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Glossary
Every part named in Auto Expert. One line on what it does, where it is, and a link to see it on the car.
- 1
- 12-volt battery
- Stores energy to start the engine and steadies the electrical system. On hybrids and EVs a 12-volt battery still runs the computers and locks. Usually a corner of the engine bay; sometimes the trunk or under a seat. See it on the car
- A
- A/C compressor
- The pump of the air conditioning system, driven by the serpentine belt (or an electric motor on hybrids and EVs). It compresses refrigerant so it can shed heat outside and absorb it inside. On the front or side of the engine, on the belt. See it on the car
- ABS module
- Anti-lock braking: wheel speed sensors tell the module when a wheel is about to lock, and it pulses that brake many times a second so you keep steering. A block of valves with a pump and electronic unit in the engine bay, with brake lines in and out, plus a speed sensor at each wheel. See it on the car
- Air filter & box
- A paper or fabric element in a plastic box that stops dust and grit before air reaches the engine. It is the cheapest protection the engine has. A black plastic box at the front or side of the engine bay with a large tube running to the engine. See it on the car
- Airbags
- Inflate in a fraction of a second during a crash. Sensors and a control module decide which bags fire and when. In the steering wheel, dashboard, seats, pillars and roof rails. See it on the car
- Alternator
- Generates electricity while the engine runs, powering everything and recharging the battery. On the side of the engine, driven by the serpentine belt. See it on the car
- B
- Ball joints
- A ball-and-socket joint that lets the wheel steer and bounce at once, like a hip joint. Where the control arm meets the wheel hub, inside the wheel well. See it on the car
- Blower motor
- The fan that pushes air through the vents. Behind the glovebox or under the dash on the passenger side. See it on the car
- Brake booster
- Multiplies the force of your foot using engine vacuum or an electric pump, so stopping takes a light push. The large round canister on the firewall behind the brake pedal, with the master cylinder bolted to its front. See it on the car
- Brake caliper
- A hydraulic clamp. Brake fluid pushes its piston, which squeezes the pads against the rotor. It slides on pins so both pads wear evenly. Straddling the rotor behind each wheel, connected to a flexible brake hose. See it on the car
- Brake lines & hoses
- Steel tubes along the body and flexible rubber hoses at each wheel that carry brake fluid pressure from the master cylinder to the calipers. Along the underside of the car, with short hoses at each wheel. See it on the car
- Brake pads
- Friction blocks that clamp the rotor. They are designed to wear away and be replaced; their thickness can be measured. Inside each caliper, one on each side of the rotor. See it on the car
- Brake rotor
- The metal disc that spins with the wheel. The pads squeeze it to slow the car; friction turns motion into heat. Behind each wheel, on the hub, inside the caliper. See it on the car
- Bumpers & crash structures
- The bumper cover you see is plastic; behind it a metal beam and crush cans absorb low-speed impacts, and behind those the crumple zones manage a real crash. Front and rear, under the painted covers. See it on the car
- Bushings
- Rubber (or polyurethane) sleeves at every suspension pivot that let parts move while absorbing noise and vibration. At the body end of control arms, trailing arms, sway bars and subframes. See it on the car
- C
- Cabin air filter
- Filters the air coming into the cabin through the vents. Usually behind the glovebox. See it on the car
- Camshaft position sensor
- Tells the computer which stroke each cylinder is on so fuel and spark go to the right cylinder. On the cylinder head near the end of a camshaft. See it on the car
- Camshafts
- Shafts with egg-shaped lobes that push the valves open at exactly the right moment. Modern engines can shift their timing (variable valve timing) for power and economy. Inside the top of the cylinder head, under the valve cover, driven by the timing belt or chain. See it on the car
- Catalytic converter
- A ceramic honeycomb coated with precious metals that turns the most harmful exhaust gases into less harmful ones. Required to pass emissions and expensive because of the metals inside. In the exhaust pipe under the front floor, sometimes right at the manifold. See it on the car
- Clutch
- On a manual, the disc that connects and disconnects the engine from the gearbox when you press the pedal. It is a wear item like brake pads. Between the engine and the manual transmission. See it on the car
- Coil springs
