Reference
Semi Truck Parts Glossary
Plain-English definitions of the heavy-duty truck parts and systems we deal with every day — what each part is, what it does, and how to tell when it’s worn out. Free reference; no login. Know the part but not the number? Cross-reference lookup · VIN lookup.
Brakes & Wheel Ends
Shop brake & wheel-end parts- ABS Moduleaka ABS ECU
- The electronic control unit that monitors wheel speed sensors and modulates air pressure to prevent wheel lockup during braking. A failed module or sensor typically triggers the dash ABS warning lamp and can disable traction/stability functions. Diagnostic fault codes are read through the vehicle's diagnostic connector.
- Air Dryer
- A canister in the air supply line that removes moisture and oil from compressed air before it reaches the brake system, protecting valves and chambers from corrosion and freeze-ups. A clogged desiccant cartridge or failed purge valve leads to moisture in the tanks, causing frozen lines in cold weather or reduced air pressure buildup.
- Automatic Slack Adjusteraka ASA
- A self-adjusting slack adjuster that maintains correct pushrod stroke as brake shoes wear, eliminating manual adjustment. Internal clutch or gear mechanism resets stroke each brake application. Failure to self-adjust shows as excessive free play or out-of-spec pushrod stroke during inspection.
- Brake Chamberaka Air Chamber
- A sealed canister mounted at each wheel end that converts air pressure into mechanical force to apply the brakes via a pushrod. Service chambers handle normal braking while spring brake chambers add a parking/emergency section. Leaking diaphragms cause slow air leaks and long push-rod travel, reducing braking force.
- Brake Drum
- The cast-iron drum that rotates with the wheel hub and provides the friction surface the brake shoes press against. Heat and wear cause scoring, cracking, or out-of-round conditions. Pulsation, vibration, or visible heat cracks indicate the drum needs machining or replacement.
- Brake Rotor (Air Disc)aka Disc Brake Rotor
- The disc that rotates with the wheel hub on air disc brake systems; a caliper squeezes pads against its faces to slow the vehicle. Rotors wear thinner over time and can warp from heat. Vibration during braking or visible scoring/cracking indicates the rotor needs machining or replacement.
- Brake Shoe
- The curved friction-lined component that pivots against the S-cam and presses outward onto the brake drum to slow the wheel. Linings wear over time and must stay above minimum thickness. Reduced stopping power, grinding noise, or visible lining below spec signals replacement is due.
- Relay Valve
- A valve mounted near the rear axle that speeds brake application and release by supplying air directly from a nearby reservoir rather than routing it all the way from the treadle valve. A faulty relay valve causes slow brake response or uneven application between axles.
- S-Camaka S-Camshaft
- A rotating shaft with an S-shaped lobe that spreads the brake shoes apart when turned by the slack adjuster, applying the drum brakes. Worn cam bushings or a worn cam profile cause uneven or delayed brake application. Play or roughness felt when rotating the shaft indicates wear.
- Slack Adjuster
- The lever arm connecting the brake chamber pushrod to the S-cam shaft, converting linear chamber force into rotational force to apply the brake shoes. Manual versions require periodic stroke adjustment. Excessive pushrod travel or uneven brake application often points to a worn or misadjusted slack adjuster.
- Spring Brakeaka Parking Brake Chamber
- The rear section of a dual brake chamber containing a heavy spring that mechanically applies the brakes when air pressure is released, used for parking and emergency braking. Air pressure holds the spring compressed during normal driving. A stuck or weak spring brake causes the truck to fail to hold on a grade or not release fully, dragging the brakes.
- Wheel Bearing
- Tapered roller bearings that support the wheel hub on the spindle, allowing rotation while carrying vehicle load. Bearings require proper lubrication and preload adjustment. Growling noise, wheel wobble, or excessive play when rocking the tire indicates wear or improper adjustment.
- Wheel Hub
- The rotating assembly that mounts the wheel, bearings, and (on drum brakes) the brake drum to the spindle. Hubs may be conventional (serviceable bearings, oil or grease filled) or unitized/pre-set bearing assemblies. Leaking lubricant, looseness, or bearing noise indicates hub or bearing service is needed.
