A list of car parts/automotive terms beginning with the letters S through Z . . .

Service ManualA handbook for a specific make, model and year published by a vehicle's manufacturer, containing instructions and specifications regarding its maintenance and repair. 

Sparkplug A device screwed into the combustion chamber of a spark ignition engine. The basic construction is a conductive core inside of a ceramic insulator, mounted in an outer conductive base. An electrical charge from the sparkplug wire travels along the conductive core and jumps a preset air gap to a grounding point or points at the end of the conductive base. The resultant spark ignites the fuel/air mixture in the combustion chamber. 
Stability Control 
A type of advanced antilock brake/traction control system that uses the brakes to assist steering manoeuvres and to help improve vehicle handling and stability as driving conditions change. The system includes various sensors that monitor the driver's steering inputs and the position of the body with respect to the road. A "yaw sensor" can tell if the vehicle is starting to understeer or oversteer in a turn. The stability control system is active fulltime and will apply individual brakes to create a counter-steer effect that brings the vehicle back under control. 
Suspension System

Sway Bar 
A component that's often used in a suspension system to control body roll. A sway bar may be used on the front and/or rear suspension to help keep the body flat as the vehicle rounds a corner. This greatly improves a vehicle's cornering agility. Replacing the sway bar with one of a larger diameter can increase it even more. 
A device used to measure the rotary speed of an engine, shaft, gear, etc., usually in rotations per minute. 
TemperatureHeat intensity measured in degrees. Engine operating temperature is a critical factor in engine performance and emissions. Brake temperature can affect the operation of the brakes. 
A device that changes electrical resistance as temperature changes. A coolant sensor and air temperature sensor are thermistors. 
A valve, located in the cooling system of an engine, which is closed when cold and opens gradually in response to engine heating, controlling the temperature of the coolant and rate of coolant flow.
ToeThe difference between the front and rear edges of tyres mounted on an axle. Toe-in means the front edges are closer together than the rear edges and tyres point inward. Toe-out means the front edges are farther apart than the rear edges and the tyres point outward. 

TowingMost vehicles can tow a moderate amount of weight (1000 lbs. or less) without too much trouble. But for heavier loads, the suspension and cooling system may require beefing up (See the owners manual for towing recommendations and load limits). Overload or air-assist shocks can keep the rear end from sagging, and a stabilizer bar on the trailer hitch can reduce swaying. Automatic transmissions should be equipped with an oil cooler to protect the transmission against overheating. A larger radiator or a larger fan may be required to keep the engine from overheating. 

How the rear wheels follow the front wheels. For proper alignment, they should follow the same path. If the rear wheels don't track straight and follow slightly to one side due to rear axle or toe misalignment, the result can be off-center steering and accelerated tire wear. 
An enhancement of an existing ABS system that prevents wheel spin while accelerating on wet or slick surfaces. It uses the same wheel speed sensors to monitor wheel speed during acceleration, but requires some additional control solenoids and a pump to apply braking pressure to control wheel spin. The traction control system brakes the drive wheel that's starting to spin to shift torque to the opposite drive wheel that still has traction. Most traction control systems only operate at speeds up to about 30 mph. Additional control strategies that some traction control systems use to limit wheel spin include reducing the throttle opening, upshifting the transmission, retarding spark timing and deactivating fuel injectors. 
A device used to change a force into an electrical signal. 

A semi-conductor component that can be actuated by a small voltage to perform an electrical switching function. 
The gearbox that multiplies engine torque via gear reduction and/or torque conversion. A typical manual transmission has four or five speeds, with the final or highest gear being either a direct 1:1 drive ratio or an "overdrive" ratio (less than 1:1). 
An automatic transmission first multiplies engine torque as it passes through the fluid coupling known as the "torque converter" and then through three or four separate gear ratios. A manual transmission usually gives slightly better fuel economy than an automatic because there is a certain amount of slippage that occurs in the automatic's torque converter. A manual transmission is normally trouble-free-except for the clutch, which can be very troublesome if adjusted incorrectly or abused. With automatics, the leading problem is fluid breakdown from overheating. Fluid and filter changes every 30,000 kms can avoid premature transmission failure but few people heed such advice. Consequently, automatics often call it quits long before they realize their potential design life. 
The region of a tyre designed to contact the ground. It is molded of tough rubber for high traction and low wear. 
Tread-wear Indicators 
When a tyre's treads have been worn down to 1.5 mm, small bands of hard rubber will be appear across the treads themselves to indicate the level of wear.
Trouble Codes
A code number generated by a vehicle's onboard computer that corresponds to a specific fault. Most computerized engine control systems have a certain amount of self-diagnostic capability. When the engine is running and the computer detects a problem in one of its sensor or output circuits, or even within itself, it triggers a trouble code. In some systems, the code number is retained in memory. In others, the code is not stored but is regenerated when a mechanic runs the system through a special self-diagnostic test. The only indication of trouble is when the "Check Engine" light on the instrument panel lights up. What does it mean? It depends on the problem. Sometimes it's nothing serious, but it could signal a failure that might lead to further problems. To understand trouble codes, you have to have a reference manual that tells what the numbers mean and explains the step-by-step diagnostic procedure for isolating the fault. Codes are read out of the computer by grounding the computer's diagnostic connector or by using a scan tool to access the computer system.

A regular maintenance, usually associated with the replacement and adjustment of parts and components in the electrical and fuel systems of a vehicle for the purpose of attaining optimum performance. 
A method of increasing power and decreasing emissions by rerouting hot exhaust gasses through a turbine, which drives a pump that forces more air into the engine's cylinder. 
Turning Radius

Two-second Rule
The minimum distance that should be kept between two vehicles traveling in the same direction on the same road. When the vehicle ahead passes a road marker or sign, the car behind it should pass the same marker or sign roughly two seconds later. In the case of poor road conditions the time lapse should be three or four seconds. 
The condition that exists when there is not sufficient air pressure in a tyre to support a specific load. This causes the tire to operate with excessive deflection and rollover. 

