Glossary automotive terms and acronyms

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10W30
An SAE classification for oil. This means an equivalent viscosity of 10 when cold (zero Celsius) and 30 when hot (100 Celsius.) (see also viscosity and SAE)

10W30 motor oil label

2-wheel alignment
Wheel alignment, where measuring devices are only attached to the front wheels rather than all four, as in a four-wheel alignment. This is normally less expensive than four-wheel alignment and adequate under certain circumstances. For instance when the rear alignment is known good. (see also our Suspension Repair Category.)

4-wheel alignment
Wheel alignment, where measuring devices are attached to all four wheels rather than just the front, as in a two-wheel alignment. This has the added advantage of correcting the rear as well as front alignment and insuring that tracking is correct on the vehicle. (see also our Suspension Repair Category. )

4WD
An acronym meaning Four Wheel Drive. An arrangement where the front and rear wheels can be linked to the transmission and used to propel the vehicle. (see also FWD and RWD)

5W30
An SAE classification for oil with an equivalent viscosity of 5 when cold (zero Celsius) and 30 when hot (100 Celsius.) (see also viscosity and SAE)

5W30 motor oil label

A
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A-arm
Slang term for a control arm, especially in a short-long arm type suspension, where many resemble the shape of the letter “A.” (see also short-long arm)

A lower control arm, sometimes called an A-arm



A/S tire
Stands for all season, a tire that can be used in snow as well as on the highway, but less aggressive than an M/S designation.

A/T tire
Stands for all terrain, a tire that can be used in mud and snow as well as on the highway, more aggressive than an M/S designation.

AALA
An acronym meaning American Automobile Labeling Act. A little known law, requiring vehicles sold in the United States to be labeled as to the percentage of US/Canadian content and the Country of manufacture. This information should be provided, upon request, by new vehicle dealerships in the United States.

ABI
An acronym for already been installed. A part sold as new that has been previously installed. This often happens when a customer buys a part, installs it and then returns it as new. Especially common when trying to guess at the source of a problem rather than properly diagnosing it. (see also swaptronics)

Accumulator
A temporary storage device, between the evaporator core and the inlet of the air conditioner compressor, designed to trap any liquid refrigerant and help prevent it from damaging the compressor. Many accumulators also contain a desiccant that helps to absorb moisture in the system. Please see our Detailed Topic, How an automotive air conditioner works, for more information.

Acorn nut
An acorn style lug nutA style of wheel fastener, where the internal threads are enclosed on one end, by the design, which is roughly the shape of an acorn. For more information on lug nuts, please see our Detailed Topic Wheel Lugs, Torque and Keeping Wheels On.

Actuator
A device used to perform a mechanical action and is operated by electricity, vacuum or fluid pressure. Actuators are often used to direct air from air conditioning systems in the dash.

Aftermarket part
A part not supplied as original equipment on a vehicle, rather built by a third party. A part that is not supplied by the original manufacturer of a vehicle. (See also OEM part)

AGCO
An acronym standing for Altazan's Garage COmpany; A Good COmpany

Agconomics
Saving money, purchasing a three-year old vehicle, maintaining it well and driving it for several years, rather than buying new vehicles and trading them in. The savings can be used to retire previous debt, substantially increasing funds available for savings and enjoyment of life.

Air bag
Another name for the Supplemental Inflatable Restraint (SIR) system. A combination of computer controlled inflatable devices that help cushion the occupants of the vehicle in a collision. (See also SIR)

Air-fuel sensor
A device used to determine the amount of oxygen in the exhaust after combustion. Similar in appearance to an oxygen sensor, the output is much different. This information is used by the power control module to more accurately determine if the amount of fuel being added is correct, over a wide range of conditions. When the fuel/air mixture is rich, the air/fuel sensor produces a negative current. Correct mixture results in no flow and a lean condition is indicated by positive current flow. (see also oxygen sensor)

Alignment
Attachment of measuring devices to the vehicle’s wheels in order to measure and correct their positioning to the proper specifications. This can be done on the front wheels only, in a two-wheel alignment or all four wheels, in a four-wheel alignment. (See also 4-wheel alignment) and our Suspension Repair Category.

Antifreeze
A chemical, often comprised of ethylene glycol or propylene glycol which is combined with water and add to the engine to prevent freezing. It also contains additives to prevent corrosion and raise the boiling point of the mixture. (see also coolant.)

Antique
The word antique is very subjective. A generally accepted definition is something reminiscent of an earlier period and valued for its age. Clearly this is very non-specific. Under Louisiana law a vehicle twenty-five years old and in original condition is eligible for an antique vehicle license plate.

API
An acronym meaning American Petroleum Institute. A body that among other things, rates oil and lubricants and assigns a classification that producers can voluntarily display on products that meet the specifications. A rating beginning with “S” is for gasoline engine classification. SN is the highest rating at the time of publication. Ratings starting with “C” would pertain to diesel engines. (see also SAE)

API badge



Aspect ratio
The ratio of width to sidewall height of the tire. For instance a 235/75R16 tires is 75% as tall as it is wide. For a calculator to demonstrate aspect ratio see our Michelin Tire Category.

Aspect ratio of a tire



ASTM
An acronym meaning American Society for Testing and Materials. An organization that provides standards for testing of a multitude of products and services. As related to automotive, particularly standards for engine coolant.

ATF
An acronym meaning Automatic Transmission Fluid. It does not denote a specific fluid rather a general term. (See also Dexron and Mercon)

Just a few automatic transmission fluids in current use

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Back probe
Using a thin metal device (made for the purpose) to connect with the contacts in an electrical connector. The probe can be connected to different measuring devices used to monitor a signal in the circuit. Back probing is far superior to piercing the insulation or probing the connection area, which can result in damage.

Please see our Detailed Topic Tips on Automotive Electrical Testing for more details.

