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Abbreviation for the American Automobile Association. AAA, in conjunction with various local motor clubs, often certifies various repair facilities. The approved facilities must meet certain minimum standards of service to be listed in the local AAA directory.



Acronym for "Anti-lock Brake System." Vehicles equipped with ABS use wheel speed sensors and a computer-controlled brake pressure regulator to prevent wheel lock-up during sudden stops. When the computer senses one wheel is slowing faster than the others (indicating it is about to lock-up and skid), the computer reduces brake pressure to that wheel by momentarily isolating brake pressure, releasing pressure then reapplying pressure in rapid sequence. This allows the wheel to regain traction so the vehicle does not skid. ABS also allows the driver to maintain steering control while braking hard on wet or slick surfaces. ABS improves braking safety on wet or slick surfaces. For more information about antilock brakes, see Antilock Brakes.



An indicator light on the instrument panel that warns the driver when there is a problem with the ABS system. When the ignition is first switched on, the ABS warning light should come on and remain on for several seconds for a bulb check. If the light fails to go out or comes on while driving, it signals a potential problem with the ABS system. The ABS system is usually disabled if the ABS warning light is on while driving (this should have no effect on normal braking -- unless the brake warning light is also on). The light is also used for diagnostic purposes when retrieving flash codes (trouble codes) from the ABS module.



A container for receiving refrigerant liquid, vapor and oil from the evaporator. Its primary function is to separate the vapor from the liquid and oil, then release the vapor to the compressor. The accumulator also contains desiccant to absorb moisture.



The creation of toe-out when turning to minimize tire wear. To create the proper geometry, the steering arms are angled to turn the inside wheel at a sharper angle than the outside wheel. This allows the inside wheel to follow a smaller radius circle than the outside wheel.



A computerized hydraulic suspension system that uses hydraulic "actuators" instead of conventional springs and shock absorbers to support the vehicle's weight. A "chassis computer" monitors ride height, wheel deflection, body roll and acceleration to control ride and body attitude. Bumps are sensed as they are encountered, causing the computer to vent pressure from the wheel actuator as the wheel floats over the bump. Once the bump has passed, the computer opens a vent that allows hydraulic pressure to extend the actuator back to its original length. The only production active suspension was used on the Infiniti Q45.



A system that cools and dehumidifies air entering the passenger compartment. The system uses a refrigerant to cool the air and carry heat away from the passenger compartment. Major system components include a compressor, condenser, evaporator, accumulator or receiver/dryer, and orifice tube or expansion valve. Do not intermix different types of refrigerants in an A/C system. Use the type specified by the vehicle manufacturer (R12 for most 1994 & older vehicles, or R134a for most 1995 and newer vehicles). For more information, see Troubleshooting Air Conditioning Problems. Also see Retrofit.



Also called plenum, HVAC unit or evaporator housing. This component contains the air ducts, doors and blower fan that deliver air through or around the evaporator and heater cores. It then delivers air to various passenger compartment outlets and ducts.



A filter used to keep dirty air from entering the engine. The filter element is typically resin impregnated cellulose fibers (paper) with a mixture of synthetic fibers. The filter is located in a housing that is attached to the throttle body, or in a housing that sits atop the carburetor. See Check Your Air Filter


A device that is used in many electronic fuel injection systems (See Electronic Fuel Injection) for measuring the volume of air entering the engine. Some use a spring loaded vane while others use a hot wire or heated filament to sense air flow.



This is the relative proportion of air and fuel delivered by the carburetor or fuel injection to the engine. The "ideal" air/fuel ratio is 14.7 parts of air to every one part fuel. Less air or more fuel and the mixture is said to be rich. More air or less fuel and the mixture is said to be lean. Rich mixtures provide more power but also use more fuel and increase exhaust emissions. Lean mixtures use less fuel, but if too lean cause misfiring at idle. An engine requires a richer mixture when starting (See Choke) and while warming up. The air/fuel ratio at idle can be adjusted by turning the idle mixture screw on the carburetor (See Idle Mixture). To alter the mixture above idle, the main metering jets inside the carburetor must be changed. With electronic fuel injection, no changes can be made because the mixture is determined by the duration (on time) of the injector(s). The longer the injectors are on, the richer the mixture (See Electronic Fuel Injection). additional information can also be found here: Engine Air/Fuel Ratios.



A type of oxygen sensor that reads unburned oxygen concentrations in the exhaust to help the PCM determine the engine's air/furl ratio. Feedback from the Air/Fuel Sensor allows the PCM to fine tune the air/fuel mixture for optimum performance, emissions and fuel economy. See also Oxygen Sensors. For additional information, see Wide Band O2 Sensors.



Supplies fresh air to the exhaust system, which helps oxidize HC and CO, and, gives the catalytic converter the extra air it needs to oxidize those pollutants. Some vehicles use an air pump while others use an aspirator system to route air into the exhaust.



A movable door in the air distribution assembly that allows either passenger compartment or outside air to be delivered to the air conditioning air distribution system.



An emission control device on some engines that pumps air into the exhaust system so the catalytic converter can "reburn" pollutants in the exhaust.



A type of overload shock absorber that can be inflated with air to increase the suspension's load carrying ability.



Air-filled rubber or elastomer bags that are pressurized to provide support to the suspension. Air springs are used in place of conventional coil springs on some vehicles. Aftermarket air springs can be installed inside coil springs or between the axle and frame to provide additional lift support for handling overloads or towing.



A type of suspension that uses air springs instead of conventional steel springs. Computer operated vents on the air springs, suspension sensors and an onboard air compressor allow the system to maintain ride height and vary the suspension's ride characteristics. See Servicing Air Ride Suspensions.



This sensor measures the temperature of air in the intake stream or intake manifold. An air temperature value is needed by the PCM to calculate the air/fuel ratio, as air density changes with temperature. The Intake Air Temperature (IAT) sensor is usually mounted in the intake manifold, or incorporated into the mass airflow sensor. See Air Temperature Sensors.


Alcohol is used as a gasoline additive to boost the octane rating of the fuel (See Octane and Gasohol) and to oxygenate the fuel (makes it burn cleaner). Two types of alcohol may be used: ethanol or methanol. Ethanol is the most commonly used alcohol. It is made by distilling fermented corn, sugar beets or sugar cane. Ethanol is the same kind of grain alcohol that goes into booze. Methanol or "Wood Alcohol" is made primarily from coal, and is highly poisonous. Ethanol blend fuels typically have a 10 % ethanol content (E10), although the EPA has also approved the use of E15 (15% ethanol) for use in 2001 and newer vehicles. Methanol blends are limited to 5 percent because methanol can be corrosive in higher concentrations. For more information, see E85 Ethanol Fuel and Alternative Fuels and E15.



Although most people think of the front wheels when alignment is mentioned, it actually refers to all four wheels. All four wheels should be perpendicular to the road and parallel to one another for the best handling, traction and tire life. If the wheels are out of alignment, rapid or uneven tire wear, and/or a steering pull to one side can result. Four wheel alignment, as opposed to a basic two wheel alignment, is very important today especially on vehicles with independent rear suspensions and/or front-wheel drive. The three basic alignment angles are toe, camber and caster, but on some new cars caster and camber may not have factory adjustments. For these vehicles, aftermarket alignment kits may provide some adjustment. (See Camber, Caster and Toe). See Wheel Alignment.



