Pre-trip inspection parts

Small side and corner lights that outline a vehicle's size at night.

Side marker lights run along the trailer. Clearance lights sit at the top corners. Front and side ones are amber; the rear ones are red.

Example: Walking down the dark side of the trailer, you count the marker lights and spot one amber lens cracked and dim.

Also called: clearance lights

• headlights
• lights and reflectors
• pre-trip inspection
• turn signals

The front glass of the cab that the driver looks through.

Check it for cracks, dirt, and stickers that block your view. Keep it clean inside and out, because dirt glares at night. Clear ice and snow before you drive.

Example: Before a night run you clean the inside of the windshield so oncoming headlights do not turn the film into glare.

• heater and defroster
• mirrors
• pre-trip inspection
• windshield wipers

Arms with rubber blades that sweep rain and snow off the windshield.

Check the arms for spring tension and the blades for stiff or torn rubber. The washer must spray and have fluid. If the wipers fail in bad weather, stop safely and fix them.

Example: Rain starts on the interstate and your blades only smear; you take the next exit to swap them.

Also called: windshield wiper, wipers

• heater and defroster
• pre-trip inspection
• visibility
• windshield

The hinged cover over the engine at the front of a truck.

You raise it, or tilt the whole cab, to check the engine compartment. Secure loose items first. Lower and latch it before you drive.

Example: You tilt the hood forward, check the oil and coolant, then latch it shut and tug it to be sure.

• compressor belt
• fluid leaks
• oil level
• pre-trip inspection
• radiator

Side-mounted glass showing the driver traffic beside and behind the vehicle.

Adjust them before every trip, with the trailer straight, so each shows a bit of your vehicle. Check them often but briefly. Convex mirrors show more but make things look small and far.

Example: Pulling onto the ramp, you glance at the right mirror and catch a car hiding beside your trailer.

Also called: side mirror, side mirrors

• blind spot
• convex mirror
• lane change
• windshield

A curved mirror that gives a wider view than a flat mirror.

Most trucks and buses have a small round mirror under the big flat one. It is convex, so you see more of the road beside you. Objects in it look smaller and more distant than they are.

Example: Before changing lanes, you glance at the convex mirror to catch a car sitting low in your blind spot.

Also called: convex mirrors

• blind spot
• lane change
• mirrors

The front lights that light the road and make the vehicle visible.

Headlights have low beams and high beams. On the pre-trip you check them from outside the vehicle: lenses clean, correct color, both beams working.

Example: On a dark rural road with no traffic ahead, you switch to high beams, then dim them when another vehicle approaches.

Also called: headlight

• high beams
• lighting indicators
• lights and reflectors
• low beams
• visibility

Flashing lights that tell other drivers you will turn or change lanes.

Front turn signals are amber or white; rear ones are red, yellow, or amber. The same lamps usually work as the four-way flashers.

Example: You switch on the right turn signal well before the exit ramp and cancel it once you are on the ramp.

Also called: turn signal, signal lights

• hazard lights
• headlights
• lane change
• lighting indicators
• lights and reflectors

The bar across the front of the vehicle that takes minor impacts.

On the pre-trip you check that the bumper is secure, not loose or hanging, and has no missing parts. The guard at the rear of a trailer is the DOT bumper, a separate item.

Example: Walking up to the truck, you grab the bumper to confirm it is tight to the frame before checking the lights.

• DOT bumper
• frame
• headlights
• hood
• license plate

The registration plate that identifies the vehicle to police and inspectors.

The plate must be present, clean, readable, and firmly attached. Any registration sticker on it must be current.

Example: At the rear of the trailer you wipe road grime off the license plate so the number can be read.

Also called: licence plate, registration plate

• bumper
• lights and reflectors
• roadside inspection
• splash guards

Oil, fuel, coolant, or other liquid escaping from the vehicle.

Look under the vehicle for fresh puddles and at the underside of the engine and transmission for drips. Any leak must be found and fixed before you drive.

Example: A dark wet spot under the engine when you walk up means you look for the source before starting.

• fuel tank
• hub oil seal
• oil level
• power steering reservoir
• radiator

The rubber casings on the wheels that grip the road.