Back probing an electrical connector



Bad ground
A term for high resistance in a ground circuit. Resistance prevents current returning to the battery and causes devices to stop working or work improperly. Voltage drop testing is most often used to locate high resistance in connections, by measuring the current that flows around them, while under a load. For more information see our Detailed Auto Topics Voltage drop testing, parts I & II.

Bank
A row of cylinders, particularly on one side of a V-style engine. A V-8 has two banks of four cylinders each.

BCM
An acronym meaning Brake Control Module. The computer that controls the anti-lock brake system, also know as the Electronic Brake Control Module or EBCM. See our section on Brake Service.

Bead
The portion of a tire, comprising the inside opening and that contacts the wheel when installed. The bead is constructed of wire and cords, covered by thick rubber and seals the tire to the wheel. For more information, please see our Detailed Topic, Ruining New Tires.

Cross section of a tire showing the bead



Belt slap
A derogatory term for simply replacing a timing belt on a high milage engine, ignoring the water pump, seals and other components which also often fail around the same milage. Please see our Detailed Topic All About Timing Belts for more details.

Bleeding
Term which describes the process of removing trapped air from fluid. Often refers to brake bleeding or cooling system bleeding.

Blower motor
An electrical motor that turns the fan, creating air flow for the heater and air conditioner. Blower motors usually have squirrel cage fans, enclosed in a blower housing.

Bottom dead center
The point of least extension of a piston in a cylinder, where it is furthest from the cylinder head. The furthest point in the intake stroke. (see also Top dead center and four cycle)

Bottoming
When a suspension travels to the maximum limit of it’s downward design. When jounce bumpers contact their mating surface. (See also jounce and rebound)

Brake job
An obsolete and improper term. Often used to describe replacing certain brake components, with no regard to whether they are needed or will address the client’s problem.

Brake master cylinder
A hydraulic pump attached to the brake pedal. On pedal application it supplies hydraulic pressure into the system to apply the brakes.

Typical brake master cylinder



Brake shudder
A condition where the steering wheel moves side to side when braking and quits when the brakes are released. Feels similar to improper wheel balance except braking brings it on rather than driving a certain speed.

Butcher
A derogatory term for a person that regularly causes additional damage in the process of attempting a repair. The act of damaging a component, by improper methods, that could have been properly repaired.

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C V Boot
An acronym meaning Constant Velocity Boot. The flexible cover that protects the C V Joint by holding lubricant in and sealing contaminants out. Often used on front wheel drive and independent suspension rear wheel drive vehicles. (see also C V Joint)

One of several varieties of C V axles



C V Joint
An acronym meaning Constant Velocity Joint. A type of coupling that allows power to be transmitted to a wheel at a wide array of angles. Often used on front wheel drive and independent suspension rear wheel drive vehicles. (see also FWD and RWD)

Cabin filter
A filter on the intake side of the air conditioner blower that protects the evaporator core from dirt build up. Not all vehicles are equipped with cabin filters. For more information, please see our Detailed Topic, Automotive Air Conditioning Problems, Part I.

Camber
The angle formed by a line drawn through the vertical center of a wheel/tire and true vertical. See also included angle and SAI Zero, positive and negative camber

Carbon dioxide test
A test used to check on the presence of exhaust gas in the coolant of an engine. Carbon dioxide in the coolant indicate leakage, normally as a result of a warped or cracked cylinder head or blown head gasket.

Please also see Symptoms of a Blown Head Gasket for far more detail.



Castle nut
Castle nut A type of fastener often used in suspension work that has raised sections, much like a castle battlement. A cotter pin fits between the raised section and prevents the fastener from working loose.

A typical castle nut and cotter pin



CEL
An acronym meaning Check Engine Light. An indicator to the driver that a fault has occurred in the computer management system See our section on Engine Diagnostics. (see also Check Engine Light)

Certified used vehicle
A marketing term for a late model, used vehicle that normally has an extended warranty tacked on to the price. These may come from all the same places that any used vehicles come from (e.g., auctions, trade ins, repossessions, etc.) They should be viewed with the exact same cautions as any other used vehicle.

Check engine light
Another name for the Malfunction Indicator Light (MIL.) An indicator to the driver that a fault has occurred in the computer management system. See our section on Engine Diagnostics. (See also MIL)

Chop
A customization technique, cutting and removing a section of a vehicle body and reattaching, to modify the appearance. Chopping is most common on tops of vintage vehicles, to provide a lower and more sleek appearance.

Examples of a normal and chopped top

CKP
An acronym meaning crankshaft-position sensor. The power control module monitors the position of the crankshaft, to help determine cylinder timing.

Classic
Generally a classic is a vehicle design that routinely might command a price above what would otherwise be considered normal given the age and condition of the vehicle.

Clunker
A derogatory term for any vehicle that is not still in the new car dealer’s inventory; A vehicle on which the salesperson has already been paid their commission. For instance, a “state of the art machine” becomes a clunker as soon as the sale is passed.

Clutch master cylinder
A hydraulic pump attached to the clutch pedal. On pedal application it supplies hydraulic pressure into the system to apply the clutch. (See also slave cylinder)

Clutch slave cylinder
A hydraulic cylinder that receives pressure from the master cylinder when pressure is applied to the pedal. The hydraulic pressure is converted into movement that releases the clutch. (See also clutch master cylinder)

CMP
An acronym meaning camshaft-position sensor. The power control module monitors the position of the camshaft, in relation to the crankshaft. Knowing camshaft position eliminates the need for an ignition distributor and enables variable camshaft timing, based on the engine need.

Coarse tread
A designation of the number of threads per inch or number of millimeters between treads of a fastener. In inch style fasteners, relating to the older United States Standard or USS. Coarse thread fasteners have less threads per inch for a given size than fine thread. In metric fasteners, there are more millimeters between threads than the same size fastener with fine thread. Please see our Detailed Topic A Guide to Automotive Bolts and Tightening, Pt.1 for more details.

