Cars, Maintenance and Automotive Knowledge — Glossary¶

A living reference. Add terms as you encounter them — don't wait until you understand them perfectly. Writing a definition in your own words is itself a powerful learning tool.

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### term-name

**Definition:** A clear, concise explanation of what this term means in the context of automotive knowledge.

**Also known as:** Other names, abbreviations, or synonyms (if any).

**Related:** [[related-concept]], [[another-related-concept]]

**Example:**
A concrete, specific example of the term in use.

Keep definitions in your own words as much as possible.

A¶

alternator¶

Definition: An electrical generator driven by the engine's serpentine belt that converts mechanical energy into alternating current (AC), which is then rectified to DC to charge the battery and power the vehicle's electrical systems while the engine is running. A failing alternator cannot keep the battery charged, causing electrical systems to fail and the engine to stall.

Also known as: Generator (older terminology)

Related: serpentine-belt, battery, voltage-regulator

Example: A healthy alternator should produce 13.5–14.5 volts DC at the battery terminals with the engine running. A reading below 13 V with the engine running suggests a failing alternator or a bad voltage regulator.

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B¶

brake caliper¶

Definition: The hydraulic clamping mechanism in a disc brake system. When brake pressure is applied, the caliper squeezes brake pads against the rotor to create friction and slow the wheel. Calipers can be fixed (with pistons on both sides) or floating (with pistons on one side only). Seized calipers that won't fully release cause premature brake wear and dangerous pulling.

Also known as: Caliper assembly

Related: rotor, brake-pad, brake-fluid

Example: When inspecting brakes, a stuck caliper is often identified by one wheel's rotor being significantly hotter than the others after driving, or by uneven brake pad wear — one pad much thinner than its pair.

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C¶

catalytic converter¶

Definition: An exhaust emission control device located in the exhaust system that uses precious metal catalysts (platinum, palladium, rhodium) to convert toxic combustion byproducts (hydrocarbons, carbon monoxide, nitrogen oxides) into less harmful substances (carbon dioxide, water vapor, nitrogen). A failing catalytic converter triggers a check-engine light and may cause the vehicle to fail emissions testing.

Also known as: Cat, cat-con

Related: exhaust, OBD-II, oxygen-sensor

Example: A rattling sound from under the car that changes with RPM often indicates a broken catalytic converter substrate. Thieves also target catalytic converters for their precious metal content — a theft leaves a vehicle extremely loud and unable to pass emissions.

coolant¶

Definition: A mixture of water and antifreeze (typically ethylene glycol or propylene glycol) that circulates through the engine and radiator to absorb and dissipate heat. Proper coolant ratio prevents both freezing in winter and boilover in summer, while also providing corrosion inhibition for internal metal surfaces. Coolant degrades over time and must be flushed and replaced on schedule.

Also known as: Antifreeze, engine coolant, radiator fluid

Related: thermostat, radiator, water-pump, viscosity

Example: Most modern vehicles use long-life coolant (orange or pink, OAT-based) rated for 5 years or 150,000 miles. Older vehicles used green (IAT) coolant changed every 2 years. Mixing types can cause gel formation and clog the cooling system.

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D¶

DTC fault code¶

Definition: A Diagnostic Trouble Code — a standardized alphanumeric code stored in the vehicle's Engine Control Module (ECM) when a sensor or system reports a condition outside its normal operating range. DTCs trigger the check-engine light (MIL — Malfunction Indicator Lamp) and can be read with an OBD-II scanner. The first letter indicates the system: P (Powertrain), C (Chassis), B (Body), U (Network).

Also known as: Fault code, OBD code, trouble code, check-engine code

Related: OBD-II, ECM, scan-tool

Example: P0420 is one of the most common DTCs: "Catalyst System Efficiency Below Threshold (Bank 1)." It indicates the catalytic converter is not converting exhaust gases efficiently. Common causes include a failing cat, exhaust leaks before the downstream O2 sensor, or oil burning.

E¶

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F¶

Add F-terms here

H¶

horsepower¶

Definition: A unit of power measuring the rate at which an engine can do work. In the automotive context, one horsepower equals approximately 746 watts or the ability to lift 550 pound-feet per second. Engine horsepower is measured at the crankshaft (brake horsepower, bhp) or at the drive wheels (wheel horsepower, whp) — wheel horsepower is always lower due to drivetrain losses. Higher horsepower generally means faster acceleration.

Also known as: hp, bhp (brake horsepower), PS (Pferdestärke, metric equivalent)

Related: torque, RPM, transmission

Example: A typical modern family sedan produces 150–200 hp. A sports car might produce 300–500 hp. The relationship between horsepower and torque: Horsepower = (Torque × RPM) / 5,252. This formula shows that horsepower is a derived value — torque is the fundamental force, and power is how quickly that force is applied.

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O¶

OBD-II¶

Definition: On-Board Diagnostics, second generation — a standardized vehicle diagnostic system mandated in all US passenger vehicles sold after January 1, 1996 (and subsequently adopted globally). OBD-II defines a standard 16-pin diagnostic port (typically under the dashboard), a standard communication protocol, and a standardized set of diagnostic parameters (PIDs) and fault codes (DTCs). Any compliant scan tool can communicate with any OBD-II vehicle regardless of make or model.

Also known as: OBD2, on-board diagnostics

Related: DTC-fault-code, ECM, scan-tool, MIL

Example: A $30 Bluetooth OBD-II adapter paired with a free smartphone app can read the same fault codes as a $5,000 shop scanner. The adapter plugs into the 16-pin OBD-II port (typically under the left side of the dashboard), establishes a connection, and reports all stored and pending DTCs, along with live sensor data like coolant temperature, throttle position, and oxygen sensor readings.