Metal or plastic spacers used in the alignment process to alter camber, caster and/or toe. On rear-wheel drive applications, shims may be added to or removed from stacks of shims on the front control arms to change camber and/or caster. On front-wheel drive applications, partial shims or full contact shims may be positioned behind the rear axle spindle to vary rear toe and/or camber. Camber shims are also available for 4x4 axle applications. Some shims are adjustable or can be indexed various ways to provide incremental alignment corrections.



A vehicle (usually a car) where all four wheels are driven. Most are fulltime systems for year-round driving, and use a viscous fluid coupling center differential instead of a transfer case to route drive torque to all four wheels. This allows the front and rear wheels to turn at slightly different speeds when turning on dry pavement.



The component in a vehicle's charging system that makes electricity. The alternator's job is to keep the battery fully charged, and to provide additional current to meet the demands of the ignition system, lights and other accessories. Vehicles equipped with air conditioning and numerous electrical accessories require an alternator with a higher output capacity than a vehicle without such amenities. Alternator capacities are rated in amps, with typical outputs ranging from 50 to 80 amps. When the alternator or its control device, the "voltage regulator," goes bad, the alternator light on the dash will glow red. If a vehicle has a charge indicator, it will show a continual discharge or low voltage. Without the supply of electricity to keep it charged, the battery soon goes dead. Sometimes a slipping drive belt is all that is wrong but usually the alternator and/or regulator need to be replaced. See Alternator Testing.



Air outside the vehicle passenger compartment.



The temperature of the air outside the vehicle. The value is measured with an air temperature sensor. See Air Temperature Sensors.



Energizes the compressor clutch when ambient air temperature is above 32 degrees F. This switch also prevents compressor clutch engagement at temperatures below 32 degrees F.



Antifreeze protects the cooling system against both freezing and boiling over. When used at normal strength (50% antifreeze, 50% water), it can lower the freezing point of the coolant to -34 degrees F. and raise its boiling temperature to 276 degrees F. Never use straight antifreeze in a cooling system. Always mix it with at least 50% water. Most antifreeze is 95% ethylene glycol (EG). The only differences between brands of antifreeze are the type and/or quantity of anti-corrosion additives used. Ethylene glycol never wears out but the corrosion inhibitors do. That is why antifreeze should be changed every two years (except for long life antifreeze, which have special additive packages that allow them to go up to 5 years/150,000 miles between changes). For the environmentally concerned, propylene glycol (PG) antifreeze is also available at slightly higher cost. PG antifreeze is less toxic than ethylene glycol. See Types of Antifreeze.



Abbreviation for the National Institute for Automotive Service Excellence. ASE certifies professional automotive technicians in various areas of repair expertise. A technician who has passed one or more tests is allowed to wear the ASE Blue Seal of Excellence on his uniform, and any repair facility that employs certified mechanics can display the ASE sign. See ASE website.



A one-way valve attached to the exhaust system of an engine that admits air during periods of vacuum between exhaust pressure pulses. Used to help oxidize HC and CO, and to supply additional air which the catalytic converter may require. Can be used instead of a belt-driven air injection pump in some applications. Called "Pulse-Air" in GM systems.



Automatic Slip Regulation. See Traction Control.



Air pressure at any given altitude: 14.69 psi at sea level. This pressure decreases as altitude increases.



A type of transmission that shifts itself. A fluid coupling or torque converter is used instead of a manually operated clutch to connect the transmission to the engine. Newer automatics use electronic controls to regulate shifting and torque converter lockup. See Automatic Transmission Diagnosis.



A special kind of oil for use in automatic transmissions. There are several types: Dexron II, Dexron III, Type F, Mercon, Mercon V, Chrysler 7176 and several varieties of Chrysler ATF-Plus. All are "friction-modified" lubricants except Type F. Dexron II and Mercon have similar additive packages as do Dexron III and Mercon V. Even so, use only the type of ATF specified by the vehicle manufacturer. Using the wrong type of ATF can cause transmission problems. If you do not know what type of fluid the transmission takes, READ THE OWNER'S MANUAL! Some dipsticks are also marked as to the type of ATF required. "Universal" ATF fluids are available that supposedly meet the OEM requirements for many friction-modified applications, but make sure the label says it meets the specific vehicle requirements before using. The newer ATFs such as Dexron III and Mercon V are longer lived than earlier ATFs, but can still oxidize if the transmission runs too hot. Trailer towing is especially hard on ATF unless the transmission is equipped with an auxiliary oil cooler. Recommended change interval for older vehicles is every 24,000 to 30,000 miles. For newer vehicles, see the owners manual. See also Automatic Transmission Fluid Types & Applications.



A fully autonomous vehicle is one with Level 4 or 5 capability that can drive itself without any human input on almost any road and in any weather conditions. This is different than a Level 1, 2 or 3 self-driving vehicle that can steer, brake, accelerate and drive itself under certain conditions, but still requires human intervention if a road is not clearly marked, if there is an obstacle in the road or the weather obscures vision.



A crossbeam that supports the weight of the vehicle (typically a truck) and is connected to the spindles with king pins.



May refer to the drive axles that connect both rear wheels to a center differential in a rear-wheel drive vehicle, or a crossbeam that connects both rear wheels and supports the rear of the vehicle in a front-wheel drive application.



Refers to a blend of two or more refrigerants that will not separate fractionate) and have different temperature and pressure characteristics than any of the separate ingredients.



This is the popping or banging sound sometimes heard in the exhaust when decelerating. It can indicate a problem such as over-rich carburetion, a bad exhaust valve or an ignition problem (retarded timing or a cracked distributor cap). If the backfiring occurs through the carburetor, it may mean over-advanced timing, a bad intake valve or a cracked distributor cap.



This is the pressure that backs up in the exhaust system as a result of the restriction caused by the muffler, catalytic converter and tailpipe. The faster you drive and/or the greater the load on the engine, the higher the back pressure in the exhaust system. Back pressure inhibits the exit of exhaust gases so the engine has to work harder to push the exhaust out. This cuts down on engine power and fuel economy. Some of the causes of high back pressure include a clogged converter, a damaged or collapsed exhaust pipe or a restrictive muffler.



Some emissions control systems use a back-pressure sensor or diaphragm to monitor backpressure so that exhaust gas recirculation (EGR) flow can be increased when the engine is under maximum load (and producing maximum back pressure). See EGR.



The distance from the back edge of a wheel rim to the back of the center section.



A flexible coupling in a vehicle's suspension that connects the control arm to the steering knuckle. A ball joint is so named because of its ball-and-socket construction. Some are designed to never require grease while others should be lubed every six months. As the joint wears, it becomes loose. The result is suspension noise and wheel misalignment. See Ball Joints.