- Wheel Sealaka Hub Seal
- A seal pressed into the hub that retains lubricant around the wheel bearings and keeps out dirt and water. A failed wheel seal shows as grease leaking onto the brake drum or wheel, often causing contaminated linings and reduced braking.
Suspension & Steering
Shop suspension & steering- Air Springaka Air Bag
- A rubber and fabric bellows filled with compressed air that supports the chassis and cushions ride on air-ride suspensions. A leaking or ruptured air spring causes the truck to sag on one corner or fail to maintain ride height. Visible cracking or hissing air leaks are common failure signs.
- Ball Joint
- A pivoting joint connecting the steering knuckle to the suspension, allowing steering and vertical wheel movement while carrying load. Worn ball joints cause clunking, uneven tire wear, and looseness in the front end. Play detected during a dry-park or prybar check indicates replacement is due.
- Bushing
- A rubber, polyurethane, or bronze sleeve that cushions and allows controlled movement between two suspension or steering components, reducing metal-to-metal wear and vibration. Cracked, torn, or collapsed bushings cause clunking noises and imprecise handling.
- Drag Link
- The linkage connecting the pitman arm to the steering knuckle (or tie rod), transferring steering box motion to the wheels. Worn drag link ends cause steering slop and wandering. Clunking or excessive free play at the steering wheel is a common symptom.
- Kingpin
- The vertical pin that connects the steering knuckle to the axle beam, allowing the wheel to pivot for steering on a solid front axle. Kingpins and their bushings wear over high mileage. Excessive steering play, wandering, or clunking over bumps points to worn kingpins.
- Leaf Spring
- A stack of curved steel plates that supports axle load and controls ride height and axle location, common on solid-axle heavy trucks. Broken leaves, sagging ride height, or squeaking indicate spring fatigue or failure. Leaf springs also position the axle and resist braking/acceleration torque on many setups.
- Pitman Arm
- The lever arm connecting the steering gear output shaft to the drag link, converting steering box rotation into linear motion to steer the front wheels. Worn splines or a loose pitman arm cause steering play and wander. Movement between the arm and steering shaft indicates wear.
- Shock Absorberaka Shock
- A hydraulic damper that controls spring oscillation and tire contact with the road after hitting bumps. Worn shocks cause excessive bouncing, cab sway, or uneven tire wear. Visible oil leakage from the shock body indicates internal seal failure.
- Tie Rod End
- A ball-and-socket joint at each end of the tie rod that connects the steering knuckles, keeping the front wheels aligned as they turn together. Worn tie rod ends cause looseness in the steering wheel and uneven tire wear. Play felt when wiggling the tie rod indicates replacement is needed.
- Torque Rodaka Track Rod, Trailing Arm
- A suspension link that controls axle alignment and driveline angle by resisting axial and torsional movement of the axle under acceleration and braking. Worn bushings allow axle shifting, causing tire wear, driveline vibration, or clunking on acceleration/braking.
- U-Bolt
- A U-shaped threaded fastener that clamps the axle to the leaf spring or spring seat, holding the suspension assembly together. Loose or corroded U-bolts allow axle shift or spring movement, often heard as a clunk over bumps and visible as rust streaking or loose clamp plates.
Engine & Cooling
Shop parts by engine- Belt Tensioner
- A spring-loaded pulley assembly that maintains correct tension on the serpentine belt, compensating for belt stretch and vibration. A worn tensioner bearing causes belt slipping, squealing noise, or premature belt wear.
- Charge Air Cooleraka CAC, Intercooler
- A heat exchanger that cools compressed air from the turbocharger before it enters the engine intake, increasing air density for more efficient combustion. Cracks or leaks in the CAC cause boost loss, black smoke, or reduced power, and are often found with a pressure/smoke test.