On the pre-trip you check inflation with a gauge, tread depth in the major grooves, and look for cuts, bulges, and damaged valve stems. Steering tires need deeper tread than the others. In very hot weather, stop and check the tires every two hours or every 100 miles.

Does not mean: A tire is the rubber part only; the metal part it sits on is the rim.

Example: At a summer fuel stop you find a tire too hot to touch, so you stop and let it cool instead of driving on.

Also called: tire

• rim
• tire blowout
• tread depth
• tread depth minimums
• valve stem

The metal wheel part that the tire is mounted on.

Check rims for cracks, bends, and welding repairs. Rust streaks around the nuts can mean the wheel is loose.

Does not mean: The rim is the metal part only; the tire is the rubber.

Example: A rim with a welded repair stays in the yard; you ask for another trailer.

Also called: rims

• hub oil seal
• lug nuts
• tires
• valve stem

The nuts that hold the wheel on the hub studs.

All lug nuts must be present and tight. Rust trails or shiny metal around a nut can mean it has worked loose.

Example: After a roadside tire change you stop a few miles later and recheck the lug nuts with a wrench.

Also called: lug nut

• hub oil seal
• rim
• tires

The seal that keeps lubricant inside the wheel hub around the bearings.

Look at the hub cap and the inside of the wheel. Oil or grease running out means the seal is leaking. Many hubs have a sight glass to check the oil level.

Example: A wet, dirty streak running from the hub across the wheel tells you the hub oil seal is leaking.

Also called: hub seal, outer hub oil seal, wheel bearing seal, wheel seal

• fluid leaks
• lug nuts
• rim
• tires

The small tube on the wheel for checking and adding tire air.

Each valve stem needs a cap. Look for cuts, cracks, bends, and stems rubbing on anything. On dual wheels the stems must be reachable with a gauge.

Example: You find an inner dual with its valve stem turned away so a gauge cannot reach it, and report it.

Also called: valve stems

• dual tire spacing
• rim
• tires
• tread depth

The gap between two tires mounted side by side on one hub.

Dual tires must not touch each other or any part of the vehicle. Nothing may be stuck between them, such as rocks or debris. Both tires should be the same size.

Example: Between the rear duals you spot a wedged stone and pry it out before it cuts the sidewalls.

Also called: space between tires

• rim
• tire blowout
• tires
• valve stem

The doors that close the cargo area of the trailer or truck.

Doors must open and close properly, latch securely, and have working hinges. Side doors must be closed and locked before you drive, with any required seals in place.

Example: Before pulling out, you push the rear doors shut, drop the latch bars, and check the seal number against the paperwork.

Also called: cargo door

• cargo securement
• door ties
• frame
• header board

The steps you climb to enter the cab or reach the catwalk.

Steps must be solid, securely bolted, and clear of oil, grease, snow, and ice. Use three points of contact when you climb.

Does not mean: In a question, 'steps you should take' means actions, not the cab steps.

Example: Climbing down after a snowy night, you knock the ice off the steps first and keep three points of contact.

• catwalk
• fuel tank
• pre-trip inspection

The tank that stores the vehicle's fuel, usually mounted on the frame.

Check that the tank is mounted securely, that the cap is tight, and that nothing leaks from the tank or its lines. Fuel on the pavement is a fire hazard and very slippery.

Does not mean: Air tanks and tank vehicles are different things; 'tank' alone in a question is usually not the fuel tank.

Example: After fueling you twist the cap until it clicks and look under the tank for drips before leaving the pump.

Also called: fuel tanks

• air tank
• fluid leaks
• frame
• tank vehicle

The heat exchanger that cools the engine coolant with air flow.

Check the coolant level and the hoses on the pre-trip. Never open a hot radiator cap; the system is under pressure and can burn you.

Example: The temperature gauge climbs on a long grade, so you pull off and let the engine cool before touching the radiator cap.

• compressor belt
• fluid leaks
• hoses and wiring
• water pump
• water temperature gauge

The drive belt that turns the air compressor from the engine.

Some compressors are gear-driven and have no belt. If there is a belt, check it for cracks, fraying, and correct tension. A loose belt cannot drive the compressor properly.

Example: Pressing the compressor belt with your thumb, you feel too much give and note that it needs tightening.

