loses pressure; this is why the trailer must be chocked before disconnecting the red line.

Inspect the fifth wheel plate for adequate lubrication; a dry plate causes steering resistance and can damage the tractor's frame or the fifth wheel.

Air brake leak test sequence: build pressure to governor cut-out (about 120-125 psi), shut off the engine, release the parking brake, and watch the gauge for one minute.

Applied-brake leak test: with the engine off and pressure built, hold the foot brake for one minute; loss must not exceed 4 psi for a combination vehicle.

Low-air warning must activate before pressure drops below 60 psi; the tractor protection valve should pop out (or the trailer air supply valve close) between 20 and 45 psi.

Spring brake test: with pressure below 45 psi, the trailer spring brakes should be set; pull gently against them to confirm they hold.

Compressor build rate: with the engine at operating rpm, pressure should rise from 85 to 100 psi within 45 seconds in a combination vehicle.

Before each trip, check that the trailer ABS lamp cycles on and then off; a lamp that stays on means the trailer ABS is not working and the trailer may have reduced braking.

Governor cut-in and cut-out: the compressor should resume pumping around 100 psi and stop around 120-125 psi; a gauge that never reaches cut-out indicates a compressor or leak problem.

Check slack adjusters by pulling each brake chamber pushrod; with the brakes released, travel should be about 1 to 1-3/4 inches for a type 30 chamber, and the pushrod angle should be about 90 degrees when applied.

Stopping distance grows with weight and speed: a combination vehicle at 60 mph needs roughly 25 percent more stopping distance than a single vehicle, and speed increases stopping distance with the square of the speed.

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

Swing-out: the rear of the tractor and the front of the trailer swing outward during a turn; the longer the wheelbase and trailer, the greater the off-tracking.

Off-tracking: the rear wheels of a combination follow a shorter path than the front wheels; a tractor-trailer can cut a corner by as much as 2 feet for every 10 feet of wheelbase.

Bobtail (tractor without trailer) braking is different: the light rear axle and unloaded tractor can lock the rear wheels easily, so brake gently and allow extra distance.

Downshift before a downgrade and use the same gear going down that you used going up; use engine braking and stab braking rather than continuous light brake pressure.

Start in a gear low enough to move the loaded rig without slipping the clutch; a loaded combination may need to start in first or second and shift progressively.

Check mirrors and signal well in advance of lane changes; a combination vehicle has large blind spots directly behind and along both sides of the trailer.

Right turns: swing wide to the left before turning, but not so far that the trailer crosses the center line; keep the rear wheels of the tractor close to the curb.

Left turns: approach from the right side of your lane or the right lane if permitted, so the trailer does not swing into oncoming traffic.

Turn slowly and check both mirrors throughout the turn; the trailer's rear wheels track inside the tractor's path, so watch for curbs, poles, and pedestrians on the inside.

Lane changes: signal, check mirrors, check the blind spot over your shoulder if possible, and move smoothly; a combination vehicle needs more space and time to change lanes.

Backing: back toward the driver's side (left) whenever possible so the trailer stays visible in the left mirror; use a helper and agree on hand signals before starting.

When backing, steer the tractor's rear wheels toward the direction you want the trailer to go; if the trailer goes the wrong way, stop, pull forward, and correct.

Back slowly and use the mirrors, not the rear window; get out and look (GOAL) if unsure, and never back with a trailer unless you can see the rear.

Turning radius and clearance: a long combination needs a wider turn and more clearance at the front and rear; check overhead clearance and tail swing before entering tight areas.

Rollover is caused mainly by speed in turns, abrupt steering, and a high center of gravity; the posted curve speed is for cars, so slow well below it in a loaded combination.

Load height raises the center of gravity: a loaded trailer with a high center of gravity rolls over at lower lateral acceleration than a low, heavy load.

Liquid loads surge: partially filled tankers and bulk trailers shift weight in turns and stops, so brake and steer more gently and allow longer stopping distance.

On wet, icy, or snowy roads, reduce speed and increase following distance; a combination vehicle can jackknife when the drive wheels lose traction during braking.

Skid recovery: release the brakes to restore wheel rotation and steer in the direction you want to go; do not brake harder, which deepens the skid.

Downhill speed control: select a safe gear before the grade, use engine braking, and apply the brakes firmly enough to feel them, then release, rather than riding them.

Escape ramps are for emergency use only; if brakes fail, steer for the ramp and stay in it rather than trying to re-enter the highway.

High winds: reduce speed and grip the wheel firmly; empty trailers and high-profile loads are especially vulnerable to being pushed sideways or tipped.

Total stopping distance = perception distance + reaction distance + brake lag distance + effective braking distance; at 55 mph a combination vehicle needs about 300 feet on dry pavement.

Brake lag for air brakes is about 0.4 to 0.6 seconds; at 55 mph that alone covers roughly 32 to 48 feet before the brakes begin to work.

Brake fade: repeated or continuous use overheats the drums and reduces braking force; control speed with gears and use stab braking on long downgrades.

Emergency braking in a combination: brake firmly and progressively, keep the steering steady, and avoid locking the wheels; if the trailer starts to swing, release the brake to let the wheels roll and regain control.

If the trailer breaks away and the emergency line separates, the tractor protection valve closes and the trailer spring brakes set automatically; do not try to reconnect while moving.

Brake failure response: downshift to use engine braking, pump the brakes to build pressure, use the emergency/parking brake gradually, and look for an escape ramp or safe runoff.

Jackknife prevention: brake before entering a curve, not in it; a locked drive axle on a slick surface lets the trailer push the tractor sideways.

After any hard stop or air loss event, inspect the brake system and air lines before continuing; do not drive with the low-air warning on.