The pressure exerted by the weight of the earth's atmosphere, equal to one bar, 100 kilopascals, or 14.7 psi (often rounded off to 15 psi) at sea level. Barometric pressure changes with the weather and with altitude. Since it affects the density of the air entering the engine and ultimately the air/fuel ratio, some computerized emissions control systems use a barometric pressure sensor so that the spark advance and EGR flow can be regulated to control emissions more precisely.



A device that senses barometric pressure for the engine control system. May be combined with a Manifold Absolute Pressure (MAP) sensor.



The battery is a storehouse of electrical energy for starting the engine. All cars and light trucks today have a 12-volt battery. Most are also maintenance-free, meaning you do not have to add water to them periodically. Some even have built-in charge indicators to tell you if they need charging. A green dot in the window means the battery is at least 75% charged, no dot means it needs recharging, and a clear or yellow window means you need a new battery because the water level inside is low. Do NOT try to jump start or charge such a battery. You might be able to salvage the battery if you can pry the sealed caps open and add water, but usually the battery must be replaced. Batteries are rated according to their Cold Cranking Amp (CCA) capacity. As a rule of thumb, an engine needs a minimum of one CCA for every cubic inch of displacement, and preferably two. The higher the CCA rating of the battery, the better. A typical passenger car battery might be rated at 500 CCA or higher. See Battery Testing, and Battery Jump Starting.



Biodiesel is diesel fuel made from renewable non-petroleum resources. Most biodiesel is currently made from soybean oil, but it can also be made from sunflower seed oil, or less expensive animal fats such as beef tallow or chicken fat, or even recycled restaurant grease. Biodiesel can be blended with conventional diesel fuel much like ethanol is blended with gasoline. Most diesels can safely handle biodiesel/regular diesel mixtures of up to 10 to 15% (B10 to B15) with no modifications, and up to 100% straight biodiesel (B100) with minor modifications. The most common blends include B2 (2% biodiesel), B5 5% biodiesel) and B20 (20% biodiesel). See Biodiesel.



A condition where combustion gases literally blow around the piston rings. When air and fuel are ignited inside the combustion chamber, the resulting explosion creates tremendous heat and pressure. The piston rings are supposed to seal against the cylinder walls to prevent the hot gases from escaping. But every engine suffers a small amount of blowby anyway. If the rings and cylinders are worn, blowby can be a real problem. The gases are mostly water vapor and unburned fuel, so when they enter the crankcase they contaminate the oil. Most of the gases are sucked out through the crankcase ventilation system (See PCV Valve) before they can do much damage. But in an engine with a lot of wear, excessive blowby can lead to rapid sludge buildup. For information on how to check blowby see Engine Blowby.



The leaning or tipping of a vehicle's body to one side when turning sharply. This reduces traction and increases tire scuff due to undesirable alignment changes. Body roll is controlled primarily by a sway bar, but the stiffness of the springs and shocks also play a role.



Also called bellows, these are the protective rubber (synthetic or natural) or hard plastic (usually Hytrel) covers that surround CV joints. The boot's job is to keep grease in and dirt and water out. Split, torn or otherwise damaged boots should be replaced immediately. Old boots should never be reused when servicing a joint. Always install new boots.



This is the process of removing air bubbles from the brake system by pumping fluid through the lines. Air bubbles are bad because they compress when pressure is applied resulting in a low or spongy feeling pedal. The correct procedure for bleeding the brakes on most RWD vehicles is to start at the furthest wheel. Do the right rear then left rear brake, followed by the right front and left front brakes. On a FWD vehicle with a diagonally-split brake system, do the right rear then left front brake, followed by the left rear and right front brake. See Bleeding Brakes.



The part of the disc brake that squeezes a pair of brake pads against the rotor. A caliper is nothing more than a casting with a piston inside. When hydraulic pressure pushes the piston out, it forces the brake pads against both sides of the rotor. Some calipers are "floating" in that they slide back and forth and self-center over the rotor. Others are said to be "fixed" because they do not move in and out. See Loaded Brake Calipers.



The cast iron housing and friction surface around a drum brake. The brake shoes expand outward and rub against the inside surface of the drums when the brakes are applied. Worn drums often take on a grooved appearance. The inner surface should be turned smooth on a brake lathe when the shoes are replaced. If the drum has worn too thin, is cracked, warped or has taken on a bell-mouthed shape, it must be replaced. The spring around the outside of the drum on some vehicles is there to soak up vibrations and noise. See Drum Brake Service.



The brake system uses a glycol-based hydraulic fluid. The fluid is "hygroscopic," which means it tends to absorb moisture over time (never leave a can of brake fluid open for this reason). Moisture lowers the boiling point of the fluid and causes internal corrosion in the brake system. That is why the fluid should be replaced when brake repairs are made or every two years for preventive maintenance. There are several different types, based on the boiling temperature and other characteristics of the fluid. DOT 3 or DOT 4 are used in most passenger cars and light trucks. Use only the type of fluid specified by the vehicle manufacturer. Using DOT 3 in an application that calls for DOT 4 might create a safety hazard. DOT 5 brake fluid is different from DOT 3 and DOT 4 in that it is silicone-based. DOT 5 is NOT recommended for any vehicle equipped with antilock brakes - but it can provide long-lasting protection against corrosion for vehicles that are stored for long periods of time or are driven in wet environments. See Brake Fluid.



A typical brake job includes replacing the brake linings (new disc brake pads and shoes), resurfacing the rotors and drums, adding fresh brake fluid and bleeding the system, and inspecting/replacing any other worn components (usually at extra cost). If rotors or drums are worn beyond safe limits, they can't be resurfaced and must be replaced. Leaky disc brake calipers, drum brake wheel cylinders or the master cylinder should be rebuilt or replaced. See Brake Job.



The friction material on disc brake pads or drum shoes. A variety of materials are used including asbestos, semi-metallic fibers, Fiberglass and Kevlar. Asbestos linings are used on most older vehicles and on the rear drum brakes. Semi-metallic linings are used on the front brakes of many front-wheel drive applications. Others may be factory equipped with ceramic-based linings. The latest generation of ceramic brake pads are low copper or copper free to conform to government regulations. Never substitute one type of brake lining material for another. The linings rub against the rotors or drums to create friction. This produces a tremendous amount of heat. If the heat builds up faster than it can be shed, the brakes can fade (See Brake Fade). The linings are a high wear item. Front brakes, especially those on FWD vehicles, receive the most wear. Average life for front brakes ranges from 30,000 to 60,000 miles. For rear brakes, 60,000 to 100,000 miles is the norm. Linings should be replaced when worn down to the lining rivet heads, or when lining thickness is less than 1/8th inch or minimum service specifications.



These are the linings used in the front disc brakes. They are called pads because of their flat pad-like shape. Each brake uses a pair of pads (one inner, one outer). Replacement pads are sold in two-pair sets, and are fairly easy to change (See Brake Squeal). Calipers should be inspected for leaks (See Calipers), and the rotors resurfaced to restore a smooth surface (See Brake Rotors). See Brake Pads.



The flat disk-like plates that provide the friction surface in a disc brake. When hydraulic pressure is applied to the caliper, the brake pads are squeezed against both sides of the rotor producing friction and heat. Some rotors have cooling fins between both faces and are called "vented" rotors. The rotors should always be resurfaced when new pads are installed. If worn beyond safe limits, cracked or severely warped, the rotor must be replaced. See Brake Rotors.