• air compressor
• alternator
• hoses and wiring
• water pump

The small tank that holds the fluid for the power steering system.

Check the fluid level between the add and full marks, and look at the hoses for leaks. Low fluid makes steering heavy.

Example: You wipe the dipstick on the power steering reservoir and see the fluid sitting below the add mark.

Also called: power steering fluid

• fluid leaks
• hoses and wiring
• oil level
• steering gear box

The gearbox that turns steering wheel motion into steering linkage movement.

Check that the box is securely mounted and not leaking, with no missing nuts or bolts. The power steering hoses connect to it.

Example: You find a wet film of fluid on the steering gear box housing and trace it to a weeping hose fitting.

Also called: steering box

• pitman arm
• power steering reservoir
• steering linkage
• steering wheel play
• tie rod

The arms and rods that carry steering motion to the wheels.

The linkage includes the pitman arm, drag link, tie rod, and the joints between them. Nothing may be bent, loose, or cracked, and every nut needs its cotter pin.

Example: Grabbing the drag link and shaking it, you feel play in a joint and write it up as a steering defect.

• drag link
• pitman arm
• steering gear box
• steering wheel play
• tie rod

The lever on the steering gear box that moves the drag link.

The pitman arm is the first link after the gear box. Check that it is not bent or cracked and that its nut and cotter pin are in place.

Example: With the engine off you have a helper rock the steering wheel while you watch the pitman arm for lost motion.

• drag link
• steering gear box
• steering linkage
• tie rod

The rod that connects the pitman arm to the wheel's steering arm.

The drag link runs from the pitman arm to the steering knuckle. It must be straight, with tight joints and cotter pins in place.

Example: You shake the drag link by hand; movement at the ball joint means a worn socket to report.

• pitman arm
• steering gear box
• steering linkage
• tie rod

The rod that links the two front wheels so they steer together.

The tie rod runs across the front axle between the steering knuckles. Bent, loose, or broken tie rods are a listed steering defect.

Example: After hitting a deep pothole you check the tie rod ends for play before pulling back onto the highway.

• drag link
• pitman arm
• steering gear box
• steering linkage

The engine-driven pump that circulates coolant through the engine and radiator.

Check that the pump is securely mounted, its hoses are tight, and nothing leaks. If a belt drives it, check the belt too.

Example: A coolant trail under the front of the engine leads you to a weeping water pump seal.

• alternator
• compressor belt
• fluid leaks
• radiator
• water temperature gauge

The belt-driven generator that charges the battery and powers the electrical system.

Check its mounting, wiring connections, and belt. Inside the cab, the ammeter or voltmeter shows whether the alternator is charging.

Example: The voltmeter needle sits low after start-up, so you suspect the alternator or its belt.

• ammeter
• battery
• compressor belt
• hoses and wiring
• water pump

The amount of engine oil in the engine, read on the dipstick.

Pull the dipstick, wipe it, put it back, and pull it again. The oil should sit between the add and full marks.

Example: Before a long run you check the oil level on the dipstick and top it up to the full mark.

Also called: dipstick

• fluid leaks
• oil pressure gauge
• power steering reservoir
• radiator

The rubber hoses and electrical wires routed around the engine compartment.

Hoses must be attached, not cracked, and not leaking. Wiring insulation must not be cracked, worn, or frayed. Nothing should rub on moving or hot parts.

Example: A heater hose rubbing on the fan shroud has worn a shiny flat spot, so you flag it before it bursts.

• air lines
• battery
• compressor belt
• fluid leaks
• radiator

A hydraulic or gas damper that controls the bounce of the suspension.

Shocks sit beside the springs or air bags. Check that each one is securely mounted, not bent, and not leaking oil.

Example: Oil streaks down the body of a shock absorber mean the seal has failed and the shock needs replacing.

Also called: shock absorbers

• air bags
• control arm
• leaf spring
• spring mounts
• torsion bar

A stack of curved steel strips that carries the vehicle's weight.

Each strip is a leaf. Leaves must not be cracked, broken, missing, or shifted so they could hit a tire. If one-fourth or more of the leaves are missing or broken, the vehicle is out of service.

Example: A leaf has slid sideways out of its stack and is almost touching the tire, so the truck stays parked.

