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The automotive term for dichlorodifluoromethane, also known as "Freon," a type of manmade CFC refrigerant used in all 1992 and earlier automotive A/C systems. R12 is being phased out because of its harmful effects on the ozone layer when it leaks or is vented into the atmosphere. See Alternative Refrigerants.



A new environmentally friendly refrigerant that has a much lower Global Warming Potential (GWP) than R-134 (4 versus 1400). The new refrigerant has cooling characteristics similar to R-134 but requires a special compressor lubricant. It is being phased in to replace R-134 on 2013 and newer vehicles. See R-1234yf.



The automotive term for tetrafluoroethane, also known as "SUVA," a manmade refrigerant that contains no chlorine and is considered to be "ozone safe." Used in most 1995 and newer automotive A/C systems. See Troubleshooting A/C Cooling Problems.



Rear Antilock Brake System. Ford's name for rear wheel ABS. See RWAL Rear Wheel Antilock Brakes.



A type of lightweight steering gear that uses a worm-like gear (the pinion) to drive a horizontal bar (the rack). The primary advantage of rack & pinion steering is that it is lightweight and uses fewer parts than a reciprocating ball steering gear. See Servicing Variable Assist Power Steering.



Variation (out-of-round) in the radius or circumference of a wheel or tire. It is measured by placing a dial indicator on the inside edge of the rim or tire tread. Too much radial runout can cause up-and-down vibrations similar to those caused by a static imbalance. See Diagnosing Tire Problems.



A type of tire that is constructed with the reinforcing belts sideways under the tread rather than lengthwise. This makes the tire more flexible which reduces rolling resistance to improve fuel economy (See Tire Ratings). A radial tire can be identified by looking for the letter "R" in the size designation on the tire's sidewall.



The part of the cooling system that gets rid of the engine heat. Coolant from the engine flows past the thermostat and into the radiator where it is cooled by air passing through the fins. Internal corrosion and hairline cracks caused by vibration are the two primary causes of radiator leaks. "Stop leak" can be dumped into the radiator to temporarily plug small leaks but larger ones usually require professional repair or replacement. "Recoring" a radiator means replacing the heat exchanger section between the end tanks.



A steering pull or lead to one side caused by misalignment of the rear wheels or axle. Misalignment creates a thrust angle that causes the vehicle to lead to one side resulting in an off-center steering wheel and accelerated toe wear in the front tires.



A type of ABS system that only involves the rear wheels. Commonly used on pickup trucks and vans, rear-wheel ABS provides skid control with varying vehicle loads. This type of ABS system uses only a single speed sensor in the transmission or differential for both rear wheels. See RWAL Rear Wheel Antilock Brakes.



A method of driving a vehicle whereby engine power is applied to the rear wheels. Power from the engine flows through the transmission, down the driveshaft, through the differential to the rear axles and wheels.



The toe setting of the rear wheels. Rear toe is not adjustable on rear-wheel drive cars with solid axle housings but is adjustable on many front-wheel drive cars and minivans. If rear toe is unequal, it can produce a diagonal wear pattern (heel and toe wear) on the rear tires. See Wheel Alignment.



This term has nothing to do with basketball. What it refers to is the suspension springing back after it has been compressed (See Jounce).



Rebuilt parts are those that have been salvaged and reconditioned to good-as-new condition. Rebuilt parts include alternators, starters, water pumps, clutches, brake calipers, brake shoes, master brake cylinders and fuel pumps. Savings compared to equivalent new parts range from 20 to 50 percent.



A type of steering gear normally used with a parallelogram steering linkage. So named because of the ball bearings that are recirculated in the gear box between the worm and sector gears to reduce friction.



A container for storing liquid refrigerant from the condenser. This component also contains a bag of desiccant that absorbs small amounts of moisture from the refrigerant.


A mandatory requirement for all facilities that perform A/C service work. Venting refrigerant into the atmosphere is no longer permitted. All refrigerant (R12 or R134a) must be recovered from a vehicles A/C system prior to opening the system for repairs. The refrigerant must then be recycled to meet certain purity standards.



A part of an alternator that used diodes to convert alternating current into direct current. It usually consists of three pairs of diodes.



In computerized engine management systems, a five volt signal sent out from the computer to a variable resistance sensor such as a TPS. The computer then reads the voltage value of the return signal. Called "V-ref."



The section of a three-way catalytic converter that breaks NOx down into harmless nitrogen and oxygen through a reduction reaction. See Catalytic Converters.



The working agent in an A/C system that absorbs, carries and releases heat. The two primary automotive refrigerants are R12 and R134a, but many other substances have similar properties (primarily a low boiling temperature) that allow them to be used as "alternative" refrigerants. But most of these substances are not "approved" for use in mobile A/C systems because of safety (flammability) or incompatibility concerns. See Alternative Refrigerants and Troubleshooting A/C Cooling Problems.



The removal of heat by mechanical means.



The complete course of refrigerant back to its starting point. During the refrigeration cycle, refrigerant circulates through the system changing temperature, pressure and physical state (liquid & vapor). This allows heat to be absorbed from air entering the passenger compartment and carried to the condenser where it is released. The compressor provides the pumping action necessary to move the refrigerant and create the desired changes. See Troubleshooting A/C Cooling Problems.



The actual moisture content of the air in relation to the total amount of moisture the air can hold at a given time. If air contains three-quarters of the maximum moisture content it could possibly hold at a given temperature, the relative humidity is said to be 75 percent. Warm air is capable of holding more moisture than cold air. Humidity affects the cooling performance of the A/C system and the engine's octane requirements.