Condenser
A heat exchanger, normally located in front of the engine radiator. The condenser is used to convert hot gaseous refrigerant to cooler liquid refrigerant. This is accomplished by allowing air flow to remove the heat from the refrigerant. For more information, please see our Detailed Topic, Automotive Air Conditioning Problems, Part IV.

Conicity
A manufacturing defect in a tire, where the belts are shifted to one side of the tread. This makes one side of the tire stronger than the other and causes it to inflate in a cone shape. The shape causes a vehicle to pull to one side when driven. Also referred to as a tire pull or ride disturbance. Please see our Detailed Topic Tire conicity and radial pull for more details.

Constantly variable transmission
A type of automatic transmission that does not used conventional stepped gears, to change ratio. Various designs may use either wheels, pulleys or belts to continuously vary the output and input ratio.

Coolant
A mixture of anti-freeze and water used to cool the engine, prevent corrosion and help with over heating.(See also Antifreeze)

COP
An acronym meaning coil on plug. An ignition system design where the ignition coil mounts directly to the spark plug and is fired by a low voltage signal.

Typical example of a coil on plug design



Cost
The initial amount paid, plus other expenses involved, divided by how long the product or service last. (See also price and The Difference In Price and Cost in our AGCO Philosophy Category)

Counterfeit parts
Cheap substitute parts, often substandard, copied from OEM parts and represented to be OEM parts. Often imported from countries with no enforcement of counterfeiting laws. (See also OEM)

Coupe
A passenger car having two doors and with or without a center post, between the front door glass and the rear quarter window.

Crank
When the starter motor causes the engine to turn in an effort to start.

Crankcase
On an automobile, a synonym for the engine block, particularly the portion between the cylinder head or heads and the oil pan. The crankcase is so named because the crankshaft is housed and supported by its structure.

Creeper
Any of several wheeled devices with a platform on which to lay or sit and roll. Before lifts the flat creeper was useful for the technician to quickly roll under a vehicle on jack stands. Creeper stools are still occasionally used when working on a partially lifted vehicle.

Cubic inches
The US or English equivalent of liters, or a measurement of volume, as in the volume of the cylinders of an engine. Engine volume is the size of a cylinder, with the piston at the lowest possible position, times the number of cylinders the engine has. For a calculator to convert cubic inches to liters, please see our Cost Savings Calculators.

Curb weight
The full weight of a vehicle, with all standard equipment and a full tank of fuel, but without passengers.

CVT
An acronym meaning Constantly Variable Transmission. (See Constantly variable transmission)

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Dealership
An organization that has purchased the rights to sell a certain line of new vehicles. The agreement also normally requires them to perform warranty work on those vehicles. Dealerships often operate an auto repair shop and a parts store as part of their operation.

Detonation
A rattling, knocking type noise, normally experienced on acceleration and under load. Detonation results when unburned fuel in the combustion chamber, spontaneously combust, after ignition has occurred. Left unattended engine damage can result. For more information see our Detailed Auto Topics Valve clatter, spark knock, pinging and pre-ignition. (see also spark knock, pinging and labor knock)

Dexos
The name of an oil specification license, used by General Motors after 2011. Oil producers are required to pay a fee to General Motors in order to display the dexos logo on their product. The charge is passed on to customers and required even if the oil exceeds dexos specifications.

Dexron
A type of automatic transmission fluid, widely used by General Motors and several other manufacturers, until recent years. Often mispronounced as dex-tron, there is no “T” in the name. Actual pronunciation is dex ron. (see also Mercon and ATF)

Direct injection
A very high pressure device, which applies fuel into the combustion chamber, rather than at the intake port, as with the conventional design. For more information see our Detailed Auto Topics Direct injection, yes or no. (see also fuel injector)

Dog tracking
When the front and rear wheels do not track in the same path causing the vehicle body to appear to travel in an angled path to forward motion. Often the result of mis-aligned suspension, body or frame components.

DOHC
An acronym standing for double overhead camshaft. An engine that uses two camshafts per cylinder bank or group, normally one for intake and one for exhaust. With an over head cam engine, the camshafts or in the cylinder heads, normally on top of the valves, as opposed to being in the engine, as with a push rod engine.

DOT
An acronym meaning Department Of Transportation. A governmental body that regulates some safety aspects of vehicles.

Dragster
A custom built vehicle especially for drag racing. Occasionally, a term used to describe people who drag race vehicles.

DTC
An acronym meaning Diagnostic Trouble Code. A combination of numbers and letters, stored by the vehicle’s computers to give an indication of the circuit in which a fault may be exist. See our section on Engine Diagnostics and our Vehicle Questions Category for What do OBDII codes actually mean?

Dynamic balance
Balancing a wheel in both dimensions, side to side and from one face to the other. Because wheels have width, they need balance not only from side to side but also from one side of the width to the other. Dynamic imbalance normally manifest as a wobble from side to side, at higher speeds. Please see our Detailed Topic Wheel Balance, Shimmy and Vibration for more details. (see also wheel balance, static balance and wheel weight)

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E-10
A blend of ten-percent ethyl alcohol and 90% gasoline, for use as a motor fuel. Please see our Detailed Topic Ethanol, Good or Bad? for more details. (see also ethanol)

E-85
A blend of 85-percent ethyl alcohol and 15% gasoline, for use as a motor fuel. Please see our Detailed Topic Ethanol, Good or Bad? for more details. (see also ethanol)

ECM
An acronym meaning Electronic Control Module. ECM could describe any computer on the vehicle but is often used interchangeably with PCM or power control module. (see also PCM)

ECT
An acronym meaning Electronic Coolant Temperature, often used to describe the sensor employed to check it. The ECT sensor provides an electrical output that varies with temperature and informs the power control module of coolant temperature.