- Coolant Hose
- Reinforced rubber hoses that carry coolant between the engine, radiator, and heater core. Hoses degrade with age and heat cycling, becoming soft, cracked, or bulging. A burst or leaking hose causes rapid coolant loss and overheating.
- DEFaka Diesel Exhaust Fluid, AdBlue
- A urea-water solution injected into the exhaust stream ahead of the SCR catalyst to break down NOx into nitrogen and water vapor. Low DEF level or quality faults trigger dash warnings and can cause engine power derate if not corrected.
- DPFaka Diesel Particulate Filter
- An exhaust after-treatment device that traps soot particulates from diesel exhaust, periodically burning them off during regeneration. A clogged or damaged DPF causes high exhaust backpressure, reduced power, and frequent regeneration cycles or derate warnings.
- EGR Cooler
- A heat exchanger that cools recirculated exhaust gas before it re-enters the intake, using engine coolant to lower gas temperature. Cracked internal passages allow coolant into the exhaust or intake, causing white exhaust smoke, coolant loss, or coolant-contaminated intake components.
- EGR Valveaka Exhaust Gas Recirculation Valve
- A valve that routes a controlled portion of exhaust gas back into the intake to lower combustion temperatures and reduce NOx emissions. Carbon buildup can cause the valve to stick open or closed, triggering rough idle, power loss, or check engine lights.
- Fan Clutch
- A temperature-controlled coupling between the engine and cooling fan that engages the fan only when additional airflow is needed, saving power and fuel. A fan clutch stuck engaged causes constant loud fan noise and reduced fuel economy; one stuck disengaged causes engine overheating.
- Fuel Injector
- A precision component that atomizes and delivers pressurized fuel directly into the combustion chamber at a specific timing and volume. A failing injector causes misfires, rough idle, poor fuel economy, or excessive smoke from one or more cylinders.
- Oil Cooler
- A heat exchanger that uses coolant or airflow to remove excess heat from engine oil, maintaining proper lubrication temperature. A leaking oil cooler can allow oil and coolant to mix, showing up as milky oil or coolant loss without an external leak.
- Serpentine Beltaka Accessory Belt
- A single continuous ribbed belt that drives multiple engine accessories such as the alternator, water pump, and air compressor from the crankshaft pulley. Cracking, glazing, or fraying of the belt indicates it is near failure and can cause sudden loss of charging, cooling, or air supply.
- Thermostat
- A temperature-sensitive valve that regulates coolant flow to the radiator, keeping the engine within its normal operating temperature range. A stuck-closed thermostat causes overheating; a stuck-open one causes the engine to run cold and take longer to reach operating temperature.
- Turbochargeraka Turbo
- A turbine-driven forced induction device that uses exhaust gas energy to compress intake air, increasing engine power and efficiency. Worn turbo bearings or seals cause oil consumption, blue/black smoke, whining noise, or loss of boost pressure.
- Water Pump
- A belt or gear-driven pump that circulates coolant through the engine block, heater core, and radiator to maintain operating temperature. A failing water pump often shows as coolant leakage from the weep hole, bearing noise, or engine overheating.
Electrical & Lighting
Shop electrical & lighting- Alternator
- A belt-driven generator that produces electrical current to recharge the batteries and power electrical accessories while the engine runs. A failing alternator causes dimming lights, a charge warning light, or a dead battery despite normal driving.
- Marker Lightaka Clearance Light
- An exterior light, typically amber or red, mounted on the cab, trailer, or body to indicate the vehicle's size and position to other drivers, required by DOT regulation. A burned-out bulb or cracked lens/housing allowing water intrusion are common failure points.
- NOx Sensoraka Nitrogen Oxide Sensor
- A sensor mounted in the exhaust stream, typically before and after the SCR catalyst, that measures nitrogen oxide levels to verify emissions system performance. A failed sensor triggers fault codes, incorrect DEF dosing, and can lead to engine derate.
- Solenoid
- An electromagnetic switch that uses a small control current to engage a larger electrical or mechanical circuit, commonly used in starter circuits and various valves. A failed solenoid often causes a clicking noise with no follow-through action, such as a no-crank starter condition.