An electrical device that uses an electromagnetic switch and contact points to turn on and off various high amperage electrical accessories. Most vehicles have a horn relay, a headlight relay, a relay for the rear window defogger, and relays for various other things such as the blower motor. When an accessory goes dead, it is often the relay that needs to be replaced. See Electrical Relays & Fuses.



To replace an older component, system or refrigerant with a newer one. With respect to A/C systems, retrofit refers to replacing R12 with R134a. Changing refrigerants requires changing compressor lubricants and service fitting, and may also require other system modifications. See Retrofit Guide.



The distance between a specified point on the chassis, suspension or body and the ground. Measuring ride height is an indirect method of determining spring height, which is important because it affects camber, caster and toe. Low ride height indicates weak or sagging springs. Ride height should be within specifications before the wheels are aligned. See Ride Height.



The slope of a road surface to the outside for proper drainage. Excessive road crown can cause a vehicle to lead to the right. Reducing caster on the left front wheel is sometimes used to compensate for road crown.



Abbreviation for Revolutions Per Minute. Engine speed is often expressed as so many rpm.



A method of driving a vehicle whereby engine power is applied to the rear wheels. Power from the engine flows through the transmission, down the driveshaft, through the differential to the rear axles and wheels.



The about of variation or wobble in a wheel, tire, shaft or pulley. See Diagnosing Tire Problems.



Rear Wheel Anti-Lock brakes. A term used by General Motors and Chrysler for rear-wheel antilock braking. See RWAL Rear Wheel Antilock Brakes.



The process of applying rust-inhibiting chemicals, waxes or sealers to the underside and inside of the vehicle body as well as any other rust-prone areas. Not to be confused with undercoating which treats only the underside of the vehicle. Commercial rustproofing treatments usually include a guarantee for a certain number of years. Be aware that some guarantees require annual "checkups" to touch up any areas where the rustproofing may have been damaged.



A diagnostic tool that is plugged into a vehicle's diagnostic connector to read fault codes, sensor data and other system information. The software in the tool must be compatible with the vehicle application, and may only be able to access or display limited information. OEM scan tools can access all vehicle data and test functions but typically cover only one make of vehicle. Aftermarket scan tools can be used on a wider variety of makes and models, but may not have all the capabilities of the OEM scan tool. A scan tool is necessary for vehicle diagnostics and to clear fault codes on most 1996 and newer vehicles with OBD II. See Scan Tool Help.



A type of valve fitting that opens when depressed. Schrader valves are used in tire valve stems, on air conditioning hoses and on the fuel rails of many fuel injection systems.



The distance between the extended centerline of the steering axis and the centerline of the tire where the tread contacts the road. If the steering centerline is inboard of the tire centerline, the scrub radius is positive. If the steering centerline is outboard of the tire centerline, the scrub radius is negative. Rear-wheel drive cars and trucks generally have a positive scrub radius while FWD cars usually have zero or a negative scrub radius because they have a higher SAI angle. Using wheels with different offset than stock can alter the scrub radius. See Wheel Alignment.



A type of brake lining that uses steel wool instead of asbestos as a reinforcing fiber. Semi-metallic brakes give better high temperature performance and wear characteristics then conventional asbestos linings. They are commonly used on the front disc brakes of front-wheel drive passenger cars. Asbestos pads should never be substituted for semi-metallic pads when relining the brakes. Rapid brake wear will result. See Choosing Friction Linings.



The toothed ring that generates a signal in a wheel speed sensor. It may be mounted on the back of the wheel hub, inside the rotor or brake drum, or mounted on the transmission output shaft or differential pinion shaft. The number of teeth or notches in the ring determines the signal frequency in the sensor as the wheel rotates. For this reason, any replacement rings must have the same number of teeth. See Antilock Brakes.



A type of fuel injection system that uses a separate fuel injector for each cylinder, and pulses the injectors individually. See Understanding Today's Fuel Systems.



A type of flat rubber drive belt that is used to turn multiple accessories on the front of an engine. It is called a serpentine belt because of the way it snakes around the various pulleys. Many vehicles now have a single serpentine drive belt because it eliminates the need for several separate V-belts. A spring-loaded pulley maintains tension on the serpentine belt. This does away with the need to retension the belt when it is replaced. Serpentine belts generally last 25% to 50% longer than conventional V-belts. See Belt & Hose Service.



The amount by which one front wheel is further back from the front of the vehicle than the other. It is also the angle formed by a line perpendicular to the axle centerline with respect to the vehicle centerline. If the left wheel is further back than the right, setback is negative. If the right wheel is further back than the left, setback is positive. Setback should usually be zero to less than half a degree, but some vehicles have asymmetrical suspensions by design. Setback is measured with both wheels straight ahead, and is used as a diagnostic angle along with caster to identify chassis misalignment or collision damage. The presence of setback can also cause differences in toe-out on turn angle readings side-to-side.



An operating strategy on ABS systems that have one wheel speed sensor for each rear wheel. The control module selects the wheel that is turning the slowest to initiate antilock braking. See Antilock Brakes.



A link that connects a leaf spring to the chassis or frame. The shackle allows the length of the spring to change as the suspension moves up and down.



A back and forth vibration that is felt in the steering wheel, sometimes violent. It can be caused by a bent wheel, excessive radial runout in a wheel, a dynamic wheel imbalance or loose steering parts.