ECU
An acronym meaning Electronic Control Unit. ECU could describe any computer on the vehicle but is often used interchangeably with ECM and PCM or power control module. (see also PCM)

EGR
An acronym meaning Exhaust Gas Recycle. A system that cools the fuel air mixture in the engine, under certain conditions to help prevent pre-ignition. For more information see our Detailed Auto Topics How does and EGR valve work. (see also spark knock, detonation, pinging and labor knock)

Emergency brake
An outdated term to describe the vehicle parking brake. In early days before vehicle hydraulic brake systems employed redundant design, mechanical emergency brakes were used. In the case of brake failure they could be used to stop the vehicle. (see also parking brake)

Engine Flush
A service often marketed to clean the engine crankcase. Very seldom actually needed and usually no more effective than more frequent oil changes. (see also wallet flush)

EP
An acronym meaning Extreme Pressure. A lubricant that has been designed to withstand very heavy loading, often used in rear differentials. (see also viscosity)

EPDM
An acronym for Ethylene Propylene Diene Monomer, a very tough and flexible material, often used in construction of high-quality automotive belts and hoses. EPDM is much tougher than neoprene and requires a different method of checking for wear, on belts. For more information on EPDM belts, please see our Detailed Topic, Symptoms of A Bad Serpentine Belt and EPDM Belts.

ESC
An acronym meaning Electronic Skid Control or Electronic Stability Control. The vehicle is equipped with sensors that detect slide or slip. When detected the computer reduces engine power and combines with anti-lock braking to help maintain control.

ESIM
An acronym meaning evaporative system integrity monitor. A device that verifies the system containing fuel vapors is working as designed.

ET
An acronym standing for elapsed time. The amount of time, in seconds, that it take a drag racer to accelerate from a stop and cover a quarter mile distance. The lower the ET, the greater the velocity of acceleration.

Ethanol
An alcohol produced by fermenting and distilling organic materials, such as grains high in starch. Commonly used as an intoxicant or blended with gasoline as a motor fuel. Please see our Detailed Topic Ethanol, Good or Bad? for more details. (see also E-10 and E-85)

Evaporator
A heat exchanger, normally located in the passenger compartment. Used to remove heat from the passenger area, by absorption, using a low pressure gaseous refrigerant. For more information, please see our Detailed Topic, How Automotive Air Conditioners Work.

Expansion valve
A mechanical device used to regulate the flow of liquid refrigerant into the evaporator core, thus helping to regulate the temperature of the core. (see also orifice tube and evaporator core)

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Fan clutch
A device installed on mechanical fans, between the fan blade and the drive pulley, that allows the output speed to be varied, based on temperature. Fan clutches allow a large fan to slip when not needed, saving noise and power and tighten up when needed, to boost air flow.

typical fan clutch and pulley



Fine tread
A designation of the number of threads per inch or number of millimeters between treads of a fastener. In inch style fasteners, relating to the older National Fine Standard. Fine thread fasteners have more threads per inch for a given size than fine thread. In metric fasteners, there are less millimeters between threads than the same size fastener with fine thread. Please see our Detailed Topic A Guide to Automotive Bolts and Tightening, Pt.1 for more details.

Flash update
An update to or replacement of computer software on the vehicle’s computer(s) performed by a repair shop. Vehicles that support flash updates can be reprogrammed, using the proper type of scan tool and without replacement of parts. Replacement computers must also be flashed after installation.

Flat head
An engine design, where the valves are in the engine block, rather than in the cylinder head. The most popular rendition of a flat head design was the Ford flat head V-8 also known as the Ford V8.

Flat rate
A method of billing, where a predetermined amount of time is charged rather the actual time a repair takes to complete. The time billed is often taken from a guide, known as a flat-rate guide. For more information, please read our Detailed Topic Flat Rate Automotive Pricing.

Flat rate tech
A technician paid a predetermined amount of time for a job, rather the actual time it takes to complete. The time paid is often taken from a guide, known as a flat-rate guide. Express your opinion on flat rate pay methods.

Four-cycle
The descriptive name for the most prevalent type of automotive internal combustion engine. This engine has four stages or strokes. A complete sequence requires two revolutions of the crankshaft or four strokes.

Starting with the piston at the top of the cylinder, the first cycle is the intake. The intake valve opens, and air is drawn in by the piston moving down the cylinder. Cycle two is the compression stroke where the piston compresses the fuel and air, by rising in the cylinder. Third is the power stroke, where the piston is pushed down the cylinder by the fuel and air explosion. Finally, the exhaust valve opens, and the waste is expelled by the piston traveling up the cylinder.

Frame
A vehicle construction technique where the body is bolted to a separate frame. The frame provides the rigidity and an attachment points for suspension components, similar to the skeleton of the human body. Very few cars but several trucks and SUV’s use frame type construction. (see also uni-body)

Frameenstein
A derogatory term for a frame repair person that causes additional damage by improper, often short-cut methods, rather than performing a proper repair.

Free
A marketing term, used to deceive people who do not look long-term, into thinking they can get something for nothing. The notion of free is contrary to the common law of business sense, which recognize that someone pays for everything. For any group to get something for nothing, requires that others pay more.

Fuel injector
An electro-magnetic solenoid, which operates a valve, that controls the amount of fuel that enters the engine. Flow is determined by the size of the injector orifice. The amount of flow is regulated by how long it is kept open and the pressure at which it operates. For more information see our Detailed Auto Topics Fuel injectors and Wallet Flushing. (see also direct injection)

FWD
An acronym meaning Front Wheel Drive. An arrangement where the front wheels are linked to the transmission and are used to propel the vehicle. (see also RWD)

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Gap insurance
A policy that pays the difference between what is still owed on a lease vehicle and the actual cash value, in the event of a total loss. Due to fees and depreciations the person leasing a vehicle may owe significantly more than the vehicle is worth, particularly in the early years. If the vehicle is declared a total, liability or collision insurance is obligated to pay the actual cash value, creating a gap for the person with the lease. For more information, please see our Detailed Topic, Problems With Automobile Leasing.