The brake system uses hydraulic pressure to stop the vehicle when you step on the brake pedal. Pushing the pedal down pumps fluid from the master cylinder to the brakes at each wheel. This squeezes the brake linings against the rotors and drums, creating friction which brings the vehicle to a halt. The only maintenance the system requires is to check the fluid level periodically, and to replace the fluid every couple of years, or when brake repairs are performed. See Brake Job.



The brake linings used in drum brakes (the rear brakes on most cars). Each drum contains two shoes (a primary or leading shoe, and a secondary or trailing shoe). Replacement shoes are sold in sets of four, one pair for each brake. When shoes are replaced, the condition of the mounting hardware and return springs should be carefully inspected. Replace any worn, damaged or stretched components. Drums should also be turned on a lathe to restore a smooth surface. See Drum Brake Service.



The annoying high pitched screech that is sometimes heard when braking. A common ailment on many disc brake-equipped cars, it is caused by vibration between the brake pad and rotor. It causes no harm, but metallic scraping sounds should be investigated because it usually means the brake linings are worn down to their metal backing plates. If not replaced, the metal-to-metal contact can ruin the rotors or drums. Brake squeal can be eliminated by installing shims on the backs of brake pads, by applying anti-squeal compound or a moly-based brake grease never ordinary chassis grease) to the backs of the pads, and/or resurfacing the rotors. Applying a nondirectional swirl finish on the rotors can provided added noise suppression. See Brake Noise.



Abbreviation for British Thermal Units. One BTU is the amount of heat it takes to heat one pound of water one degree Fahrenheit. The energy value of various fuels is often expressed in so many BTUs per gallon. Gasoline, for example, has around 120,000 BTUs per gallon.



The tendency of a vehicle to suddenly veer or swerve to one side when hitting a bump or dip in the road. The condition is caused by uneven toe changes that occur as a result of the steering linkage or rack not being parallel with the road surface. This causes the wheels to change toe unevenly as the suspension undergoes jounce and rebound.



Rubber bumpers (often cone or wedge shaped) on the chassis that limit suspension travel. "Bottoming out" the suspension means hitting the bump stops.



A liner, grommet or sleeve made of rubber, plastic or metal that fits around a bolt or bar to support, position and in some instances cushion the part. Bushings are used around the pivot bolts that attach the control arms to the chassis. They are also used around sway bars, the links that connect the ends of the sway bar to the control arms, and on the ends of strut rods. Rubber or soft elastomer bushings provide "compliance" in the suspension to help dampen road noise, vibrations and feedback. Hard plastic (usually polyurethane) bushings "firm" up the suspension for improved handling but also increase ride harshness.



A scan tool PID that indicates engine load. It is the percentage of engine capacity being used based on current airflow divided by maximum airflow.



A wheel alignment angle that refers to the inward or outward tilt of the wheels as viewed from the front. Outward tilt is called "positive" camber while inward tilt is called "negative." Ideally, the wheels should have zero rolling camber (perpendicular to the road) when the vehicle is loaded. Camber changes as the vehicle is loaded and the suspension sags. To compensate, the static alignment specifications may call for a slight amount of positive or negative camber depending on how the suspension is built (See Alignment). On vehicles with independent rear suspensions, excessive negative camber often results with the vehicle is overloaded. Excessive camber can cause uneven tread wear on the tires (one side will be worn more than the other). Camber can be affected by worn suspension components such as control arm bushings and ball joints, or by bent parts such as a MacPherson strut. Camber is changed by adding or subtracting shims from the control arm pivot mounts, or on strut cars by moving the top or bottom of the strut in or out. See Wheel Alignment.



The change in camber that occurs when the front wheels on a vehicle with an independent suspension are steered to either side. The amount of camber change that occurs is affected by the amount of caster. Some camber change is good because it causes the tires to lean into a turn for better handling and traction. But too much camber change can accelerate shoulder wear on the tires.



Tire wear that occurs on one side of the tread because the tire is leaning in or out. The underlying cause may be worn control arm bushings, a weak or sagging spring or a badly worn ball joint.



A bolt fitted with an eccentric that is turned to change a wheel's camber setting.

Camber bolts are typically used on control arms and lower strut mounts.



A shaft inside an engine that has lobes to operate the engine's valves. In "pushrod" engines, lifters ride on the cam lobes. The up and down motion is transferred through push rods and rocker arms to actuate the valves (See Lifters). In an "overhead" cam engine, the cam may push directly on the tops of the valves or work the valves through short rocker arms. Loss of lubrication (low oil) or dirty oil can cause scuffing and lobe wear on a cam. The result is loss of engine power because the affected valves do not open completely. The only cure is to replace the cam, a job that requires more advanced skills. The cam may also be changed to improve performance and/or fuel economy. Aftermarket camshafts offer a wide range of different lobe profiles from which to choose. A higher lift, longer duration cam generally provides more power and moves the engine's peak power point up the rpm scale. See Camshafts.



A harmless, odorless gas composed of carbon and oxygen. It is the byproduct of complete combustion. But it is also a greenhouse gas that contributes to global warming and climate change.



A deadly gas that results from the incomplete burning of gasoline inside the engine, carbon monoxide is considered to be a serious air pollutant. You can't see it or smell it, but it can kill in very small concentrations. Because of this you should never run an engine inside a closed garage. Various means are used to reduce the amount of CO produced by an engine, and primary among these is the catalytic converter. The converter "reburns" CO in the exhaust and converts it into harmless carbon dioxide.



A component used to deliver air and fuel on older engines. It mixes air and fuel in varying proportions according to the position of the throttle opening and engine vacuum. Carburetors were replaced by fuel injection in the 1980s, but are still found on many older engines and performance engines. Carburetor adjustments include idle speed, idle fuel mixture and choke setting. Most carburetor problems are due to choke misadjustment or dirty air or fuel. Dirt can plug up the tiny metering orifices, resulting in a variety of drivability problems. Wear around the throttle shafts or warpage or vacuum leaks around the base plate can also cause problems. Overhaul kits are available, but many carburetors can be very difficult to rebuild correctly. A better alternative is a factory rebuilt carburetor that can be easily installed. See Carburetor Diagnosis.



Also known as a Hooke Joint, Universal Joint or U-Joint, it is a simple flexible coupling using a double yoke and four-point center cross. Cardan joints are used as couplings in the driveshafts of rear-wheel drive cars. Because they can produce uneven shaft speeds when operated at joint angles of more than a few degrees, they are usually not used with front-wheel drive (because the front wheels also steer and create large operating angles).



A wheel alignment angle that refers to the forward or rearward tilt of the steering axis on the front wheels (See Alignment). A forward tilt of the steering axis is called "negative" caster while a rearward tilt is called "positive." The caster angle has no affect on tread wear but it does affect steering return and stability. Most vehicle have a certain amount of positive caster. The higher the caster angle the more steady the car feels at high speed (Mercedes, for example, uses a very high caster setting). But the higher the caster angle, the greater the steering effort. The caster angle on many strut suspensions is fixed at the factory and is not adjustable. See Wheel Alignment.