- Holds the weight of the car and lets the wheel move up and down. Around the strut at the front; beside or around the shock at the rear. See it on the car
- Condenser
- A radiator for the A/C: hot refrigerant from the compressor gives up its heat to the outside air here. In front of the engine radiator, behind the grille. See it on the car
- Connecting rods
- Steel links that join each piston to the crankshaft, turning the straight push of the piston into a turn of the crank. Inside the block, one per cylinder, between the pistons and the crankshaft. See it on the car
- Control arms
- Hinged links that connect the wheel hub to the body so the wheel can move up and down while staying upright. Bushings at the body end and a ball joint at the wheel end. Low in each wheel well, running from the subframe out to the hub. See it on the car
- Control modules
- The computers: the engine module (ECM/PCM) runs fuel and spark, the transmission module runs shifts, the body module runs lights and locks, the ABS module runs the brakes. Trouble codes come from here. The engine module is usually in the engine bay or behind the dash; others are scattered through the car. See it on the car
- Coolant reservoir
- A translucent tank that gives coolant room to expand when hot and shows you the level. On many cars it is also the pressurized cap for the whole system. A plastic tank near the radiator with MIN and MAX lines. See it on the car
- Coolant temperature sensor
- Reports engine temperature so the computer can adjust fuel, run the fans and light the gauge. Threaded into the cylinder head or thermostat housing, in the coolant. See it on the car
- Cooling fans
- Pulls air through the radiator when the car is not moving fast enough to do it on its own, such as in traffic or at idle. Mounted on the engine side of the radiator. See it on the car
- Crankshaft
- The spinning shaft at the bottom of the engine that converts the pistons' up-and-down motion into rotation, the way bicycle pedals turn a wheel. Everything the car does starts here. Along the bottom of the block, above the oil pan, running the length of the engine. See it on the car
- Crankshaft position sensor
- Tells the computer exactly where the crankshaft is and how fast it is turning, so spark and fuel happen at the right instant. Low on the engine near the crankshaft pulley or flywheel. See it on the car
- CV axles & joints
- Shafts that carry power from the transmission to the front (and on all-wheel drive, rear) wheels, with constant-velocity joints at each end that let the wheels steer and bounce while spinning. From each side of the transmission out to the wheel hubs. See it on the car
- Cylinder head
- The lid on top of the block. It holds the valves, camshafts and spark plugs, forms the top of each combustion chamber, and carries the intake and exhaust ports. Bolted on top of the engine block under the valve cover. V-shaped engines have two. See it on the car
- D
- Differential
- A gearbox at the driven axle that lets the inside and outside wheels turn at different speeds in a corner while both receive power. In the center of the rear axle on rear- and all-wheel drive; built into the transaxle on front-drive. See it on the car
- Doors
- Hold the side windows, locks, speakers and side-impact beams that protect you. Hinged into the body on each side of the cabin, between the front and rear wheels. See it on the car
- Downstream oxygen sensor
- Checks the exhaust after the converter to confirm the converter is working. In the exhaust pipe just after the catalytic converter. See it on the car
- Driveshaft
- A long tube that carries power from a front engine to a rear differential on rear- and all-wheel-drive vehicles, with universal or CV joints at each end. Under the center of the car, running from the transmission (or transfer case) back to the rear axle. See it on the car
- E
- EGR valve
- Lets a little exhaust back into the intake to lower combustion temperature and cut nitrogen oxide emissions. Between the exhaust manifold and the intake manifold, on or near the cylinder head. See it on the car
- Electric drive motor
- An electric motor that turns the wheels directly (EV) or alongside the engine (hybrid). It also acts as a generator when braking, putting energy back into the battery. Between the drive wheels, in the transaxle or on the axle. See it on the car
- Engine block
- The cast metal foundation of the engine. The cylinders are bored into it, the crankshaft spins in its base, and coolant and oil run through passages inside it. The lowest, heaviest part of the engine, sitting on the engine mounts in the middle of the engine bay. See it on the car
- Engine mounts