Gas filled shock absorber
A suspension damper that uses a gas, such as nitrogen, to hold pressure on the hydraulic fluid. This helps prevent aeration of the fluid in severe use and may improve performance of the shock absorber. For more information, please see our Detailed Topic, How to Know When You Need Shocks or Struts. (see also shock absorber)

GDI
An acronym meaning gasoline direct injection. With GDI the fuel is injected into the combustion chamber as opposed to the intake port, as with conventional injection. For more information see our Detailed Auto Topics Direct injection, yes or no. (see also fuel injector)

Gear head
Slang for a person that is very interested in mechanical operation of devices, especially cars and trucks.

Gear ratio
The numerical relationship of a driven gear to the drive gear, relating to the number of times each rotates when turned. With a 3.50 ratio rear differential, the pinion gear rotates 3.5 times, for each single revolution of the ring gear.

Gov-lock differential
A locking type rear differential that uses centrifugal force to move a counter-weight and lock the rear wheel together under wheel slip conditions. Widely used in trucks, by General Motors, starting around 1973. (see also limited slip and posi-traction)

Governor
A mechanical device that helps to control something else, normally based on RPM. A governor is used on older automatic transmissions to control shift points. Later units use electronic sensors to accomplish this function.

Gray market
A vehicle or part imported through other than the official distribution network. Such vehicles may not meet safety, emission or lighting standards of the country into which they are imported.

Grease plug

Grease plugs are used to block the lubrication opening in components. The plug may be removed, allowing a zerk fitting to be inserted for lubrication as needed. The plug protect the component and seals it while service is not needed.

A grease fitting plug on a tie rod

H
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Half shaft
A part of an axle assembly, normally containing the outer constant velocity (CV) joint and the axle shaft. A complete axle assembly would contain a half shaft and the inner flexible joint as well. (see also CV joint)

Hardtop
A vehicle that does not have a center post between the driver side window and the rear window or quarter glass. When both side window are down the side of the vehicle is open. The same model, with the center post is referred to as a sedan or coupe.

Harmonic balancer
A mass of weight, attached to the front of the crankshaft and used to dampen the resonance that results from twisting and releasing in the shaft, during the four cycles of engine operation. The harmonic balancer normally has a center hub, bolted and pressed to the crankshaft snout and a heavy outer ring, mounted with rubber. Torsional twisting and spring back in the shaft are absorbed by the balancer. Please also see our Detailed Topic The importance of the harmonic balancer for more information

Head gasket
Material used to seal, coolant, oil and compression in the area where the cylinder head(s) attach to the engine block. For more information see our Detailed Auto Topics Symptoms of a blown head gasket.

Head pressure
The pressure of refrigerant in an air conditioner, between the outlet of the compressor and orifice tube or expansion valve. Also referred to as high-side pressure. Please also see our Detailed Topic Air conditioner problems, part II for more information.

High side
The part of an air conditioner system, between the outlet of the compressor and orifice tube or expansion valve. The name refers to high pressure generated by the compressor, in contrast to the suction side. Also called the discharge side. Please also see our Detailed Topic Air conditioner problems, part II for more information.

Hot tank
A metal tank, filled with a heated caustic solution and used to clean cast iron and other ferrous materials. The hot tank is effective at removing grease, dirt and some rust. It is not appropriate for aluminum or soft metals and will cause damage to them. For this and for environmental reasons, the use of hot tubs has diminished greatly in recent years. (see also jet wash)

HSLA
An acronym meaning high strength low allow, referring to a type of steel used in automobile construction.

HSS
An acronym meaning high strength steel, referring to a type of steel used in automobile construction.

Hydroplane
A loss of traction, between the tires of the vehicle and the road surface, due to a thin layer of water coming between them. The surface tension of water on the road exceeds the weight applied by the tires. Hydroplaning is extremely hazardous as the driver has almost no control of the vehicle.

Hygroscopic
The tendency of a material to readily absorb moisture. Brake fluid is made of alcohol and is hygroscopic, which is why it becomes contaminated over time. Please also see our Detailed Topic Brake fluid and proper brake service for more information.

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IAT
An acronym meaning intake air temperature. Also used to refer to the sensor that reads the intake air temperature. The power control module monitors the temperature of the air to help with adjusting fuel-air mixture. It is also assumed to be near ambient air temperature and compare with the coolant temperature sensor to determine warm-up time.

Idle speed
The speed of an engine, before the throttle is opened. and without a load applied. Idle speed is expressed in revolutions per minute or RPM and is specified by the vehicle manufacturer.

Idler pulley
A pulley used to support, guide or hold a belt tight but that does not drive any accessory. Idler pulleys are often added to provide additional stabilization on longer belts. For more information on belts, please see our Detailed Topic, Symptoms of A Bad Serpentine Belt and EPDM Belts.

A serpentine belt idler pulley



Included angle
The angle formed by a combination of camber and steering axis inclination (SAI). (see also camber and SAI)

Independent repair shop
An organization designed to service vehicles and owned independently, as opposed to a franchise or mass-merchandiser.