Small wedge shaped shims that fit between a leaf spring and solid axle to change caster. Used primarily on trucks with a solid front axle or four-wheel drive.



The OBD II system monitors the operating efficiency of the catalytic converter to make sure it is reducing emissions and that the vehicle is in compliance with tailpipe emission standards. The OBD II system does this by using a "downstream" O2 sensor mounted behind or in the converter. It compares the readings of the downstream O2 sensor with the "upstream" O2 sensor(s) in the exhaust manifold(s) to estimate converter efficiency. If efficiency drops below a certain threshold, it sets a fault code (p0420) and turns on the Check Engine light. The catalyst monitor only runs under certain driving conditions. The catalyst monitor will NOT run if any O2 sensor codes are present.



The converter is an emissions control device in the exhaust system that reduces the amount of pollutants that come out the tailpipe. It does this by reburning certain pollutants and reforming others. Platinum, palladium and rhodium catalysts act as triggers for the chemical reactions. Catalytic converters were first used on 1975 model year cars to reduce hydrocarbon and carbon monoxide emissions. In 1981, a new type of "three-way" converter was installed to also reduce oxides of nitrogen. The converter does a superb job of reducing pollutants, but the catalyst can be contaminated with lead (from leaded gasoline) and phosphorus (from burning oil), or silicone (from internal coolant leaks). The converter is covered by an 8 year/100,000 mile emissions warranty. It is illegal to remove a catalytic converter. If replacement is necessary, it must be replaced with the same type of converter as the original. See Catalytic Converters.



The bolt that maintains the alignment of the leaves in a leaf spring, and the position of the axle on the springs.



The geometric center of the suspension defined by a line that runs the length of the vehicle and bisects the midpoints of the front and rear axles. Used as a reference line in alignment for measuring toe and thrust angle.



The center bar or link in a parallelogram steering system that connects the pitman arm and idler arm. Also called a "relay rod."



An imaginary point around which the weight of a vehicle is centered. A lower center of gravity improves handing stability and cornering agility. The center of gravity can be lowered by installing shorter suspension springs and/or low profile tires.



A mechanical means of advancing spark timing with flyweights and springs to compensate for changing engine speed (rpm). The weights are located inside the distributor on older vehicles with electronic (noncomputer) ignition systems. The size of the weights, the amount of spring tension, and engine rpm determine the rate and amount of advance. Advancing the spark timing as engine speed increases is necessary for good fuel economy and performance.



The frame or undercarriage of a vehicle. On unibody vehicles, the lower structure to which the suspension is attached.



A process whereby technicians take an EPA approved course on R12 recovery and recycling, and pass a written examination. CFC certification is required to work on all A/C systems, but no additional certification is required for servicing R134a systems.



The rearrangement of the molecular structure of matter as it changes from one physical state to another (solid, liquid or gas). Also called a "phase" change.



A storage device in the evaporative emissions control system. It is a small cylindrical or rectangular container that contains activated charcoal particles. The charcoal traps gasoline vapors from the fuel tank (and carburetor on older vehicles). Later, the vapors are purged and drawn into the engine when the vehicle is being driven. See EVAP System.



A specific amount of refrigerant or compressor oil by weight. This is specified by the vehicle manufacturer for individual A/C system applications.



The charging system includes the alternator, voltage regulator which is often a part of the alternator itself), the battery, and the indicator gauge or warning light on the dash (See Alternator, Battery and Voltage Regulator). The charging system's job is to generate enough current to keep the battery fully charged, and to satisfy the demands of the ignition and electrical systems. The voltage regulator senses the demands on the electrical system, and controls alternator output so sufficient current is produced. A loose V-belt, or a defective alternator or voltage regulator can cause the dash warning light to glow red (or the amp gauge to show and steady discharge). If the problem is not corrected, the battery will run down and eventually go dead. See Charging System Checks.



A warning light that comes on if the computerized engine control system detects an engine performance or emissions problem. Also called the "malfunction indicator lamp" (MIL). To determine the nature of the problem, the computer system must be accessed to read a fault code. See Check Engine Light.


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A valve which permits the passage of a gas or fluid in one direction, but not in the other. For example, the check valve between the air pump and exhaust manifold in an air injection system allows air to flow to the manifold, but stops exhaust gas from entering the air pump in the event that the pump belt breaks. A check valve in the master brake cylinder allows brake fluid to flow in one direction only.



A little flap-like valve in the top of a carburetor that opens and closes to control the amount of air entering the carburetor when the engine is cold. The choke's purpose is to artificially enrich the fuel mixture (by choking off the air supply) during starting and engine warm-up. If the choke is not adjusted correctly, it can make the engine hard to start and/or stall. See Carburetor Diagnosis.



A family of manmade chemicals containing chlorine that include R12 automotive air conditioning refrigerant. CFCs have been blamed for a deterioration of the Earth's protective ozone layer. CFCs have been phased out of production by international agreement.



A protective device that�s often used in a wiring circuit to protect against overloads. A circuit breaker has a bimetallic arm and a pair of contact points. When the current exceeds its preset limits, the arm gets hot, bends and opens the contact points. This shuts off the current through the circuit and protects against damage or fire. Most circuit breakers automatically reset themselves after they cool down, but some have a button that must be manually reset to restore power. Circuit breakers are often used in the headlight and air conditioning circuits.



The basic principle of electronic engine management in which input from an oxygen sensor allows the engine control computer to determine and maintain a nearly perfect air-fuel ratio. To enter closed loop operation, the oxygen sensor must be producing a voltage signal and the engine must have reached a certain operating temperature. Sell also Open Loop.


A device that couples the engine to the transmission. The clutch consists of a friction-lined disk (called the "clutch disk") and a spring-loaded "pressure plate" that presses the clutch disk tightly against the flywheel (See Flywheel). When you push in on the clutch pedal, the linkage releases the spring pressure allowing the clutch disk to slip. The clutch disk is subjected to a tremendous amount of friction and heat, which eventually wears it out. At this point it starts to slip. Oil or grease on the flywheel, weak or broken springs in the pressure plate, or overadjusted linkage can also make it slip. If it fails to release, the most likely cause is a broken clutch cable or a leaky hydraulic linkage. See Common CLutch Problems.



A type of distributorless ignition system where individual ignition coils are mounted directly over each spark plug. No spark plug wires are used. See COP Ignition.



A type of spring made of wound heavy-gauge steel wire used to support the weight of the vehicle. The spring may be located between the control arm and chassis, the axle and chassis, or around a MacPherson strut. Coil springs may be conical or spiral wound, constant rate or variable rate, and wound with variable pitch spacing or variable thickness wire. Coil springs sag with age, and sometimes break. Replacement in pairs is recommended to maintain even ride height side-to-side.



The "give" or flexing that occurs in the suspension and steering due to the compression of rubber bushings and joint play. A small amount of compliance is desirable because it absorbs shocks and dampens vibrations to reduce steering feedback and harshness. But too much compliance can make the steering feel vague and mushy (unresponsive), while also contributing to toe wear by allowing excessive changes in toe alignment.