Also called: leaf springs

• air bags
• out of service
• shock absorber
• spring mounts
• torsion bar

The brackets and U-bolts that fasten the springs to frame and axle.

Spring hangers hold the ends of the leaf spring to the frame. U-bolts clamp the spring to the axle. All must be present, tight, and not cracked.

Example: You see a gap where a U-bolt nut has backed off and the axle has shifted slightly on the spring.

Also called: spring hangers

• air bags
• frame
• leaf spring
• shock absorber
• torsion bar

The hoses and tubes that carry air or brake fluid to each brake.

Lines must be securely mounted and not leaking. Look for cracks, cuts, chafing, and fraying, and make sure every fitting is tight.

Example: You hear a faint hiss at a rear wheel and find a chafed air hose rubbing on the frame.

Also called: brake hoses, hoses and lines

• air leakage rate
• air lines
• brake chamber
• hydraulic brakes
• slack adjuster

The storage battery that starts the engine and powers the electrical system.

Batteries may sit in the engine compartment or in a box on the frame. Check that they are secure, the connections are tight and clean, and the box cover is latched.

Example: You open the battery box, find white corrosion on a terminal, and clean it before it causes a hard start.

Also called: battery box

• alternator
• ammeter
• hoses and wiring
• spare fuses

The metal platform behind the cab where the trailer lines connect.

The catwalk must be solid, bolted firmly to the frame, and kept clear of objects. Keep it free of oil, grease, snow, and ice.

Example: Standing on the catwalk to connect the air lines, you first kick off the snow so your boots grip.

• air lines
• electrical cord
• frame
• glad hands
• steps

The steel rails and cross members that carry the body, axles, and load.

Look for cracks, bends, broken welds, and holes in the frame rails and cross members. Illegal welds and missing bolts are also defects.

Does not mean: In a question, 'time frame' means a period of time, not the vehicle frame.

Example: Under the trailer you spot a hairline crack spreading from a cross member bolt hole and report it.

Also called: trailer frame, vehicle frame

• cargo securement
• fifth wheel platform
• header board
• landing gear
• spring mounts

The spinning shaft that sends engine power to the drive axle.

Check the drive shaft for bends and cracks, and make sure the U-joints at each end are secure and free of debris.

Example: Looking under the tractor you see a strap of plastic wrapped around the drive shaft and clear it.

Also called: driveshaft

• drive axle
• exhaust system
• frame
• tandem axles

The pipes and muffler that carry exhaust gases away from the cab.

Parts must be securely mounted, not leaking, and not touching fuel lines, wiring, or tires. A leak lets poison fumes reach the cab or sleeper.

Example: Black soot streaks at a pipe joint under the cab show an exhaust leak you must get fixed before driving.

Also called: exhaust

• drive shaft
• frame
• fuel tank
• hoses and wiring
• sleeper berth

The outside lamps and reflectors that make the vehicle visible to others.

Lights and reflectors must be clean, working, the correct color, and not broken or missing. Front and side ones are amber; rear ones are red.

Differs by state: Mounting height for reflectors is state law. Texas, for example, requires reflectors on larger vehicles to be mounted between 24 and 60 inches above the ground.

Does not mean: Emergency reflectors (the triangles) are separate equipment, not the reflectors fixed to the vehicle.

Example: On your walk-around you wipe road film off the side reflectors so they shine at night.

Also called: reflectors, reflector

• hazard lights
• headlights
• marker lights
• turn signals
• warning devices

Flaps behind the wheels that stop tires from throwing mud and water.

Also called mud flaps. They must be present, not torn, properly mounted, and not dragging on the ground or rubbing the tires.

Differs by state: Mud flap height and coverage rules are state law, not federal. Texas, for example, requires flaps behind the rear wheels of trucks and trailers with four or more rear tires. They must reach to within 8 inches of the road.

Example: A torn mud flap slapping against the tire at highway speed is something you fix at the next stop.

Also called: mud flaps, splash guard, mud flap

• DOT bumper
• license plate
• lights and reflectors
• tires

A suspension arm that links the axle to the frame and guides it.

Control arms and their mounts must be securely attached and not bent, cracked, or broken. They are suspension parts, not steering parts.

Does not mean: A control arm is suspension; it does not steer the wheels.