Injector flush
The practice of cleaning fuel injectors with a chemical and some sort of pressurized device. Often sold as necessary or as maintenance but only beneficial when there is an actual problem. (see also wallet flushes)

Inner tie rod
The flexible pivot that connects the steering mechanism to the outer tie rod, usually by a threaded connection. Steering-box type systems usually use a tie rod similar to the outer connecting each with a threaded sleeve. Rack and pinion systems normally use a rod as below. Threaded to the rack on one end and with threads to provide adjustment and attachment to the outer tie rod on the other end. (see also outer tie rod and rack and pinion)

Typical inner tie rod design



Intake manifold
On an injected engine, the device that connects and seals the throttle body and cylinder head intake ports, used to transfer only metered air into the engine and keep foreign materials out. With a carburetor, the intake transfers fuel and air mixture. Intake manifolds on modern vehicles are often made of plastic and are prone to leakage after a period of time. This can produce a rough idle, particularly when the engine is cold. For more information, please see our Detailed Topic, GM V8 Rough Idle When Cold.

Interference engine
An engine design where the valves and pistons occupy the same area, at different times. This improves performance, but has the risk of engine damage when the timing device, such as the chain or belt fails. Please see our List of popular Asian and Domestic interference engines for more details.

Interference fit
One component retained by the slightly smaller internal dimensions of another. This is often the case where a shaft of slightly greater diameter is pressed into a bearing. Force is applied to overcome the size differential and friction holds the parts in place.
An interference or press fit

IPC
An acronym meaning Instrument Panel Cluster. The computer that operates the dash instruments and often the security system and overall communications between computers on the system.

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J-bolt
A fastener with threads on one end, a long shank and a hook on the non-threaded end. Often used to secure batteries, where the J portion hooks into a hole in the lower pan, and the threaded portion secures a top mount.

Typical battery hold down J-bolt



Jack
A mechanical device that uses levers, screws or hydraulics to multiply effort and lift a vehicle with a minimum of force.

Jack stand
A device designed to lock and support the weight of a vehicle at variable heights, once it has been lifted with a jack. Jack stands normally lock by means of a rachet and pawl, a pin inserted through holes or a treaded support with an adjustable collar. They are also referred to as safety stands.

Three types of jack stands in common use



Jalopy
A slang term for an old vehicle, particularly one in disrepair

Jet wash
A cabinet fitted with high pressure nozzles and a rotating floor, used to clean automotive components. Hot soapy water is blasted against the components as they rotate inside the cabinet. The jet wash is very effective for cleaning aluminum, iron and plastic components. The cleaning solution may be recycled, is non-toxic, and the jet wash is considered environmentally friendlier than other cleaning methods. (see also hot tank)

JIS
An acronym meaning Japanese Industrial Standard. A cross-drive type screw driver, that resembles a Phillips, but is not designed to cam out or slip when torque is applied.

Jounce
Downward travel of the suspension; pushing the vehicle down in the front or rear to settle the suspension, as in alignment. For more information, please see our Detailed Topic, How to Know When You Need Shocks or Struts. (see also rebound)

Jounce bumper
Flexible devices, often rubber, located at points on the suspension where contact might otherwise occur between components that move on jounce and rebound. Flexible cushions that slip over the shaft of McPherson struts and dampen bottoming. (see also jounce and rebound)

Jump timing
A malfunction where a timing chain, belt or sprocket breaks or slips and the camshaft is no longer properly synchronized with the crankshaft.

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KAM
An acronym meaning Keep Alive Memory. Battery powered backup memory used by the various control modules on a vehicle.

Keyless entry system
Systems designed to unlock vehicles without the use of a key. Often a remote transmitter with a push button, but numeric key pads have also been employed. Very often linked to the onboard security system and can be used to activate personalize radio and seat position settings as well.

Killed
When an engine unexpectedly quits running and the ignition is still on. (i.e., The engine killed)

Kink
A type or degree of bend in a frame resulting in a fold of metal with a radius of 3MM or less. Generally accepted as the point at which a frame can no longer be structurally repaired.

Knock off
A cheap, usually substandard auto part, packaged to appear as a name brand product. A counterfeit part, illegally packaged and sold through unauthorized sources. Many brand name parts makers also sell second line knock off parts, in packaging similar to their name brand. Many times the part number may be the only difference apparent to the client.

Knock sensor
A device that detects the knock or pinging from engine detonation and informs the power control module. The engine computer retards ignition timing to help prevent detonation, which might damage the engine. For more information, please see our Detailed Topic, Valve Clatter, Spark Knock, Pinging and Pre-ignition.

Knuckle
On a suspension, the component between the McPherson strut or upper control arm and the lower ball joint, that contains the wheel bearing. Also sometimes referred to as a spindle.

Typical front wheel drive steering knuckle



KOEO
An acronym meaning Key On Engine Off. One state in which pre-OBDII vehicles could be tested. (see also OBDII and KOER)

KOER
An acronym meaning Key On Engine Run. One state in which pre-OBDII vehicles could be tested. (see also OBDII and KOEO)

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Labor knock
A rattling, pinging type noise, normally experienced on acceleration and under load. Left unattended engine damage can result. For more information see our Detailed Auto Topics Valve clatter, spark knock, pinging and pre-ignition. (see also spark knock, detonation and pinging)

Leading trailing brake
A drum brake system with the shoes anchored at the bottom, to prevent rotation and force multiplication as occurs with a duo servo system. For more information on drum brakes, please see our Detailed Topic Solving drum brake problems.

Leak-down test
A procedure for testing an engine, by injecting air into the cylinder and monitoring the amount and place where it leaks out. Leak-down tests are handy for finding leaking gaskets, bad piston rings and valves that do not seat properly.

Lean
A condition where the amount air to fuel is more than the ideal 14.7 to 1. Too much air or too little fuel in the ratio. Can result in loss of power, high cylinder head temperature and misfires.

Lease
A method of obtaining a vehicle, where equity in the vehicle is traded for a lower monthly payment. A lease is similar to rental, except the person leasing is generally committed to the term and conditions of the lease. Please see our Detailed Topic, Problems With Automobile Leasing for additional information.