The amount by which the air volume in a cylinder is reduced or compressed by the upward stroke of the piston. See Compression Ratio. Compression can be measured mechanically by installing a compression gauge in a spark plug hole, disabling the ignition and cranking the engine, or electronically by an engine analyzer during a cranking test. See Compression Testing.



The relationship between the piston cylinder volume from bottom dead center to top dead center. Higher compression ratios improve combustion efficiency but also require higher-octane fuels. Pre-emission control engines often had compression ratios as high as 11.5:1 whereas most of today's engines are between 8.5:1 and 9.5:1. Diesel engines have very high compression ratios, from 18:1 to 22:1.



The refrigeration system component that pumps refrigerant and increases the pressure and temperature of refrigerant vapor. The compressor is belt driven via a magnetic clutch, and may be a piston or scroll type design. A compressor failure can throw metallic debris into the A/C system that can damage a replacement compressor unless the condenser is cleaned by flushing or replaced. See Compressor Failures.



See Thermostatic Switch.



A low pressure cutoff switch in a CCOT refrigeration circuit that reacts to low head pressure and opens the compressor clutch circuit to disengage the compressor if the system loses its charge of refrigerant. Some systems also have a separate high pressure cutoff switch (or a combination high-low pressure switch) that opens the compressor clutch circuit if system pressure exceeds a preset limit.



The oil within the A/C system that lubricates the compressor. R12 systems use a special type of mineral oil. R134a systems use either a PAG or ester-based oil. A certain amount of compressor oil must be in the system at all times to prevent compressor damage. Loss of compressor oil (or failure to replace oil that was lost during the service or replacement of system components) will in compressor failure. Too much oil in the system can cause loss of cooling efficiency or compressor failure. See PAG Oil Recommendations.



A microprocessor based engine management systems that utilizes various sensor inputs to regulate spark timing, fuel mixture, emissions and other functions. Used on most vehicles since 1981 to comply with federal emission regulations. Diagnosis usually requires accessing trouble codes and/or putting the system into a special diagnostic mode. See Engine Management Systems and PCMs.



The process whereby a vapor changes to a liquid. This requires a "cooling effect" to draw heat away from the vapor. When the temperature of the vapor reaches a certain point, droplets of liquid (condensate) begin to form. Condensation of the refrigerant vapor takes place in the condenser.



The refrigeration system component that changes refrigerant vapor to a liquid by removing heat. The condenser is an air-to-air heat exchanger consisting of metal tubes and cooling fins. It is usually mounted just ahead of the radiator, and may have its own cooling fan.



The transmission of energy (heat) through a medium without perceptible motion of that medium (direct contact).



A Constant Velocity Joint is one that provides consistent driveshaft speeds regardless of the operating angle of the joint. CV joints are used primarily in on the driveshafts of front-wheel drive vehicles, and they come in two basic varieties: the Rzeppa ball type joints (which you will find on the outer end of the driveshaft) and tripod joints (which are used on the inner end). See CV Joints.



CAN is essentially an engineering standard for how computers and modules talk to one another via the serial data bus in a vehicle's wiring system. It's a high speed standard designed for powertrain control modules, antilock brakes and stability control systems. It is used on a growing number of 2003 and newer vehicles. See CAN systems.



Suspension components which connect the steering knuckles to the chassis or subframe, and allow the knuckles to move up and down.



The transfer of heat by the circulation of a liquid or vapor



The liquid inside the radiator and cooling system is called the "coolant" because it cools the engine. It circulates through the engine and soaks up heat. The coolant then flows to the radiator (See Radiator) where it sheds its heat. When the heater is turned on, coolant also flows through the heater core (which acts like a miniature radiator) to heat air entering the passenger compartment. A low coolant level can result in overheating, no heat from the heater, and/or serious engine damage. The coolant level inside the radiator should be checked periodically to replace any that has been lost. The recommended coolant for most vehicles is a mixture of 50% water and 50% antifreeze. Straight water should never be used because it is extremely corrosive, and offers no freezing or boilover protection. See Types of Antifreeze.



A variable resistance thermistor which changes resistance as the engine's coolant temperature changes. The sensor's output is monitored by the engine computer to regulate various ignition, fuel and emission control functions, and to turn the radiator cooling fan on and off as needed. In the PTC (Positive Temperature Coefficient) type of sensor, ohms go up with temperature. In the more common NTC (Negative Temperature Coefficient) type, resistance goes down as heat goes up. See Coolant Sensors.



The cooling system consists of the radiator, water pump, thermostat, heater core, heater and radiator hoses, and the water jackets inside the cylinder head and engine block (See Coolant, Radiator and Water pump). An engine produces a tremendous amount of waste heat when it runs, so some means of cooling is needed to prevent the engine from self-destructing. Some engines (such as lawn mower and small motorcycles) are air-cooled. But liquid-cooling is used for most automotive applications because it is more efficient, it allows better temperature control (for better performance and lower emissions), and it can provide heat in the winter. See Servicing Cooling System.



A structural member used in many front-wheel drive cars that supports the engine and transaxle. The cradle is bolted to the subframe, and is also connected to the lower control arms. The position of the cradle is important because it affects camber and caster.



See blowby, also Measuring Blowby



The main shaft inside the engine that turns the up-and-down motion of the pistons into rotational torque. There are two types of crankshafts: cast iron and forged steel. The cast variety are used in most passenger car engines while the stronger forged ones are used primarily in high performance applications. When an engine is overhauled, the rod and main bearing journals are reground to restore a smooth surface. Crankshaft failures are fortunately not very common, but when they happen it usually caused by excessive internal engine vibration or defects in the crankshaft itself.



A type of sensor used to monitor the position of the crankshaft. The sensor's input is used to trigger the ignition system. There are two basic types: magnetic and hall effect. The sensor reads notches in a ring mounted on the crankshaft, harmonic balancer or flywheel. See Crankshaft Position Sensors.



The difference side-to-side between camber settings. More than half a degree difference may cause a steering pull toward the side with the most (positive) camber. See Wheel Alignment and Alignment Diagnosis.



The difference side-to-side between caster settings. More than half a degree difference may cause a steering pull toward the side the least (negative) caster. Caster on the left front wheel is sometimes decreased to compensate for high road crown. See Wheel Alignment and Alignment Diagnosis.



Refers to the switching activity of the oxygen sensor as it switches back and forth from rich to lean and back again. Low cross count activity in an upstream oxygen sensor on a warm engine indicates a bad oxygen sensor.



A structural component that bolts between the frame rails or attaches to the subframe of a unibody. The lower control arms may be attached to the crossmember. The position of the crossmember is important because it affects camber, caster and setback.



A refrigerant system in which a fixed displacement compressor is engaged and disengaged to maintain the refrigeration cycle. By cycling the compressor clutch on and off, the cooling output of the system is regulated.



The communications network in a vehicle that allows multiple control modules to communicate with each other. Various protocols determine the speed or baud rate at which information flows over the bus network.



The diagnostic communications connector on a vehicle. Allows a scan tool to communicate with the Powertrain Control Module (PCM) and other onboard modules. On 1996 and newer OBD II vehicles, the connector is usually located under the dash near the steering column.