Example: You grab the control arm near its bushing and push; movement at the mount means worn bushings to report.

Also called: control arm mount

• air bags
• shock absorber
• spring mounts
• steering linkage
• torsion bar

A suspension rod that locates the axle and resists twisting forces.

On trucks the inspection item is usually the torque rod, also called a radius rod, that keeps the axle in place. Check that it is securely mounted and not bent, cracked, or broken.

Does not mean: Here it usually means the torque rod that locates the axle, not a car's torsion-bar spring.

Example: A torque rod with a missing bolt has let the axle walk back on one side, so the trailer looks crooked.

Also called: torque rod

• air bags
• control arm
• leaf spring
• shock absorber
• spring mounts

Rubber bellows filled with compressed air that act as the vehicle's springs.

Air ride suspension uses air bags instead of, or together with, steel springs. Check that each bag is securely mounted, not damaged, and not leaking; listen for hissing.

Does not mean: Not the safety airbags in a steering wheel; these are suspension parts.

Example: The tractor sits low on one side at the yard, and you find a hissing air bag on that corner.

Also called: air ride suspension, air suspension

• leaf spring
• shock absorber
• spring mounts
• torsion bar

The tractor frame structure that holds the fifth wheel skid plate.

Check the platform for cracks, breaks, and loose or missing mounting bolts. It must hold the skid plate solidly.

Example: Shining your light under the fifth wheel, you check the platform welds for cracks before coupling.

• fifth wheel
• frame
• kingpin
• skid plate
• sliding fifth wheel

The greased fifth wheel plate that the trailer apron rests on.

The skid plate must be securely mounted on the platform, properly greased, free of cracks, and able to tilt freely. The kingpin locks into it.

Example: A dry, shiny skid plate tells you the fifth wheel has not been greased, so you grease it before coupling.

• apron
• fifth wheel
• fifth wheel gap
• fifth wheel platform
• kingpin

Any space between the trailer apron and the fifth wheel after coupling.

After coupling, look between the apron and the skid plate. The trailer should lie flat on the fifth wheel with no space. Any gap means the coupling is wrong.

Example: You look into the fifth wheel gap with a flashlight and confirm the locking jaws are closed around the kingpin.

• apron
• coupling
• kingpin
• locking jaws
• skid plate

Straps or hooks that hold trailer doors shut, or hold them open.

On the pre-trip, ties must be present, not broken, and securely mounted. A loose door swinging at speed can hit other vehicles.

Does not mean: A door seal is different: the rubber weather seal, or the numbered security seal on a locked load.

Example: Loading at a dock with the doors swung open, you hook each door tie so the wind cannot slam them.

• cargo doors
• cargo securement
• frame
• tie-downs

The rear guard that stops a car from sliding under the trailer.

Also called the ICC bumper or rear impact guard. Check that it is securely mounted and not bent, broken, or damaged.

Example: Backing to a dock, you keep an eye on the DOT bumper so it does not hit the dock plate.

Also called: ICC bumper, rear impact guard

• bumper
• cargo doors
• lights and reflectors
• splash guards

The safety items a commercial vehicle must carry for breakdowns and fires.

The required set is a fire extinguisher, spare electrical fuses unless the vehicle has circuit breakers, and warning devices for a stopped vehicle. Tire chains and an accident kit are optional extras.

Example: Before leaving the yard you open the side box and confirm the extinguisher, fuses, and triangles are all there.

• fire extinguisher
• pre-trip inspection
• spare fuses
• warning devices

A portable device for putting out small fires, carried in the cab.

Check that it is fully charged, the gauge is in the green, the pin and seal are in place, and it is mounted securely. The rating letters tell you which fire classes it handles. A power unit carrying placarded hazardous materials must have an extinguisher rated at least 10 B:C.

Example: You pull the extinguisher from its bracket, see the needle in the green, and clip it back in.

• emergency equipment
• hazardous materials
• placarded vehicle
• spare fuses
• warning devices

Extra electrical fuses carried to replace a blown fuse on the road.

Required unless the vehicle uses circuit breakers. On the inspection test, say so if your vehicle has breakers instead of fuses.

Example: The marker lights die on a night run, so you swap in a spare fuse from the kit and carry on.