A part of the suspension that is designed to dampen up-and-down wheel motions that result from bumps and chassis movement. Each wheel has its own shock absorber (See MacPherson Strut), which is nothing more than a fluid-filled cylinder with a piston and valving inside. The shock absorber's job is to provide a> controlled amount of resistance every time the wheels bounce up and down or the chassis leans as it goes around a corner. The constant motion and the heat created by all the internal friction can wear out an original equipment shock in 50,000 miles. There are many different types of replacement shocks from which to choose, and selecting the one that is right depends on the application. Oil on the outside of a shock is a sign that the seal is leaking and the shock may need to be replaced. A "bounce test" can also be used to tell if the shocks are worn (the vehicle should bounce no more than once or twice after rocking the bumper up and down vigorously). See Shock Absorber & Strut Diagnosis.



A condition where loss of dampening action occurs because of fluid foaming inside a shock absorber. The rapid oscillations of the piston moving through the fluid churns it into foam, which reduces the amount of resistance encountered by the piston. This causes the dampening action to fade, resulting in loss of control, excessive suspension travel and reduced handling. Pressurizing the fluid chamber inside the shock with a gas charge can minimize foaming and prevent fade.



A common type of suspension that uses upper and lower control arms of unequal length. The upper arm is usually shorter than the lower arm to control camber changes during jounce and rebound.



A window, usually located in the top of the receiver-dryer for observing the refrigerant during diagnosis. Most newer A/C systems do not have this device.



Movable plates on an alignment rack that go under a vehicle's wheels that allow the suspension to settle prior to an alignment.



Any suspension that uses computer-controlled shock absorbers and/or air springs to vary ride characteristics and/or ride height. The advantage of such a suspension is that it can change the way the suspension reacts to changing road conditions. On a rough road, it can provide a smoother ride. On smooth roads, it can firm up to provide better handling. A computer-controlled solenoid atop each shock absorber or MacPherson strut changes the internal valving of the shock to provide a stiffer or softer suspension as needed. On suspensions that use air springs, ride height sensors allow the computer to maintain the same ride height in spite of changing loads. Air can be added or bled from the air springs by computer-controlled solenoid valves. On some vehicles, the computer lowers the vehicle for better aerodynamics at high speed. On some four-wheel drive vehicles, the suspension can be raised for increased off-road ground clearance. See Servicing Air Ride Suspensions.



A slang term for an air injection system pump. Used to pump extra air into the exhaust system to help the converter reburn pollutants. See Emission Guide.



The EPA's "Significant New Alternatives Policy." This was implemented in July, 1994 for the purpose of approving alternative refrigerants for automotive use. Under this rule, a manufacturer must submit refrigerant data to the EPA for review. If the alternative refrigerant is not approved, it cannot be used as a substitute for R12 or R134a. The EPA does not approve any flammable refrigerants any that contain butane, propane or other flammable hydrocarbons), or any that contain CFCs. See Alternative Refrigerants.



A professional association that among other things establishes industry "standards" for tools and repairs, including A/C service procedures, recovery, recycling and leak detection equipment, refrigerant purity, etc. See SAE website.



A type of electrical device that uses an electromagnet to move something. The starter on the engine uses a solenoid for engaging the flywheel. Power door locks use solenoids to pull and release the locks. A fuel injector has a built-in solenoid that opens and closes the nozzle. An idle stop solenoid may be used on the carburetor to close the throttle to prevent dieseling when the engine is shut off, or to increase idle speed when the air conditioner is running.



There are several different types of spare tires: a folding spare (which must be inflated with an air canister prior to mounting), a compact spare (which is much smaller and narrower than the other wheels on the vehicle), and a lightweight spare (which is the same diameter as the other tires on the vehicle but thinner). All of these tires are labeled "temporary" spares because of their weight-saving construction. As such, they are intended for emergency use only, and not for sustained or high speed driving. Most carry a warning not to exceed 50 mph nor to travel further than 50 miles. The only kind of spare tire that can be used without such restrictions is a conventional full-sized spare that is the same as the other tires on the vehicle.



This is the pinging or rattling noise sometimes heard during acceleration that indicates detonation is occurring inside the engine. Spark knock can be caused by a variety of things including using low octane fuel, over-advanced ignition timing, too much compression (often due to a buildup of carbon in the combustion chamber), by an inoperative EGR valve, and/or by too much heat. If switching to a higher octane fuel does not cure the problem, the cause should be investigated because prolonged or heavy knocking can damage the engine. See Spark Knock.



A component in the ignition system that ignites the fuel inside the combustion chamber. The spark plug is nothing more than a pair of electrodes with a gap in between. When high voltage from the ignition system reaches the gap, an electrical arc jumps across it and ignites the fuel. The distance across this gap is critical because if it is too wide, there may not be enough voltage to push the spark across. The center electrode gradually wears away as the spark plug accumulates miles, and deposits build up around the insulated tip that can short circuit the firing voltage. That is why spark plugs require periodic replacement. With unleaded fuel, average plug life should be around 30,000 miles. With platinum plus, the interval is 100,000 miles. See Why Spark Plugs Still Need To Be Changed and Spark Plugs.



A type of fuel injection system that does NOT use an airflow sensor, but estimates air flow based on engine speed, throttle opening, air temperature and intake manifold vacuum. See Understanding Today's Fuel Systems.



The component on which the hub and wheel bearings are mounted.



An aerodynamic add-on that goes across the trunk or back of a vehicle to deflect the direction of airflow and reduce drag. A front spoiler is technically an "air dam" because it prevents air from getting under the car and increasing drag.