A drying agent used in the refrigeration system to remove moisture. The moisture-absorbing zeolyte crystals are located in the accumulator-dryer or receiver-dryer depending on the type of system. R134a systems generally require XH-7 or XH-9 desiccant.



This is the pinging or knocking sound that is sometimes heard while accelerating. The noise is the result of erratic combustion inside an engine. Instead of burning normally, the fuel explodes in multiple flame front, and the colliding fireballs inside the cylinders shake and rattle the pistons. Mild detonation is annoying but it will not hurt anything. Severe or prolonged detonation, on the other hand, can ruin an engine. If switching to a higher octane fuel does not cure the problem, timing adjustments or other repairs may be necessary. Detonation is often a symptom of a faulty EGR system or a defective knock sensor. See Spark Knock.



Chemical name of R12 refrigerant. See Alternative Refrigerants.



Computerized engine control systems have a certain amount of built-in self-diagnostic capability to detect problems that affect engine performance and emissions. The same is true for the antilock brake system and other onboard systems that are computer controlled. When a fault is detected, the computer will store a diagnostic trouble code in its memory and illuminate the "Check Engine" light. On some vehicles, the computer can be put into a special diagnostic mode by grounding certain terminals on a diagnostic connector. This will cause the Check Engine or other lights to flash out the fault code. On many vehicles, though, a scan tool must be plugged into the computer system to access and read the codes. For more information about Trouble Codes, Click Here.



A type of engine that uses compression to ignite its fuel rather than a spark. A diesel engine has a much higher compression ratio than a gasoline engine (22:1 versus 8:1 for example), and because of this it is able to squeeze more usable power out of each drop of fuel. A typical diesel gets 30 to 50 percent better fuel mileage than a comparable gasoline engine of equal displacement. A diesel engine has no carburetor or throttle. Fuel is injected directly into the engine's cylinders through high pressure injectors. Injector timing is very important because it affects idle quality, rattling and exhaust smoke. Engine speed is governed by the injection pump which controls the amount of fuel delivered. Newer diesels use electronic injectors and computer controls to reduce emissions. Most passenger car diesel engines have a glow plug starting system that preheats the combustion chamber. The fuel system can be contaminated by water so many also have fuel/water separator filters. See Diesel Diagnosis.



This is the gear box between the drive axles that transfers torque from the driveshaft to the axles and allows the drive axles to rotate at different speeds. This is necessary because the inner wheel follows a smaller arc than the outer one when the vehicle turns. The differential always provides power to the wheel that needs it least, because the gears always allow torque to follow the path of least resistance. Locking differentials that use spring-loaded clutch packs or fluid-encased disks are available as an option on some vehicles to prevent wheel spin. This is a "must" option for any high performance or off-road vehicle.



An electrical component used to control the flow of electricity in one direction. Used in alternators to convert alternating current into direct current. Diodes are part of the alternator rectifier assembly.



A type of brake design that uses a flat disk-shaped rotor as the friction surface. A caliper squeezes a pair of brake pads against the rotor to stop the vehicle. Disc brakes are used on the front wheels of most passenger cars, and sometimes on the rear. See Disc Brake Service.



The conditioned (cooled & dehumidified) air entering the passenger compartment from the A/C system.



Connects the compressor outlet to the condenser inlet. Also called the "high side" line. High pressure refrigerant vapor flows through this line.



The pressure of the high temperature refrigerant vapor as it leaves the compressor.



The part of the A/C system from the outlet port of the compressor to the evaporator inlet.



A method of improving fuel economy under light load when full engine power isn't needed. The engine computer deactivates up to half of the engine's cylinders to reduce the engine's effective displacment. When more power is needed, the cylinders are reactivated to increase engine displacement. Used on late model Chrysler 300 Hemi engines.



The "brain" of the ignition system that "distributes" ignition voltage to each of the spark plugs. The distributor contains an electronic trigger or pickup device (older cars use contact points) that trigger the ignition coil. High voltage enters the distributor cap from the coil, travels down through the rotor to the appropriate spark plug terminal and exits out the wire. On pre-computer cars, the distributor also controls spark timing via centrifugal and vacuum advance units, but this function is performed by the computer in late model cars. The only maintenance the distributor requires is periodic replacement of the rotor and cap (older cars need annual point replacements). Most newer engines (1990s & up) do not have distributors, but use a "distributorless" type of ignition system.



An ignition system that does not use a distributor to route high voltage to the spark plugs. The high voltage plug wire runs directly from the ignition coil to the spark plug. Some DIS systems have one coil for every two spark plugs (a shared system), while others have a separate coil for each spark plug (See Coil-On-Plug Ignition). Eliminating the distributor makes the system more reliable and eliminates maintenance. See Distributorless Ignition Systems.



The propeller shaft that transmits engine torque to the differential, or from the differential to the drive wheels. In front-wheel drive vehicles, the two driveshafts are often referred to as "halfshafts."



Also called crabbing, this refers to a condition where the rear wheels do not follow straight behind the front ones because of rear axle or rear toe misalignment. The rear wheels track off to one side, which produces off-center steering and contributes to front toe wear. See Alignment Diagnosis.



Brake fluid that meets the Department of Transportation specifications for glycol based fluids with a wet boiling point (lowest allowable after it has been in use) of 284 degrees F. and a dry boiling point of 401 degrees F. DOT 3 fluid is the type commonly specified by most vehicle manufacturers. Because it is glycol based, it absorbs moisture over time (hygroscopic). This lowers its effective boiling point and promotes internal corrosion in the brake system. For this reason, the brake fluid should be replaced periodically (every two years is recommended by many experts) and every time the brakes are relined or serviced. See Brake Fluid.



A "heavy-duty" glycol based brake fluid with a slightly higher wet boiling point of 311 degrees F. and a dry boiling point of 446 degrees. This type of fluid is sometimes specified for performance vehicles or those subject to high brake temperatures. See Brake Fluid.



A silicone based fluid that does not absorb moisture and has a boiling point of at least 500 degrees F. DOT 5 fluid does not have to be changed periodically and can minimize brake system corrosion, but is very expensive compared to DOT 3 or DOT 4 fluid (it costs three to five times as much). It will not mix with DOT 3 or DOT 4 brake fluid. DOT 5 is NOT recommended for any vehicle with ABS brakes because it tends to aerate when cycled rapidly through small orifices. See Brake Fluid.



On a feedback carburetor, a solenoid that cycles many times per second to control a metering rod, hence the air/fuel mixture. The "on time" (duty cycle) of the solenoid determines the air/fuel ratio.



Wheel balance that results from the equal distribution of weight on both faces or sides of a wheel. Achieving dynamic balance requires spinning the wheel to identify the heavy spots on each side. A wheel that lacks dynamic balance will shimmy back-and-forth.



A machine that is used to measure the horsepower output of an engine. A chassis dyno has large rollers upon which the drive wheels are placed. The vehicle is run up to a certain speed and put under load so the amount of power that is being delivered to the wheels can be measured (See Horsepower and Torque). A dyno can also be used to simulate actual driving conditions when troubleshooting various derivability problems. Dynos are also used to simulate driving conditions during certain types of emissions tests.