UndersteerA handling condition in which the slip angle of the front tires is greater than the slip angle of the rears. An understeering car is sometimes said to push, because it resists turning and tends to go straight. 
Universal Joint
A joint that transmits rotary motion between two shafts that aren't in a straight line. Depending on its design, a universal joint can accommodate a large angular variation between its inputs and outputs. The simplest kind of universal joint, called a "Hooke joint," causes the output shaft to speed up and slow down twice for every revolution of the input shaft. This speed fluctuation increases with the angular difference between the shafts. Another, more common, name for a Cardan joint. 
The absence or reduction of air pressure. Vacuum is created in the intake manifold by the pumping action of the pistons. Air is pulled out of the manifold into the cylinders faster than it can be replenished by air bypassing the throttle plate. The throttle creates a restriction that allows vacuum to buildup inside the manifold. This is necessary to help pull fuel through a carburettor, and to vaporise fuel sprayed into the engine by fuel injectors. Vacuum is also used to operate various components such as the EGR valve, to pull crankcase vapors through the PCV system, to boost the power brakes and to open and close air control doors in many A/C systems.
A device that controls the pressure, direction of flow or rate of flow of a liquid or gas. 
More commonly known as a "fan belt," a V-belt is the rubber belt that drives such things as the alternator, air conditioning compressor, power steering pump and waterpump. It's called a V-belt because of its "V" shaped cross-section. The sides of the belt are what grip the pulleys. Some belts have notches in them to increase grip, to help cool the belt and to relieve stress as the belt bends around small diameter pulleys. Some vehicles use a single flat belt (See Serpentine Belt) to drive multiple accessories. Cogged rubber timing belts are used on many overhead cam engines to drive the camshaft (See Overhead Cam). After three or four years of flexing and countless cycles around the engine's pulleys, most V-belts need to be replaced. But due to the way in which many belts are constructed today, you can't determine a belt's true condition by a visual examination. Time and mileage must also be taken into consideration. That's why most experts now recommend replacing the belts as a preventive measure every three to four years regardless of how they look. 
The narrow part of the carburettor throat. When air passes this point, the restriction causes an increase in velocity and a drop in pressure that siphons fuel from the fuel bowl into the airstream. 

The rating of a liquid's internal resistance to flow. 
The basic guarantee that comes with a new vehicle. All vehicle manufacturers today offer a bumper-to-bumper (covers everything!) warranty of a certain number of years or or so many thousand kms (which ever comes first). Separate warranties may be provided on emission controls, body rust, powertrain or other components. New car and truck dealers also sell "extended" warranty packages that extend the time and mileage of coverage. Extended warranties are expensive but can easily pay for themselves if the vehicle requires major repairs. 
Water Pump
A small impeller-like pump that circulates coolant through the engine's cooling system. The waterpump is mounted on the engine and is driven by the fan belt, alternator belt or overhead cam timing belt. The pump shaft has a large bearing and seal, which after 65,000 kms or so usually starts to leak. The pump can be replaced with a new or rebuilt unit, but the degree of difficulty varies, depending on pump accessibility. 
Wheel Alignment

Wheel BalanceThe even distribution of weight around a wheel so that it rotates without vibrating or shaking. It is achieved by positioning weights on the rim that offset heavy spots on the wheel and tyre assembly.  If your steering wheel vibrates when you drive, bad wheel balance is usually the problem.
Wheel Base
The distance between the centers of the front and rear wheels. Measuring and comparing the wheelbase on both sides of a vehicle can identify rear axle misalignment or front wheel setback. 
Wheel Bearings 
Inside the wheel hubs are either roller or ball bearings that carry the vehicle's weight. On RWD vehicles with solid axles, the rear wheel bearings are mounted on the axles. The front wheel bearings on older rear-wheel drive cars and trucks usually require "repacking" (regreasing) every two years or 40,000 kms. The wheel bearings on most newer vehicles are sealed and do not require any maintenance. A bad wheel bearing will typically make grinding, whining or squealing noises, and you can often feel the looseness or roughness if you raise the suspension and rotate the wheel by hand. Worn wheel bearings should be replaced, because failure may actually cause the wheel to come off the vehicle. 
Wheel Cylinder 
Found in the automotive drum brake assembly, it is a device, actuated by hydraulic pressure, which, through internal pistons, pushes the brake shoes outward against the drums. 
Wheel Lockup 
The action of a wheel that is skidding, that is to say, one that has ceased rotating even though the car is still in motion. 
Wheel Weights 
A weight used to balance a wheel and tyre assembly. Most are metal (usually lead) and clip to the wheel rim. Wheel weights come in various sizes and styles, and must be properly attached to the rim so they don't move or fall off. Different style clips are available for various types of rims. Self-adhesive stick-on weights are also available that mount to the inside face of alloy wheels. 
The rotation of the vehicle's body around its center point as viewed from above. When a vehicle enters a turn or makes a sudden lane change, it experiences a change in yaw. A yaw sensor in the ABS stability control system senses this change to determine if the vehicle is experiencing understeer or oversteer. If the yaw rate indicates a problem, corrective actions are taken to help keep the vehicle under control. 
ZEVZero Emission Vehicle, one that produces no pollutants. Unless somebody comes up with a car that burns water, this means an electric-powered car with a battery, fuel cell or flywheel using an energy storage device.