- Starter Motoraka Starter
- An electric motor powered by the battery that cranks the engine to initiate combustion. A worn starter causes slow cranking, clicking with no crank, or intermittent no-start conditions.
- Wiring Harness
- A bundled and routed group of wires and connectors that distribute electrical power and signals throughout the chassis, cab, or engine. Chafed insulation, corroded connectors, or rodent damage cause intermittent electrical faults, shorts, or open circuits.
Body & Exterior
Shop body & exterior- Deer Guardaka Bumper Guard
- A tubular or plate steel guard mounted to the front bumper to protect the grille, headlights, and radiator area from animal strikes and minor front-end impacts. Mounting bolts and brackets should be checked periodically for looseness from vibration and impact stress.
- Fender
- The body panel that covers the top of the wheel/tire area, shielding the cab and other components from road spray and debris. Cracking or mounting bracket failure is common from road vibration and debris impact.
- Grille Guardaka Brush Guard
- A heavier-duty front-mounted guard, often combined with a bumper replacement, designed to shield the grille and front end from brush, debris, and collision damage. Similar to a deer guard but typically covers a larger area and may integrate light mounts.
- Hood
- The front body panel assembly that covers the engine compartment, often tilting or hinged for access on heavy trucks. Cracks, hinge wear, or latch failure can cause misalignment or, in severe cases, unexpected hood movement while driving.
- Mirror Assembly
- The complete mirror unit, including housing, glass, and mounting arm, providing the driver rearward and side visibility; may include heated, powered, or convex sections. Cracked housings, broken glass, or failed motors/heating elements are common failure points.
- Mudflap
- A flexible rubber or poly flap mounted behind wheels to reduce road spray and debris thrown onto following vehicles, often required by regulation for width and ground clearance. Torn, missing, or dragging mudflaps are common inspection violations.
Drivetrain
Browse the full catalog- Clutch
- A friction assembly that engages and disengages engine power from the transmission input shaft, allowing gear changes and smooth starts from a stop on manual transmissions. Slipping under load, difficulty shifting, or a soft/spongy pedal indicates worn clutch discs or a failing pressure plate.
- Differentialaka Diff, Rear End
- An axle assembly that transmits engine torque from the driveshaft to the wheels while allowing them to rotate at different speeds during turns. Whining, grinding, or oil leakage from the housing indicates internal gear or bearing wear.
- Driveshaftaka Propshaft
- The rotating shaft assembly that transmits torque from the transmission to the differential(s), incorporating u-joints and a slip joint to accommodate length and angle changes. Vibration at speed or visible wobble often indicates an out-of-balance shaft or worn joint.
- U-Jointaka Universal Joint
- A cross-shaped joint with bearing caps that connects driveshaft sections, allowing torque transfer through changing driveline angles. Worn u-joints cause clunking on acceleration/deceleration and vibration; play felt when twisting the shaft by hand indicates wear.
Fitment & Identification
Cross-reference lookup- Cross-Referenceaka Cross-Ref
- The process or listing of matching an OEM or competitor part number to an equivalent aftermarket part number to confirm fitment and function. Used by counter staff to locate replacement parts when only an OEM or competitor number is known.
- Interchange
- A listing showing which parts are functionally and dimensionally compatible across different makes, models, or brands. Used to find substitute parts when the original number is discontinued or unavailable.
- MPNaka Manufacturer Part Number
- The part number assigned by the manufacturer of a specific component, aftermarket or OEM, used for ordering and inventory identification. Distinct from OEM numbers when the part is produced by a supplier under its own numbering system.
- OEM Numberaka OE Part Number
- The part number assigned by the original vehicle or component manufacturer to identify a specific factory part. Used to verify correct fitment when cross-referencing aftermarket replacement parts.
- VINaka Vehicle Identification Number
- The unique 17-character number assigned to a specific vehicle, encoding manufacturer, model, engine, and production details. Used to decode exact factory specifications when ordering parts that vary by build configuration.
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