• battery
• emergency equipment
• fire extinguisher
• marker lights
• warning devices

Reflective triangles or flares that warn traffic your vehicle is stopped.

Carry three red reflective triangles; set them out within 10 minutes of stopping and hold each one toward traffic as you walk. On a one-way or divided highway, space them 10, 100 and 200 feet behind the vehicle, toward the traffic. On a two-lane road, put one within 10 feet of the vehicle, one 100 feet behind and one 100 feet ahead.

Differs by state: Placement distances and the triangle count are the same in every state manual. Georgia and North Dakota do not list fusees or liquid flares as alternatives, so 'three red reflective triangles' is the answer that is correct everywhere.

Example: Stopped on a two-lane shoulder, you carry a triangle facing traffic as you walk back to place it.

Also called: reflective triangles, emergency warning devices

• emergency equipment
• fire extinguisher
• hazard lights
• lights and reflectors
• spare fuses

The belt that holds you in your seat during a crash.

Federal rules require you to wear the seat belt whenever you drive a commercial motor vehicle. On the pre-trip, check that it is mounted securely, adjusts, latches, and is not ripped or frayed.

Example: Before releasing the parking brake you buckle the seat belt and tug it to make sure it is latched.

Also called: safety belt

• emergency equipment
• parking brake
• passenger vehicle
• pre-trip inspection

Pedal that disconnects the engine from the transmission so you can shift gears.

Press it with your left foot before you move the shifter, then let it out smoothly. Most heavy trucks need double clutching: press, shift to neutral, release, press again, shift into gear. On the in-cab check you make sure it is not loose, sticking or damaged.

Example: Stopped at a red light on a hill, you partly engage the clutch pedal before releasing the brake so the truck does not roll back.

Also called: clutch

• accelerator
• brake pedal
• pre-trip inspection
• shifter

The pedal that controls engine power; pressing it speeds the truck up.

Drivers also call it the gas pedal. Press it gently so the truck does not jerk; on wet or icy roads too much power makes the drive wheels spin. If the truck's retarder is switched on, letting the pedal all the way up makes the retarder brake.

Example: Pulling away on packed snow, you feel the drive wheels start to spin, so you ease off the accelerator until they grip again.

Also called: accelerator pedal

• brake pedal
• clutch pedal
• hydroplaning
• retarder
• traction

The lever you move to select a gear in a manual transmission.

Before you start the engine, put the shifter in neutral, or in park on an automatic. On trucks with extra gear ranges, a selector knob or switch on the shifter changes range.

Example: Climbing into the cab, you check the shifter is in neutral and the parking brake is set before you turn the key.

Also called: gearshift, gear shift

• accelerator
• clutch pedal
• downgrade
• pre-trip inspection

Dash gauge showing how hot the engine coolant is.

After you start the engine, the needle should rise slowly into the normal range and stay there. A reading that keeps climbing means the engine is overheating; stop and find the cause.

Example: Halfway up a long grade, the water temperature gauge creeps toward the red, so you ease off the accelerator and let the engine cool.

Also called: coolant temperature gauge

• ammeter
• oil pressure gauge
• pre-trip inspection
• radiator
• water pump

Dash gauge showing the pressure of the oil circulating through the engine.

The needle should move into the normal range within seconds of starting the engine. If it stays low or a warning light stays on, shut the engine off; running without oil pressure destroys it.

Example: You start the engine and watch the oil pressure gauge: the needle lifts into the normal band at once, then you check the air gauge.

• air pressure gauge
• ammeter
• oil level
• pre-trip inspection
• water temperature gauge

Dash gauge showing whether the alternator is charging the batteries.

An ammeter shows current flowing in or out of the batteries; a voltmeter shows system voltage. After start-up the needle should sit in the normal range and the charging warning light should go out.

Example: At a fuel stop you notice the voltmeter reading lower than usual, a sign the alternator belt may be slipping.

Also called: voltmeter

• alternator
• battery
• compressor belt
• oil pressure gauge
• water temperature gauge

Dashboard lights that show which lamps are on: turn signals, four-way flashers, high beams.

In the vehicle inspection test you name each indicator and show it works when you switch the lamp on. The ABS light and, on some trucks, the DEF light are checked in the same step.