Left side
With the front of the vehicle represented by zero on a scale of 360 degrees, the portion from 180 degrees to 360 degrees is the left side. On US vehicles this is also the driver side.

Left side of a vehicle

Limited slip
A type of differential that attempts to apply equal force to both drive axles. This is usually accomplished by linking them together through a series of clutches and drive plates that only slip when necessary. (see Posi-traction)

Limp mode
A pre-programmed set of running instructions, in a vehicle computer, that allows the vehicle to operate at a minimum level, even when certain inputs are loss or not present. This can allow the driver to ‘limp’ home, in a situation where they may otherwise be stranded.

Liter
The metric equivalent of cubic inches or a measurement of volume, as in the volume of the cylinders of an engine. Engine volume is the size of a cylinder, with the piston at the lowest possible position, times the number of cylinders the engine has. For a calculator to convert liters to cubic inches, please see our Cost Savings Calculators.

Load rating
The amount of weight, a properly inflated tire can carry. This number is on the sidewall of the tire and expressed in kilograms and pounds. (see also Ply rating and Speed rating)

Tire sidewall showing the load rating



Load test
An electrical procedure that places the object tested under an electrical load to test it in an operating state. Load test are often used with batteries to test their capacity, or with starters to determine their efficiency. A load test can find problems that do not show up in static testing of the same components.

Lock-up (torque converter)
A clutch in the torque converter that “locks” the input to the output at a specified time. This prevents further slippage of the torque converter and improves fuel mileage. Lock-up clutches are normally controlled by the computer through solenoids and hydraulic pressure. (see also Torque converter)

Low side
The part of an air conditioner system, between the inlet of the compressor and orifice tube or expansion valve. The name refers to low pressure generated by the compressor, in contrast to the higher pressure of the discharge or high side. Also called the suction side of the system. Please also see our Detailed Topic Air conditioner problems, part II for more information.

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M/S tire
Stands for mud and snow, a tire that can be used in mud and snow as well as on the highway.

MAF sensor
An acronym meaning mass air flow. The MAF sensor measures the amount of air flowing into the intake manifold of an engine. Air flow is used to help determine the amount of fuel to be added, to achieve the proper mixture.

Maintenance
Service performed and parts replaced in the hope of preventing other, more expensive parts from failing. For instance coolant is replaced in the hope of preventing corrosion damage to radiators, heater cores and engines. The cost of the coolant is a fraction of the cost of the repairs it can prevent. Read more on vehicle maintenance.

MAP sensor
An acronym meaning manifold absolute pressure. The MAP sensor measures the vacuum present in the intake manifold of an engine. Vacuum is used to help determine the status of the engine (e.g.; high vacuum indicates idle or deceleration.) MAP sensors may also be used to help detect flow of an EGR valve

McPherson strut
A suspension component that contains a shock absorber but also supports the weight of the vehicle through a spring or torsion bar. For more information, please see our Detailed Topic, How to Know When You Need Shocks or Struts. (see also shock absorber)

Menu pricing
A method of assigning a price to a specific service decided on in advance. A name is assigned to the service with no regard to whether it is needed or will address a problem. Menu prices are often set at a level geared to get the client in the shop and never intended as what will be charged. (see also spiking the job, brake job and tune up) and how much is a brake job and how much is a transmission.

Mercon
A brand name for a type of automatic transmission fluid, widely used by Ford Motor Company. The name is also used, with a suffix, for several other specialty automatic transmission fluids, such as Mercon Premium, Mercon LV, Mercon V, etc. (see also Mercon V, Mercon SP, Dexron and ATF)

Mercon SP
A brand name for a type of specialized synthetic automatic transmission fluid, with additional additives, and used by Ford Motor Company in certain transmissions. Mercon SP may not be replaced with Mercon V, which would not meet the same specifications. (see also Mercon V, Dexron and ATF)

Mercon V
A brand name for a type of synthetic automatic transmission fluid, with special additives and widely used by Ford Motor Company. Mercon V may not be replaced with Mercon, which would not meet the same specifications. (see also Mercon, Dexron and ATF)

Metal on metal
The characteristic grinding noise made when the friction material wears off of brake shoes or pads and the metal backing scores the rotor or drum surface.

MIL
An acronym meaning Malfunction Indicator Light. An indicator to the driver that a fault has occurred in the computer management system See our section on Engine Diagnostics. (see also Check Engine Light)

Misfire
A malfunction in the combustion of the fuel-air mixture of an internal-combustion engine. A misfire occurs when any of the needed components, such as compression, fuel, spark or timing is incorrect or not present. For more information see our Detailed Auto Topics Diagnosing cylinder misfires, part I and II.

Multi viscosity
In oil, the ability to change the resistance to flow over a range of temperatures. For example 5W30 is the SAE designation for an oil that can flow like a five-weight oil at low temperature (0 degrees Celsius) and like a thirty-weight oil at high temperature (100 degrees Celsius.) (see also viscosity)

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NEFR
An acronym meaning Not Economically Feasible to Repair. A vehicle that has deteriorated to a state where repair would far exceed the value of the vehicle. A condition often brought on by following extended maintenance intervals. (see also our Detailed Topic entitled The Sad Truth)

Negative
The smaller of the two tapered battery post on top mount batteries. Normally attached to ground and color coded black. (see also positive)

Neutral safety switch
An electrical device to prevent a vehicle from cranking, unless the gear selector is in neutral or park position. More commonly referred to as a manual lever position sensor on modern vehicles.

NHTSA
An acronym meaning National Highway Traffic Safety Administration. A division of the Department of transportation that regulates automotive recalls and safety issues. See our Links Category for current recalls and more information.

NIST
An acronym meaning National Institute for Standards and Technology. A national organization concerned with setting standards for automotive air-conditioning systems.