A tool for compressing and holding a coil spring so it can be removed or replaced, or to allow the disassembly of a MacPherson strut.



A suspension component that supports the weight of the vehicle. Basic types include coil springs, leaf springs, air springs and torsion bars. Spring height affects ride height, which in turn affect wheel alignment. Weak or sagging springs should be replaced in pairs to restore and maintain proper ride height and wheel alignment.



A type of advanced antilock brake/traction control system that uses the brakes to assist steering maneuvers 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. See Electronic Stability Control.



Wheel balance that depends on an equal distribution of weight around the circumference of the wheel and tire assembly. Static balance can be achieved without spinning the wheel by using a bubble balancer. A wheel that lacks static balance will shake or tramp up-and-down.



The arms on the steering knuckles (or struts) to which the tie rods are attached to steer the wheels.



The angle formed by a line that runs through the upper and lower steering pivots with respect to vertical. On a SLA suspension, the line runs through the upper and lower ball joints. On a MacPherson strut suspension, the line runs through the lower ball joint and upper strut mount or bearing plate. Viewed from the front, SAI is also the inward tilt of the steering axis. Like caster, it provides directional stability. But it also reduces steering effort by reducing the scrub radius. SAI is a built-in nonadjustable angle and is used with camber and the included angle to diagnose bent spindles, struts and mislocated crossmembers. See Wheel Alignment.



The amount of driver input or muscle it takes to turn or steer the wheels. Excessive effort can be caused by loss of power assist, binding in the steering gear, worn upper strut bearing plates or binding in ball joints or tie rod ends. Excessive caster can also increase steering effort as can underinflated tires.



A hydraulic device similar to a shock absorber attached to the steering linkage to absorb road shock and steering kickback.



A general term used to describe the angular relationships between the wheels, steering linkage and suspension. See Wheel Alignment.



A forging that usually includes the spindle and steering arm, and allows the front wheel to pivot. The knuckle is mounted between the upper and lower ball joints on a SLA suspension, and between the strut and lower ball joint on a MacPherson strut suspension.



The ability of the steering wheel to self-center after turning. Causes of poor return include excessive caster or binding in the steering column, steering gear, ball joints, upper strut bearing plates or tie rod ends. See Correcting Steering Pulls.



This type of driving is especially hard on a vehicle because the engine spends most of it time at idle where it works less efficiently. Because the water pump is turning slowly at idle, the cooling system can overheat on a hot day. Continual stopping and starting also accelerates wear on the brakes, clutch and automatic transmission. When combined with short trips, the engine never gets a chance to reach full operating temperature so the oil becomes contaminated much more rapidly. Therefore, this kind of driving usually means more frequent oil changes and more frequent brake, clutch and transmission repairs. See How Often Should You Change Your Oil?.



The panels or structural members in a unibody to which the upper strut mounts are bolted. The position of the towers is important because it affects camber and caster readings.



The process of removing heat from refrigerant after condensation.



The lower frame rails and structural members that comprise the lower elements of a unibody. Steering and suspension components may be attached directly to the subframe, or to a "cradle" or "crossmember" that bolts to the subframe.



Connects the evaporator outlet and compressor inlet. Low pressure refrigerant vapor is drawn from the evaporator to the compressor through this line.



The portion of an A/C system under low pressure, the area between the evaporator and compressor inlet.



Compressor intake pressure as indicated by a manifold gauge set.



Also called a "blower," a supercharger is a device that forces more air and fuel into the engine to increase horsepower. Unlike a turbocharger (See Turbocharger), a supercharger is belt or gear driven and provides instant boost pressure to the engine at all speeds. See Basics of Supercharging.



The difference between A/C evaporator inlet and outlet temperatures. It is created in the evaporator as liquid refrigerant changes into vapor.



Refrigerant vapor at a temperature that is higher than its boiling point at a given pressure.



The part of a vehicle that carries the weight. This includes the springs, control arms, ball joints, struts and/or shock absorbers.



DuPont's trade name for tetrafluoroethane or R134a refrigerant.



A component 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.



Abbreviation for Top Dead Center. This is the point at which the piston reaches its uppermost position in the cylinder. Ignition timing is usually expressed as so many degrees before top dead center (BTDC) or after top dead center (ATDC). A timing mark on the crankshaft pulley or flywheel corresponds to the top dead center position of the number one engine cylinder.



Heat 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 short piece of exhaust pipe that is designed to replace a catalytic converter in an exhaust system, supposedly while you test the results of the switch (See Catalytic Converter). Test pipes are illegal and you can be fined if you are caught with one on your vehicle.



Chemical name of R134a refrigerant


A catalytic converter that oxidizes hydrocarbons and carbon monoxide, and also reduces oxides of nitrogen emissions. Usually, it has separate chambers, the one upstream handling reduction, and the one downstream handling oxidation. The noble metals used as the catalytic agents are platinum, palladium, and, for reduction, rhodium. See Catalytic Converters.



A device that changes electrical resistance as temperature changes. A coolant sensor and air temperature sensor are thermistors.



A component in the refrigeration system that controls the rate of refrigerant flow into the evaporator. This is done by means of a temperature sensing bulb that causes the valve to open or close in response to temperature changes in the evaporator.



A temperature control device in the engine's cooling system that speeds engine warm-up and helps the engine run at a consistent operating temperature. Thermostats come in various temperature ratings must most engines today use ones that open between 190 and 195 degrees. The thermostat is usually located in a small housing that connects the upper radiator hose to the engine. Sometimes a thermostat will stick shut, causing the engine to overheat because it blocks the flow of coolant back to the radiator. If a thermostat sticks open, the engine will warm-up slowly and may never reach its normal operating temperature. This can result in little or no heat from the heater. Running an engine without a thermostat is not recommended because excessive cooling can lead to increased blowby and ring wear.