R¶

rotor¶

Definition: In the brake system: a disc-shaped metal component attached to the wheel hub that rotates with the wheel. Brake pads clamp against the rotor faces when the brakes are applied, converting kinetic energy into heat through friction. Rotors have a minimum thickness specification — below this threshold they cannot safely dissipate heat and must be replaced. Rotors can also warp from excessive heat, causing a pulsating or vibrating sensation when braking.

Also known as: Disc, brake disc, brake rotor

Related: brake-caliper, brake-pad, brake-fluid

Example: A warped rotor is felt as a pulsation through the brake pedal when stopping. To check rotor thickness, measure with a micrometer at multiple points around the rotor — if thickness varies by more than 0.001 inch (0.025 mm), the rotor has lateral runout and should be replaced (not resurfaced, as this removes material and may put it below minimum thickness).

RPM¶

Definition: Revolutions Per Minute — the number of complete rotations the engine crankshaft makes per minute. RPM is displayed on the tachometer. Most gasoline passenger car engines idle at 600–900 RPM, operate at 1,500–3,000 RPM during normal driving, and have a redline (maximum safe RPM) of 5,000–8,000 RPM. Operating consistently near redline causes accelerated wear; operating at very low RPM under heavy load (lugging) stresses the engine differently.

Also known as: Engine speed, revs

Related: horsepower, torque, transmission, throttle

Example: When climbing a hill in too high a gear, the engine "lugs" at low RPM under high load — this is hard on the drivetrain and should be avoided by downshifting. A properly driven car maintains RPM in the efficient range (typically 1,500–2,500 RPM for most fuel-efficient driving).

S¶

serpentine belt¶

Definition: A long, ribbed belt that loops around multiple engine accessories — the alternator, power steering pump, air conditioning compressor, and water pump (in some engines) — to drive them all from the engine's crankshaft pulley. Called "serpentine" because it winds in an S-shape around all these components. A broken serpentine belt causes immediate loss of all these accessories and rapid engine overheating if the water pump is belt-driven.

Also known as: Drive belt, accessory drive belt, poly-V belt

Related: alternator, water-pump, timing-belt

Example: Signs of a failing serpentine belt: squealing on startup (especially in cold or wet weather), visible cracking or fraying on the belt's ribbed surface, or the belt looking glazed and shiny. Most manufacturers recommend replacement every 60,000–100,000 miles. A belt that breaks while driving leaves you stranded — budget-quality belts should be avoided.

strut¶

Definition: A structural suspension component that combines the function of a shock absorber and a coil spring into a single unit, also serving as the upper pivot point for the front wheel steering. Because struts are structural (they support vehicle weight and are part of the steering geometry), their replacement requires realignment afterward. Struts differ from shock absorbers, which are standalone dampers on vehicles with separate spring and shock designs.

Also known as: MacPherson strut (the most common design type)

Related: shock-absorber, alignment, suspension, coil-spring

Example: Signs of worn struts: the car "nose-dives" excessively when braking, bounces more than twice after hitting a bump, or pulls to one side. The "bounce test" (push down hard on each corner of the car and let go — it should return to normal height and stop in one motion) can indicate worn struts if the car keeps bouncing.

T¶

timing belt¶

Definition: A toothed rubber belt (or in some engines, a timing chain) that connects the crankshaft to the camshaft(s), synchronizing the valve timing with piston movement. This timing is critical: if the belt breaks or jumps teeth, valves and pistons can collide, causing catastrophic (often irreparable) engine damage in "interference" engines. In "non-interference" engines, a belt failure stops the engine but does not damage it. Timing belts must be replaced on schedule — typically every 60,000–105,000 miles.

Also known as: Cam belt, cambelt

Related: serpentine-belt, camshaft, crankshaft

Example: A 2005 Honda Accord 4-cylinder with a timing belt (interference engine) must have it replaced at 105,000 miles per the manufacturer. Skipping this service to save $600 in labor is one of the most costly mistakes an owner can make — belt failure causes valve-to-piston contact and may require a full engine replacement costing $3,000–8,000+.

torque¶

Definition: A rotational force — specifically, the twisting force an engine produces at the crankshaft. Torque is measured in pound-feet (lb-ft) or Newton-meters (Nm). High torque at low RPM feels like strong "pulling power" — it's what makes a truck feel effortless when hauling a load or a diesel feel strong off the line. High horsepower at high RPM is what enables high top speeds. The relationship: Horsepower = (Torque × RPM) / 5,252.

Also known as: Engine torque, twist, pulling power

Related: horsepower, RPM, transmission

Example: A typical gas-powered family sedan might produce 150 lb-ft of torque at 4,000 RPM. A diesel pickup truck might produce 400+ lb-ft at 2,000 RPM — this lower-RPM peak torque is why diesels feel stronger when towing at low speeds, even if their peak horsepower is similar to a gas engine.

V¶

viscosity¶

Definition: A measure of a fluid's resistance to flow — its "thickness." In motor oil, viscosity is specified using the SAE (Society of Automotive Engineers) multi-grade system: a label like "5W-30" means the oil has a cold-weather viscosity of 5W (flows easily at low temperatures) and a hot operating viscosity of 30. Using oil of the wrong viscosity — too thin or too thick — can cause inadequate lubrication or sludge formation, both of which damage the engine over time.

Also known as: Oil weight (colloquially), oil grade

Related: motor-oil, coolant, engine

Example: Most modern engines specify 0W-20 or 5W-30. Older engines may specify 10W-40. Using 10W-40 in an engine that requires 0W-20 can cause the oil to be too thick at startup, starving the engine of lubrication during the critical first seconds after a cold start. Always use the viscosity specified on the oil cap or in the owner's manual.

W¶

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X Y Z¶

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Symbols & Notation¶

Glossary Stats¶