- Rubber-and-metal cushions that hold the engine and transmission to the body while soaking up their vibration. Some are filled with fluid or controlled electronically. Two to four points where the engine and transmission meet the body or subframe, around the edges of the engine bay. See it on the car
- EVAP canister & purge valve
- A charcoal canister that stores fuel vapor from the tank and a purge valve that feeds it into the engine to be burned, so vapor never escapes to the air. Near the fuel tank, with a purge valve near the intake manifold. See it on the car
- Evaporator
- The cold part: refrigerant expands here and absorbs heat from cabin air blown across it. Inside the dashboard, in the heater box, next to the heater core. See it on the car
- Exhaust manifold
- Collects hot exhaust from each cylinder into one pipe. On turbo engines the turbo bolts directly to it. Bolted to the side of the cylinder head, usually under a heat shield. See it on the car
- Exhaust piping
- Carries exhaust from the manifold, through the converter and muffler, to the tailpipe. Rubber hangers isolate it from the body. Running the length of the underside. See it on the car
- F
- Fuel filter
- Catches rust and debris before fuel reaches the injectors. Many modern cars build it into the pump module as a lifetime strainer. Along the fuel line under the car, or inside the tank with the pump. See it on the car
- Fuel injectors & rail
- Precise electric valves that spray a measured mist of fuel into the intake port or directly into the cylinder. The fuel rail feeds them. On the intake manifold or cylinder head, one per cylinder. See it on the car
- Fuel lines
- Steel and reinforced plastic tubes that carry fuel from the tank to the engine and, on older designs, return it. Running along the underside of the car, usually beside the brake lines. See it on the car
- Fuel pump
- An electric pump inside the tank that pushes fuel forward under pressure to the engine. Direct-injection engines add a second, engine-driven high-pressure pump. Inside the fuel tank, reached through a panel under the rear seat or by lowering the tank. See it on the car
- Fuel tank
- Holds the fuel. Modern tanks are plastic, sealed so vapor cannot escape, and contain the pump and level sender. Under the rear seat or trunk floor, between or just ahead of the rear wheels. See it on the car
- Fuse box
- Distributes power and protects each circuit with a fuse or relay. One in the engine bay, one or more inside the cabin. See it on the car
- H
- Head gasket
- The seal between the block and the cylinder head that keeps combustion, coolant and oil in their separate paths. A thin layer clamped between the block and the head, invisible from outside. See it on the car
- Heater core
- A small radiator inside the dash that hot engine coolant flows through to warm the cabin. Deep in the dashboard, with two hoses through the firewall. See it on the car
- High-voltage battery
- A large battery pack that stores the energy that drives the car on a hybrid or EV. It runs at hundreds of volts and is cooled and managed by its own electronics. Under the floor on most EVs; under or behind the rear seat on many hybrids. See it on the car
- Hood
- Covers the engine bay and is designed to fold in a crash rather than enter the cabin. The large hinged panel over the engine bay at the front of the car. See it on the car
- I
- Ignition coils
- Transformers that turn 12 volts into the tens of thousands of volts a spark plug needs. Most engines have one coil per cylinder. On top of each spark plug under the engine cover. See it on the car
- Intake manifold
- Splits incoming air into a path for each cylinder. On many engines it also holds the fuel rail and sensors, and vacuum lines branch from it. On top of or beside the engine, between the throttle body and the cylinder head. See it on the car
- Intercooler
- A radiator for air. Compressed air from the turbo is hot; cooling it makes it denser and safer for the engine. Usually low at the front behind the bumper, or on top of the engine, with large pipes to the turbo and throttle body. See it on the car
- Inverter / power electronics
- Converts the battery's direct current into the alternating current the motor needs, and back again when braking. Also charges the 12-volt system on most hybrids and EVs. Usually a finned box in the engine or motor bay, with thick orange high-voltage cables. See it on the car
- K
- Knock sensor
- A microphone bolted to the block that hears detonation (fuel igniting too early) so the computer can retard spark and protect the pistons. On the side of the block, often under the intake manifold. See it on the car
- L
- Lighting
- Headlamps, tail lamps, signals and the lamps inside. Modern LED units are sealed assemblies rather than replaceable bulbs. Front and rear corners, mirrors, and the cabin. See it on the car
- M
- Mass airflow sensor