A component (sometimes adjustable) used in a cycling clutch A/C system to engage and disengage the compressor clutch. It prevents water (condensate) from freezing on the evaporator core. It also controls the temperature of air flowing out of the evaporator fins.



A type of electronic fuel injection system that uses a single injector or pair of injectors mounted in a centrally-located throttle body. The throttle unit resembles a carburetor except that there is no fuel bowl, float or metering jets. Fuel is sprayed directly into the throttle bore(s) by the injector(s).



A little gadget on the carburetor throttle linkage or fuel injection throttle body that keeps the engine control computer informed about the throttle opening (See Computerized Engine Controls). The TPS is a variable resistor that changes resistance as the throttle opens wider. The computer needs this information to change the air/fuel mixture. Adjustment is very critical and is best left to a qualified professional. TPS sensors are also used on throttle-by-wire applications for throttle position feedback to the PCM. See Throttle Position Sensors, Throttle-By-Wire and Sensor Guide.



The angle between the thrust line and centerline. If the thrust line is to the right of the centerline, the angle is said to be positive. If the thrust line is to the left of center, the angle is negative. It is caused by rear wheel or axle misalignment and causes the steering to pull or lead to one side or the other. It is the primary cause of an off-center or crooked steering wheel. Correcting rear axle or toe alignment is necessary to eliminate the thrust angle. If that is not possible, using the thrust angle as a reference line for aligning front toe can restore center steering. See Correcting Steering Pulls.



Aligning front toe to the rear thrust angle instead of the vehicle's centerline to compensate for rear axle steer. See Correcting Steering Pulls.



A line that bisects total rear toe. It defines the direction the rear wheels are pointed. The thrust line should correspond to the centerline for the vehicle to steer straight. See Correcting Steering Pulls.



A part of the steering linkage that connects the steering arms on the knuckles to the steering rack or center link.



A flexible coupling in the steering linkage that connects the tie rods to the steering knuckles. Some require periodic greasing (twice a year or every 6,000 miles) while others are sealed. A loose or worn tie rod will cause a feathered wear pattern on tires, and is probably the leading cause of rapid tire wear. Worn tie rod ends can be detected by raising the suspension and rocking the front wheel back and forth. If there is any free play, it probably means the tie rod ends are bad. Toe alignment must be reset once the new tie rods ends have been installed.



A part of the tie rod assembly that is threaded internally and is turned to shorten or lengthen the tie rod to adjust toe alignment.



A strobe light for checking ignition timing. The light is connected to the number one spark plug wire so every time the plug fires the light flashes. The light is then aimed at the timing marks on the crankshaft pulley or flywheel to read timing.



On the sidewall of every tire is information about tire size, maximum load rating, maximum inflation pressure, tire construction (See Radial Tire) and performance standards. Treadwear is a comparative rating of how long the tire will last compared to other tires. The higher the number, the longer the predicted life of the tread. A tire with a 200 rating should go twice as many miles as one with a 100 rating. The numbers do not correspond to a fixed mileage figure because there are so many variables that affect the life of the tread (maintaining the correct inflation pressure is one of the most important). The traction rating is a measure of the tires ability to stop on wet pavement. An "A" is the best rating, "B" is average, and "C" is the lowest acceptable rating. The temperature rating is an indication of how cool the tire runs as highway speeds. Again, an "A" is the best while "C" is the lowest acceptable rating. Performance tires also carry a speed rating: "H" rated tires are good for speeds up to 130 mph, and "V" rated tires are certified for speeds above 130 mph.



Changing the relative positions of the tires on a vehicle periodically to even out tread wear. Rotation is recommended every 5,000 miles for optimum tire life. When tires are not rotated, they can develop wear patterns particular to their wheel location that shortens tread life and may cause vibrations or a rough ride. See Tire Rotation: When & How To Rotate Your Tires



A wheel alignment angle that refers to the parallelism of the tires as viewed from above (See Alignment). Toe-in means the leading edges of the tires are closer together than the rear edges. Toe-out means the leading edges of the tires are farther apart than the rear edges. A vehicle should have zero running toe (perfect parallel alignment) when driving. But because the rubber bushings and joints in the suspension "give" a little (called "compliance"), most rear-wheel drive vehicles call for a slight amount of toe-in when the wheels are initially aligned. Front-wheel drive vehicles are just the opposite, Most call for a slight amount of toe-out because the drive wheels tend to bow in as they pull the vehicle down the road. Toe alignment is very important because it greatly affects tread wear. If toe alignment is off, it will produce a feathered wear pattern across the tire tread. Toe is adjusted by turning the tie rods or tie rod ends to shorten or lengthen the steering linkage. On front-wheel drive vehicles, the rear toe setting can often be changed by adding shims behind the wheel hub, or by changing the pivot position of the control arms. See Wheel Alignment.



Toe-in means the leading edges of the tires are closer together than the rear edges. A small amount of toe-in is usually specified for rear-wheel drive vehicles to compensate for suspension compliance that allows the wheels to toe-out slightly as the vehicle is pushed down the road. Too much toe-in accelerates tire wear and causes the outside edges of the tread to wear more quickly. See Wheel Alignment and Fixing Wheel Alignment Problems.