The EGR valve is the main emissions control component in the exhaust gas recirculation system (See Exhaust Gas Recirculation). The valve is located on the intake manifold, and opens a small passageway between the exhaust and intake manifold to allow a metered amount of exhaust to flow back into the engine. This reduces combustion temperatures and helps control the formation of oxides of nitrogen (See NOX). The EGR valve is opened by the application of vacuum to its control diaphragm. Some also require a certain amount of exhaust back pressure before they will open. On newer vehicles, the valve is electronic and uses one or more solenoids or a small stepper motor. The valve should remain closed while the engine is cold and at idle. It should only open once the engine has warmed up and is running at part-throttle. If the valve sticks shut (or is disconnected), NOX emissions will soar and detonation will often result (See Detonation and Spark Knock). If it sticks in the open position or fails to close all the way, it acts like a vacuum leak resulting in a rough idle, hesitation and possible stalling. See EGR.



The battery, wires and electrically-operated accessories in a vehicle. All modern passenger cars, light trucks and most large motorcycles have 12-volt electrical systems. Farm tractors, most small motorcycles, antique cars and pre-1967 Volkswagens have 6-volt electrical systems. Most heavy-duty trucks use 24-volt systems. The electrical system uses the battery and charging system as its power source, with wires and switches routing the voltage to where it is needed. The metal body serves as the ground or return path for the voltage back to the battery. The electrical system is protected against damage by various devices (See Circuit Breaker, Fuse and Relay). Most electrical problems fall into one of three basic categories: poor ground connections (loose or corroded), opens (breaks in circuit wires, connectors or switches), or shorts (grounded circuit wires or switches). A test light, ohmmeter and/or voltmeter can be used to find the fault. See Troubleshooting Electrical Problems.



Abbreviation for Electronic Fuel Injection. This type of system uses computer-controlled fuel injectors to spray fuel into the engine rather than mechanically controlled injectors or a carburetor. EFI comes in several varieties: "throttle body injection" (See TBI), "multi-port injection" (See MFI or PFI) or Sequential Fuel Injection (SFI). Electronic fuel injection is considered to be superior to carburetion because it allows more precise fuel metering for easier starting, lower emissions, better fuel economy and performance.



The vehicle components that are responsible for reducing air pollution. This includes crankcase emissions, evaporative emissions and tailpipe exhaust emissions. Crankcase emissions consist of unburned fuel and combustion byproducts. These gases are recirculated back into the engine for reburing by the Positive Crankcase Ventilation (PCV) system (See PCV system). Evaporative emissions are the fuel vapors that seep out of the fuel tank and carburetor. They are prevented from escaping into the atmosphere by sealing the fuel system and storing the vapors in a vapor canister (See Charcoal Canister) for later reburning. Tailpipe exhaust emissions consist of carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOX)(See Carbon Monoxide, Hydrocarbons and NOX). These formation of these pollutants is minimized by various engine design features, careful control over fuel calibration (see Air/Fuel Ratio) and ignition timing, and the EGR system (See Exhaust Gas Recirculation). The pollutants that make it into the exhaust are "reburned" before they exit the tailpipe by the catalytic converter (See Air Pump and Catalytic Converter). The emission control system is an integral part of the engine, and should not be tampered with or disconnected. This is especially true on vehicles with computerized engine controls and/or those that must be subjected to mandatory emissions testing. See Emissions Testing.



The specific operating conditions that must be met before OBD II will run self-diagnostic checks and/or set a diagnostic trouble code. The criteria will vary depending on the year, make and model year of the vehicle, and the system or sensor being monitored.



The computer or electronic control mudule that regulates engine functions such as spark timing, fuel delivery and other emissions functions. Also called a Powertrain Control Module (PCM) if it incorporates transmission control functions. See Engine Management Systems and PCMs.



The size of an engine or its volumetric displacement is a function of cylinder bore diameter, piston stroke and the number of cylinders. Displacement = Surface area of the bore Diameter x stroke x number of cylinders. Engine displacement is usually expressed in Cubic Inches Displaced (CID) or Liters (L). As a rule, the larger the engine, the more power it is capable of making. For more information on this subject, see Engine Displacement.



Abbreviation for the Environmental Protection Agency, the government agency responsible for enforcing anti-pollution rules. The EPA requires all vehicle manufacturers to certify their new car as being in compliance with the applicable clean air standards for the year of manufacture. The manufacturer, in turn, must provide an "emissions" warranty on every vehicle they sell that guarantees free replacement of any emissions control device that might fail during that time. This coverage usually extends to such items as the computer control system, catalytic converter, fuel and ignition system (except the spark plugs and normal wear items). See EPA website.



To create a vacuum within a refrigeration system for the purpose of drawing out air and moisture. The system may be evacuated as part of a refrigerant recovery process, to check for leaks or to purge the system of unwanted air and moisture prior to recharging it with refrigerant.



Gasoline fuel vapors that are released into the atmosphere from a vehicle's fuel system.



The emission control system that prevents the escape of fuel vapors from a vehicle's fuel system. Fuel vapors are routed by hoses to a charcoal canister for storage. Later, when the engine is running a purge control valve opens allowing intake vacuum to siphon the fuel vapors into the engine. See EVAP System



The change from a liquid to a vapor. This process absorbs heat and has a cooling effect. Refrigerant evaporates inside the evaporator to cool the air flowing through it.



The component in the refrigeration system that absorbs heat from air entering

the passenger compartment to produce a cooling effect. It is an air-to-air heat exchanger.



A piece of test equipment used to analyze the composition of vehicle exhaust gases. A 5-gas analyzer measures carbon dioxide (CO), carbon monoxide(CO), oxygen (O2), hydrocarbons (HC) and oxides of nitrogen (NOX). The gas readings can be used to determine emissions compliance, and to diagnose various engine performance problems. See Emissions Testing.



This is an emissions control technique for reducing oxides of nitrogen emissions in the tailpipe. A small amount of exhaust gas is recirculated back into the intake manifold to dilute the incoming air/fuel mixture. Contrary to what you would think, it has a cooling effect on combustion temperatures which helps reduce the formation of oxides of nitrogen (See NOX). The EGR valve is the main control device in this system. See EGR



The exhaust system consists of the exhaust manifold, exhaust pipe, catalytic converter, muffler and tailpipe. The system performs three important jobs: it carries exhaust gases away from the engine, it quiets the engine (See Muffler), and it helps control pollution (See Catalytic Converter). The exhaust systems one weakness is its vulnerability to corrosion. Original equipment exhaust systems usually have stainless steel headpipes (the pipe between the exhaust manifold and catalytic converter) and converter shells, and aluminized pipe to resist corrosion. But after three or four years, the muffler and tailpipe often need to be replaced. Many newer vehicles are equipped with stainless steel exhaust systems which typically last 7 to 10 years. See Troubleshooting Exhaust Problems and How to Check Exhaust Backpressure



Same as TXV valve, a control device that meters the amount of refrigerant to the evaporator to regulate cooling.

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