Example: With the engine running, you flick the left turn signal and point to the blinking green arrow on the dash for the examiner.

• anti-lock braking system
• hazard lights
• high beams
• pre-trip inspection
• turn signals

The electric or air-powered device that sounds a warning to other road users.

Use the horn when it helps avoid a crash, for example a light tap before passing someone who may not see you. Do not use it to vent anger; a sudden blast can startle other drivers and cause the crash you wanted to avoid.

Does not mean: In coupling text, the horn of a pintle hook is a metal lug, not the warning device.

Example: A cyclist ahead is drifting toward your lane, so you tap the horn lightly well before you pass.

Also called: horns

• aggressive driving
• blind spot
• passing
• pintle hook
• pre-trip inspection

Cab controls that warm the driver and clear frost or fog from the windshield.

In the inspection test you turn the switch from heat to defrost and show both work. In winter, check them before you leave; if you cannot see out, you cannot drive safely.

Example: On a frosty morning you run the defroster for a few minutes so the windshield is fully clear before you pull out.

Also called: defroster

• pre-trip inspection
• visibility
• windshield
• windshield wipers

Depth of a tire's main grooves; shallow grooves mean less grip.

Front (steering) tires need at least 4/32 inch of tread in every major groove; all other tires need at least 2/32 inch. No fabric may show through the tread or sidewall. Measure with a tread depth gauge in every major groove; on the inspection test, say that you use a gauge.

Example: Before a winter trip you check each groove of the steer tires with a gauge; one reads below the limit, so the tire is replaced.

• hydroplaning
• pre-trip inspection
• tires
• traction
• tread depth minimums

Free movement of the steering wheel before the front wheels respond.

A little play is normal. Too much play makes the truck hard to steer and means worn or loose steering parts; it must be fixed before you drive. The manual gives the limit in degrees and in inches of movement at the rim.

Example: With the engine running, you rock the steering wheel back and forth; it moves a hand's width before the tires react, so you report it.

Also called: steering play

• pitman arm
• pre-trip inspection
• steering gear box
• steering linkage
• tie rod

Timed air-pressure drop with the engine off and all brakes released.

Build full air pressure, shut the engine off, release the parking brakes, wait for the gauge needle to settle, then time one minute. If pressure drops more than the manual allows, there is a leak to find before you drive. The allowed loss is in the air leakage rate entry.

Differs by state: Most manuals print 2 psi per minute for a single vehicle and 3 psi for a combination. California adds a third tier: 5 psi for a combination of three or more vehicles. Colorado and Wisconsin describe only the applied (brakes-held) test; their manuals have no static test and no 2 psi figure. A question keyed to the static drop cannot be answered from those two manuals.

Does not mean: Not the applied test: here the brake pedal stays up; the applied test holds it down.

Example: Before leaving the yard you run the static air leak test; the needle holds steady for the full minute, so the system is tight.

Also called: air leak test (static)

• air leakage rate
• air pressure gauge
• applied air leak test
• pre-trip inspection
• spring brake pop-out test

Air-pressure drop timed while the brake pedal is held fully down.

With the engine off and parking brakes released, press the brake pedal all the way, let the gauge settle, then hold for one minute. The allowed loss is a little higher than in the static test; see the air leakage rate entry. It is one of the four air brake checks; missing any one fails the vehicle inspection test.

Differs by state: Every manual describes this test with the same limits, so it is the safer leak question nationally. Colorado and Wisconsin describe only this test and omit the static one.

Example: You stand on the brake pedal for a minute with the engine off and watch the gauge fall only slightly, well inside the limit.

Also called: applied pressure test

• air leakage rate
• brake pedal
• pre-trip inspection
• spring brake pop-out test
• static air leak test

Fanning air pressure down until the parking-brake valve pops out on its own.

With the engine off, pump the brake pedal to bleed air down. First the low-air warning should come on; keep pumping and the yellow and red knobs should pop out, which sets the spring brakes. The pressure range where this must happen is in the spring brake pop-out range entry.

Example: During the air brake check you fan the pedal; the buzzer sounds, then both knobs snap out with a hiss, right where the examiner expects.

Also called: pop-out test

• low air pressure warning
• parking brake
• spring brake pop-out range
• spring brakes
• trailer air supply control