- Measures how much air is entering the engine so the computer can add the matching amount of fuel. In the intake tube just after the air filter box. See it on the car
- Master cylinder
- Turns the pedal's push into hydraulic pressure, in two separate circuits so one leak cannot take out all four brakes. The brake fluid reservoir sits on top. On the booster on the firewall, driver's side. See it on the car
- Muffler
- Quiets the exhaust with chambers and perforated tubes. At the very back of the exhaust, near the rear bumper. See it on the car
- O
- Oil filter
- Traps particles in the oil before they reach bearings. Changed with every oil change. A canister or cartridge on the side or top of the engine. See it on the car
- Oil pan
- The reservoir bolted to the bottom of the engine that holds the oil when the engine is off and feeds the oil pump. The lowest part of the engine, visible from underneath; the drain plug is here. See it on the car
- Oil pump
- Pushes oil under pressure to every bearing and moving surface. Without it, metal touches metal within seconds. At the bottom front of the engine, driven by the crankshaft or timing chain, drawing from the oil pan. See it on the car
- P
- Parking brake
- Holds the car when parked, by cable or an electric motor acting on the rear brakes. At the rear wheels, operated by a lever, pedal or button. See it on the car
- PCV valve
- Lets the fumes that leak past the pistons out of the crankcase and back into the intake to be burned, so pressure does not build up inside the engine. On or near the valve cover, with a hose to the intake manifold. See it on the car
- Pistons
- Pistons are the plungers that burning fuel pushes down. That push is the only power the engine makes. Rings around each piston seal it against the cylinder wall and scrape oil back down. Inside the cylinders, within the block, moving up and down beneath the cylinder head. See it on the car
- Power steering assist
- Reduces the effort to turn the wheel: a belt-driven hydraulic pump and fluid on older cars, an electric motor on most newer ones. Hydraulic: a pump on the engine with a reservoir and hoses to the rack. Electric: a motor on the column or rack. See it on the car
- R
- Radiator
- A grid of thin tubes and fins at the front of the car. Hot coolant from the engine flows through it and air passing through the fins carries the heat away. Right behind the front grille, in front of the engine, with a fan behind it. See it on the car
- Radiator hoses
- Large rubber hoses that carry coolant between the engine and the radiator, plus smaller hoses to the heater core and reservoir. The big hoses at the top and bottom of the radiator, running to the engine. See it on the car
- Resonator
- A chamber in the exhaust that cancels specific droning frequencies. It changes the sound, not the emissions. In the exhaust pipe between the converter and the muffler. See it on the car
- S
- Seat belts
- The primary restraint. Pretensioners tighten them in a crash and load limiters let them give slightly so your chest is not injured. At each seat, anchored to the body. See it on the car
- Serpentine belt & tensioner
- One long ribbed belt driven by the crankshaft that spins the alternator, water pump, power steering pump and A/C compressor. A spring-loaded tensioner keeps it tight. On the front (or side) of the engine, wrapped around several pulleys. See it on the car
- Shock absorbers (rear)
- A damper that controls spring bounce. The rear of many cars uses a separate shock and spring instead of a strut. At each rear wheel, between the body and the axle or control arm. See it on the car
- Spark plugs
- Make the spark that lights the fuel-air mixture in each cylinder, many times a second. Screwed into the cylinder head under the ignition coils, one per cylinder. See it on the car
- Starter motor
- A powerful electric motor that spins the engine until it fires. Low on the engine where it meets the transmission. See it on the car
- Steering column
- Connects the wheel to the rack, with joints to route around the dashboard, the ignition lock, and on many cars the electric power-steering motor. From behind the steering wheel down through the firewall to the rack. See it on the car
- Steering rack
- Turns the column's rotation into the side-to-side motion that points the wheels. Hydraulic racks use pump pressure; electric ones have a motor on the rack or column. Across the car behind or below the engine, between the front wheels. See it on the car
- Steering wheel
- Your input. It also houses the driver airbag and the controls on its spokes. In front of the driver, on top of the steering column. See it on the car
- Struts (front)