Toe-out means the leading edges of the tires are farther apart than the rear edges. A small amount of toe-out is often specified for front-wheel drive cars to compensate for suspension compliance that allows the wheels to toe-in slightly when the front wheels pull the vehicle down the road. Too much toe-out accelerates tire wear and causes the inside edges of the tread to wear more quickly. See Wheel Alignment and Fixing Wheel Alignment Problems.



The change in toe that occurs when the wheels are steered to either side. The change in toe allows the inside wheel to follow a smaller circle than the outer wheel to reduce tire scuffing and wear. The toe angle is nonadjustable and is determined by the geometry of the steering arms and linkage. A toe-out on turn angle is usually specified for the outer wheel when the inner wheel is turned 20 degrees. If the angle is not within specifications, it usually means a steering arm is bent. See Wheel Alignment.



Wear across the face of the tire tread caused by slippage or scrubbing as the tire rolls along. Toe wear can produce a feathered wear pattern (bias ply tires primarily) as well as shoulder wear on radial tires. It results from too much toe-in or toe-out, which in turn may be caused by toe misalignment, worn tie rod ends, a worn idler arm or a worn or bent center link. See Wheel Alignment and Fixing Wheel Alignment Problems.



Turning or twisting force. Torque is usually expressed as so many foot/pounds (a one pound force exerted on a lever one foot in length). A torque wrench measures how much twisting force is being applied to a nut or bolt. The torque output of an engine is expressed as the maximum force exerted by the engine at a given engine speed. Large cubic inch displacement engines and engines with long throw crankshafts produce high torque outputs. For more information on this subject, see Horsepower & Torque.



A fluid coupling that connects the engine to an automatic transmission. The torque converter contains a three sets of bladed wheels that face one another. One wheel (the impeller) is attached to the converter housing and turns at the same speed as the engine. The other wheel (the turbine) is attached to the transmission input shaft. As the impeller spins, it slings automatic transmission fluid at the turbine, and makes it turn. The third wheel (the stator) is positioned between the turbine and impeller to redirect fluid flow. When starting out, the stator remains stationary and multiplies torque from two to two-and-a-half times (much like a reduction gear) by recirculating fluid back through the impeller. But when the speed of the turbine wheel starts to catch up with the impeller, the stator starts to spin and the converter "locks up," becoming a direct drive fluid coupling. Many late model vehicles are equipped with a "lockup" torque converter that contains an electrically-operated computer-controlled clutch mechanism. The mechanical clutch eliminates the slight amount of slippage that occurs in an ordinary torque converter fluid coupling to improve fuel economy. The lockup solenoid is engaged when the vehicle reaches a predetermined speed and/or engine load. See Diagnosing Automatic Transmission Problems.



The annoying tendency of some front-wheel drive vehicles to pull to one side when engine torque is applied. In other words, you step on the gas and the car wants to steer right or left. By redesigning the powertrain to use equal length halfshafts between the transaxle and wheels, the tendency towards torque steer can be greatly reduced. The other cure is to keep off the gas. See Torque Steer and Front-Wheel Drive Guide.



A special wrench with a built-in indicator that shows you how much force you are applying to a bolt. A torque wrench should always be used when doing any type of major engine work, when tightening fasteners on the brake system or suspension, when tightening wheel lug nuts or when you do not want to risk breaking a bolt. See Tightening Cylinder Head Bolts.



A steel bar that is twisted to support the weight of the vehicle. Torsion bars are used in place of coil or leaf springs on some vehicles, and allow ride height to be adjusted to compensate for sage that occurs over time.



The combined toe reading of a pair of wheels on a given axle. Total toe is the difference between the leading and trailing edges of both tires with respect to one another. It may be specified in inches, millimeters or degrees.



Most 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 is 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. See also Traction Control



Components in the rear suspension that connect the rear axle or spindles to the chassis.



The transmission in a front-wheel drive vehicle. It combines both transmission and differential into one assembly.



An electronic component using a semiconductor to amplify or switch current. Used in voltage regulators, computers and other electronic accessories.



The gear box 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" (appropriate name, huh?) 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 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 24,000 miles can avoid premature transmission failure but few people heed such advice. Consequently, automatics often call it quits long before the realize their potential design life. See Diagnosing Automatic Transmission Problems and Common Clutch Problems.



The electronic control module or computer that regulates the operation of the transmission. This function may be integrated into the Powertrain Control Module, eliminating the need for a separate TCM control module.



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 is 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 computers diagnostic connector or by using a scan tool to access the computer system. See Trouble Codes.



An obsolete term used to describe the periodic maintenance that is performed when "tuning" an engine to its original specs. With electronic ignition systems that require no periodic adjustments, sealed carburetors and non-adjustable fuel injection, there is not much left to adjust. Today's tune-up, therefore, consists primarily of replacing the spark plugs. It may also include replacing the air and fuel filters and inspecting the emissions control system but as far as "tuning" is concerned, there is little left to tune. See Tune-ups Today.