- A strut combines the spring that holds the car up with the damper that stops it bouncing, and it is also part of the steering pivot on most front suspensions. Running vertically from the top of the wheel well down to the wheel hub at each front corner. See it on the car
- Sway bar & end links
- A torsion bar linking left and right wheels so the car leans less in corners. Short end links connect it to the struts and are a very common cheap source of clunks. Across the front (and often rear) of the car under the engine or trunk, with links at each end. See it on the car
- T
- Thermostat
- A temperature-controlled valve that keeps coolant inside the engine until it is warm, then opens to let it reach the radiator. In a housing where the upper radiator hose meets the engine. See it on the car
- Throttle body
- A valve that controls how much air the engine breathes when you press the accelerator. Most are now moved by an electric motor rather than a cable. Where the intake tube meets the intake manifold. See it on the car
- Tie rods
- Rods from each end of the rack to the wheel hubs. Their length sets the toe alignment; worn ends make the steering loose and wear tires. Behind each front wheel, from the rack to the steering knuckle. See it on the car
- Timing belt or chain
- The belt or chain that links the crankshaft to the camshafts so the valves open in step with the pistons. A chain is meant to last the engine's life; a belt is replaced on a schedule. At the front (or side) of the engine behind a cover, running from the crankshaft pulley up to the camshaft sprockets. See it on the car
- Tire pressure sensors
- A small battery-powered sensor in each wheel that reports tire pressure to the dashboard. Inside each tire, attached to the valve stem. See it on the car
- Tires
- The only part of the car that touches the road. Grip, braking, steering and comfort all depend on four patches of rubber about the size of your hand. Mounted on each of the four wheels, at the corners of the car. See it on the car
- Torque converter
- A fluid coupling between the engine and an automatic transmission that lets the engine keep running while the car is stopped, then locks up at speed for efficiency. Between the engine and the automatic transmission, inside the bell housing. See it on the car
- Transfer case
- On four- and all-wheel drive, splits power between the front and rear axles. Some have a low range for off-road. Bolted to the back of the transmission, with a driveshaft to each axle. See it on the car
- Transmission
- Chooses a gear ratio so the engine can stay in its happy range whether you are crawling or cruising. Automatics, manuals, dual-clutch and CVTs do it differently. Bolted to the engine; across the car on front-drive vehicles, behind it on rear-drive. See it on the car
- Turbocharger
- Uses exhaust gas to spin a turbine that forces extra air into the engine, so a small engine can make the power of a larger one. Bolted to the exhaust manifold, with pipes running to the intercooler and intake. See it on the car
- U
- Unibody / frame
- The structure. Most cars are a unibody, where the body shell itself is the frame; trucks use a separate ladder frame. Engineered crumple zones fold in a crash so the cabin does not. Everything the panels bolt to: rails, pillars, floor and roof. See it on the car
- Upstream oxygen sensor
- Measures oxygen left in the exhaust before the converter so the computer can trim the fuel mixture dozens of times a second. Screwed into the exhaust manifold or the pipe just after it. See it on the car
- V
- Valve cover & gasket
- The cap over the top of the cylinder head that keeps oil in and dirt out. Its gasket is one of the most common oil leaks. The very top of the engine; what you see first when you open the hood. See it on the car
- Valves
- Doors into each cylinder. Intake valves open to let air (or air and fuel) in; exhaust valves open to let burnt gas out. Each opens and closes thousands of times a minute. In the cylinder head, between the ports and the combustion chambers. See it on the car
- W
- Water pump
- Circulates coolant through the engine and radiator. Driven by the serpentine belt, the timing belt or, on some cars, an electric motor. On the front or side of the engine, often behind the timing cover. See it on the car
- Wheel hub & bearing
- The hub carries the wheel; the bearing inside lets it spin with almost no friction. On most cars it also carries the ABS wheel speed sensor ring. At the center of each wheel, behind the brake rotor. See it on the car
- Wheels
- The metal rim the tire mounts on. It must be round, true and balanced. At each corner, bolted to the hub. See it on the car
- Wiring harness
- The nervous system: thousands of wires and connectors carrying power and signals between every part. Everywhere, bundled and routed along the body, engine and doors. See it on the car