A means of increasing horsepower (up to 50 percent or more) by using an exhaust-driven air pump (the turbocharger) to force more air and fuel into the engine. Hot exhaust gases coming out of the engine spin an impeller on one end of the turbocharger. On the other end is a second impeller that pumps air into the engine. A "wastegate" (a small trap door that opens to bleed off exhaust pressure) limits the amount of pressure boost the turbo can produce (See Intercooler and Wastegate). A little boost is a good thing, but too much boost can destroy the engine. Generally speaking, the higher the boost pressure, the greater the horsepower produced. It is a way of making a little engine breathe like an engine of much higher displacement. Turbochargers spin at extremely high speeds, sometimes over 100,000 rpm. A steady supply of clean oil is essential to lubricate the turbo shaft bearings. Because of this, a turbocharged engine should never be revved up and shut off abruptly. The high temperatures in the turbo are hard on oil, so more frequent oil changes are usually recommended. Special "turbo oils" are also available that offer better high temperature resistance. If the turbo bearings go bad, the impellers will not turn freely and boost pressure will drop. A turbo can be inspected by removing the plumbing from either side and seeing if the impeller spins freely when turned by hand. Any looseness, roughness or sign of rubbing means it is shot and needs to be replaced. See Turbocharger Diagnosis & Repair.



Plates on an alignment rack that go under the front wheels and allow the wheels to be steered 20 degrees to either side to measure toe-out on turns.



The diameter of the smallest circle in which a vehicle can complete a U-turn. Turning radius depends on the wheelbase of the vehicle (longer vehicles usually need more space to turn around), and maximum steering angularity.



A type of independent front suspension used on Ford pickup trucks that used two parallel I-beam axles (one for each wheel). The design combines the superior strength of an I-beam suspension with the flexibility and ride comfort of an independent suspension.



A bolt in the shape of a "U" that attaches an axle housing to a leaf spring.



Another name for a Cardan joint (See Cardan Joint or Universal Joint).



The application of a sound-deadening and/or rust-inhibiting chemical, wax or sealer to the underside of a vehicle. Do not confuse it with rustproofing (See Rustproofing) which includes coating the inside body panels and other rust-prone areas of the vehicle, too.



A condition where a tire contains less air pressure than the recommended amount. This increases rolling resistance (which may contribute to a steering pull or lead), tire wear and the risk of tire failure due to overheating from excessive flexing of the sidewalls. See Tire Inflation Tips.



A steering condition where the vehicle does not respond quickly to steering changes. If a vehicle understeers, it wants to continue going straight when the steering wheel is turned (See Oversteer). Under normal driving conditions, understeer is not a problem. But when the vehicle is driven at high speed into a curve, the front of the car will tend to plow to the outside. Some vehicles are more prone to understeer than others. Front-wheel drive vehicles fall into this category as do over-powered rear-engine Porsches.



Another name for a Cardan joint (See Cardan Joint).



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 water pump. It is 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 (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 pulleys, most V-belts need to be replaced. But due to the way in which many belts are constructed today, you cannot determine a belt's true condition by a visual examination. Time and mileage must also be taken into consideration. That is why most experts now recommend replacing the belts as a preventive measure every three to four years regardless of how they look.



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 carburetor, and to vaporize 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. See Manifold Vacuum. See Vacuum Leaks.



This has nothing to do with pushing a vacuum cleaner forward. Actually it is the name of a device on the distributor that changes ignition timing in response to engine load. When an engine is cruising under light load, there is very strong vacuum in the intake manifold. This pulls on the vacuum advance diaphragm and advances timing for better fuel economy. When the engine is under heavy load, the throttle is opened wide and vacuum falls. This releases the diaphragm and eliminates the extra timing advance. Where the extra advance not canceled, the engine would likely experience spark knock. See Spark Knock and Distributors.



An orifice-controlled valve which delays a vacuum signal to a diaphragm, such as in the distributor vacuum advance unit. Used to improve derivability and emissions when the throttle suddenly changes position.



Same as "vacuum actuator" and "vacuum power unit." It is a device that opens valves (heater controls) or doors (air control doors in the HVAC plenum) using vacuum as a power source.



This is when the engine valves are reconditioned. It requires removing the cylinder head, disassembling the head and checking it for cracks or warpage (a common problem on aluminum cylinder heads), regrinding the valve faces and seats, replacing or restoring the valve guides, installing new valve guide seals, inspecting the springs, and other valve hardware, then reassembling the heads and putting them back on the engine. See Valve Seat Repairs.



A type of airflow sensor that uses a mechanical flap to measure engine airflow. As the flap moves, the sensor produces a variable voltage signal that changes in proportion to airflow. See Vane Airflow Sensors.



Lines carrying refrigerant vapor. See "suction line" and "discharge line." May also refer to hoses in the evaporative emission control system that route fuel vapors to the charcoal canister.



When gasoline overheats and boils inside the carburetor bowl or fuel pump of a hot engine, it ceases to flow. This can cause stalling or hard starting. This is called vapor lock, and it usually happens during hot weather. If a hot engine will not start, all you can do is let it sit and cool off. You should check the cooling system to see if anything is causing the engine to run unusually hot (a bad thermostat or cooling fan, for example). Switching brands of gasoline may also help.



A type of power steering system where electronics are used to vary the amount of power assist provided as vehicle speed changes. Most such systems provide maximum assist at low speed to make parking maneuvers easier, and reduce assist at higher speeds to increase road feel and stability. System inputs include the vehicle speed sensor and sometimes a steering angle sensor. See Servicing Variable Assist Power Steering.



A type of spring that changes stiffness as it deflects. A variable rate spring uses coils of varying thickness or spacing to provide a soft ride when the vehicle is lightly loaded, but a firmer ride when the load increases. Only a few vehicles have variable rate springs as original equipment. On most vehicles, the rear coil springs can be easily replaced with variable rate springs to reduce bottoming and to increase the vehicle's load carrying capacity. Variable rate springs are also available for the front suspension.



A method that advances or retards camshaft timing to improve engine performance. A hydraulic mechanism on the cam drive uses oil pressure to rotate the cam�s position slightly as engine speed changes. This increases valve duration to produce more horsepower at higher rpms.



The narrow part of the carburetor 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.



This is a term used to describe the thickness of motor oil. The higher the number, the thicker the oil. Common straight grade viscosity ratings are 10, 20, 30 and 40, with 10 being the thinnest and 40 the thickest. A low viscosity oil provides better lubrication at low temperatures and reduces internal drag on the engine. But they lack the staying power for high temperature or high speed protection. The heavier grade oils such as 30 and 40, on the other hand, are much better for high speed and high temperature lubrication, but they may be so thick at low temperatures as to inhibit easy cranking. The best motor oils take advantage of each. These are the "multi-viscosity" oils such as a 5W-20, 5W-30, 10W-30 and 10W-40. By using a blend of different viscosity oils, they have the flow characteristics of a low viscosity oil when cold but offer the protection of a heavy oil when hot. See Motor Oil Viscosity.



Abbreviation for Vehicle Identification Number. This is a vehicle's serial number. You�ll find it stamped on a small metal plate affixed to the dash at the base of the windshield. The number may also be stamped on various body parts, the engine and transmission. It is sometimes necessary to refer to the VIN number when ordering replacement parts.



A part of the charging system that controls how much electricity the alternator puts out (See Alternator). The voltage regulator on today's cars is an electronic black box, which means you cannot adjust it or repair it if anything goes wrong with it. On most newer vehicles the voltage regulator is located inside the alternator and cannot be replaced separately. On some cars, the powertrain control module (PCM) regulates the alternator. A defective regulator can cause the alternator to produce too much voltage (which can damage the battery, lights and electronic components) or it can prevent it from making enough voltage to keep the battery fully charged. The toughest challenge when diagnosing a charging problem is to figure out whether it is the voltage regulator or alternator that is at fault. Using a procedure called "full fielding the alternator" that causes the alternator to put out maximum current will reveal which component is at fault. See Charging System Checks.



The basic guarantee that comes with a new vehicle. All vehicle manufacturers today offer a bumper-to-bumper (covers everything!) warranty of 3 years or 36,000 miles (which ever comes first). Separate warranties may be provided on emission controls, body rust, powertrain or other components. On 1995 and newer vehicles, the emissions warranty is 8 years and 80,000 miles on the catalytic converter and engine computer, and 2 years/24,000 miles on all other emission control 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. For more info, see New Car Warranty Coverage: What Is Covered, What Is Not



A trap door-like device on the exhaust side of a turbocharger that limits the amount of boost a turbo can produce (See Turbocharging). The wastegate consists of a spring-loaded diaphragm. A control pressure hose connects the diaphragm to the plumbing between the turbo and intake manifold. When boost pressure starts to exceed the rating of the wastegate, pressure moves the actuator diaphragm and opens a bypass flap in the turbo housing. This allows some of the exhaust to bypass the turbine wheel, slowing the turbo and reducing boost output. A wastegate can be checked by applying pressure to the hose with a hand-held pump. If it does not move at the specified pressure (which you can look up in a manual), the diaphragm is probably ruptured and the wastegate needs to be replaced. See Turbocharger Diagnosis & Repair.



No, this is not a new type of life preserver. It refers to the hollow space inside the engine block and cylinder head where coolant flows.



A small impeller-like pump that circulates coolant through the engine's cooling system. The water pump 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 40,000 miles 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. See Water Pump Diagnosis & Replacement.



The even distribution of weight around a wheel so that it rotates without vibrating or shaking. See static and dynamic balance. It is achieved by positioning weights on the rim that offset heavy spots on the wheel and tire assembly. See Wheel Balancing.



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.



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 24,000 miles. 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 cause the wheel to come off the vehicle. See Wheel Bearing Service.



This is the hydraulic component that pushes the brake shoes out in a drum brake.

The wheel cylinder consists of a small casting with two outward facing pistons. When hydraulic fluid from the master cylinder is forced into the cylinder, it pushes the two pistons out and applies the brakes. Leaks sometimes develop around the cup-like piston seals. The cheapest way to fix a leaky wheel cylinder is to install a "kit" that contains new piston seals.



This is when one drive wheel spins uselessly while the other does not turn. It can happen when one wheel is on a slippery surface (ice, snow, mud, slush) and the other on dry pavement. The reason it happens is because the differential always routes power to the wheel that needs it the least (See Differential). The only way to eliminate it is to buy a vehicle with a locking differential or traction control.



A weight used to balance a wheel and tire assembly. Most are metal (lead, zinc or steel) and clip to the wheel rim. Wheel weights come in various sizes and styles, and must be properly attached to the rim so they do not 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. See Wheel Bearing Service.



The up-and-down bouncing motion of a wheel or spindle due to static imbalance or an out-of-round tire or wheel. See Curing Wheel & Tire Vibrations.



The process of preparing a vehicle for the ravages of winter. The annual fall ritual includes checking, replacing and/or replenishing the antifreeze in the cooling system (See Antifreeze), mounting the snow tires, waxing the body to protect it against road salt, and sometimes a tune-up to aid starting. See Winterizing Tips.



Wide Open Throttle. Some carburetors and throttle bodies have a switch that signals the engine computer when the throttle is wide open.



A polished steel pin that attaches a connecting rod to a piston. Some wrist pins are press fit into the small end of the connecting rod while others are a "free floating" loose fit.



The rotation of the vehicle body around its centerpoint 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. See Antilock Brakes.



Zero emission vehicle, one the 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. See Alternative Fuels.

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