Non-repairable
The state of a vehicle or automotive component, where repair is not feasible, due to economic, reliability or safety issues. Non-repairable does not mean someone cannot repair it. Non-repairable means replacement offers an overall lower cost. (see also NEFR)

Non-socialized pricing
Billing for the actual time spent repairing a vehicle, rather than a flat rate. With non-socialized pricing, people who maintain their vehicles get a lower price than a non-maintained vehicle, which takes more time to repair. (see also flat rate pricing and socialized pricing)

Normal service
As defined by most manufacturers, should more properly be called ideal service. For example longer trips at highway speed with little stop and go driving. Under these conditions the normal service schedule is okay. (see also severe service and maintenance) Read more on vehicle maintenance.

NOS
An acronym meaning new old stock. An unused part, in new condition and that was produced by the original manufacturer, but is no longer supplied, because it is outdated. NOS parts are highly sought after by restorers of vintage vehicles. They represent the original quality and do not detract from the value of the vehicle, as a reproduction part would.

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OBD II
An acronym meaning On Board Diagnostics II. This is a diagnostic standardization of fault codes and connections to the vehicle diagnostic system. Implementation began around 1994 and was implemented fully by 1996. (see also our Vehicle Questions Category for What do OBDII codes actually mean?)

Octane
The word octane is often interchanged with octane rating. In reality octane is a hydrocarbon and octane rating is a point of pressure at which gasoline will spontaneously combust. This is important in an engine, because when fuel combust before the ignition ignites it, damage may occur. For more information see our Detailed Auto Topics Valve clatter, spark knock, pinging and pre-ignition of. (see also octane rating and posted octane rating)

Octane rating
Octane rating refers to the ability of fuel to resist spontaneous combustion (pre-ignition.) Iso-octane has a rating of 100 (great resistance to spontaneous combustion) and n-heptane has a rating of 0 (little resistance to spontaneous combustion. Basically, fuel with a rating of 87 octane would have at minimum, the same spontaneous combustion point as a mixture of 87 percent iso-octane and 13 percent n-heptane. (see also octane and posted octane rating) For more information on octane ratings, please see our Detailed Topic is high octane gas really needed.

OEM
An acronym meaning Original Equipment Manufacturer. For instance a part that is the same manufacturer as the original part that came on the vehicle may be called an OEM part. See also aftermarket part

OHV
An acronym standing for overhead valve. An engine with the camshaft beneath the cylinder head, normally in the engine block. The overhead valve engine uses a push rod to transfer motion from the camshaft to the valves.

Orifice tube
A small fixed tube used to regulate the flow of liquid refrigerant through the evaporator core, thus helping to regulate the temperature of the core. For more information, please see our Detailed Topic, How Automotive Air Conditioners Work. (see also evaporator core and expansion valve)

Outer tie rod
The flexible pivot that connects the steering arm to the remainder of the steering mechanism. Usually tapered and threaded at the connection with the steering arm and threaded to provide adjustment on the other end. Steering-box type systems usually use a similar rod on the inner side connecting each with a threaded sleeve. (see also inner tie rod)

Typical outer tie rod design



Overall lowest cost
A philosophy of looking at the entire amount an item or service cost, rather than only the price. Price is the initial amount paid for an item or service. Cost by comparison, includes the price paid, plus other expenses involved, divided by how long the item or service last. Buying on overall lowest cost saves substantial money over time.

Oxygen sensor
A device used to determine the amount of oxygen in the exhaust after combustion. This information is used by the power control module to help determine if the amount of fuel being added is correct for idle, cruise and moderate acceleration. Oxygen sensors are also used to check the efficiency of the catalytic converter, by comparing the exhaust, before and after passing through.

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Parking brake
A series of mechanical devices that use a pedal or lever and cables to apply front or rear brakes. So called because they are intended to hold the vehicle in place when parked. (see also emergency brake)

PCM
An acronym meaning Power Control Module. The power control module is a computer that operates the engine and sometimes transmission of the vehicle. (see also ECM)

Phaser
A variable camshaft sprocket, used to change the valve timing while the engine is running. Phasers are normally operated by oil pressure and controlled by the power control module. Solenoids may vary oil pressure supplied to the phaser and enable adjustment to camshaft timing.

Phillips Screwdriver
A tool designed to install cross-drive screws and cam out or slip before they are overtightened. Named for the man that brought them to market, rather than the inventer. Please see our Detailed Topic Stripping Phillips Screws for much more information. (see also JIS.).

Pinging
A rattling, pinging type noise, normally experienced on acceleration and under load. Left unattended engine damage can result. For more information see our Detailed Auto Topics Valve clatter, spark knock, pinging and pre-ignition. (see also spark knock, detonation and labor knock)

Plug
Short for spark plug. A replaceable component in the ignition system that provides the spark necessary for combustion.

Ply rating
An alpha designation, related to the equivalent strength of the plies in the tire. Ply rating is not necessarily the same as the actual number of plies present. (see also Load rating and Speed rating)

The ply rating designation of LT tires



Posi-traction
A brand name for a type of limited slip differential that used clutches and springs to assist rear traction. The name is outdated today but is still often used to describe any limited slip differential. (see limited slip)

Positive
The larger of the two tapered battery post on top mount batteries. Normally marked with a plus sign and color coded red. See also negative

Posted octane rating
US octane ratings are assigned based on a combination of two testing methods. The results of the Research method and the Motor method are averaged for a posted octane rating. On the label below, R stands for research and M for motor methods. See also octane and octane rating For more information on octane ratings, please see our Detailed Topic is high octane gas really needed.

Typical octane rating markers



Price
The initial amount paid for a product or service without regard to other expenses involved or how long the product or service last. See also The Difference In Price and Cost in our AGCO Philosophy Category. (see also Cost)

Program car
A marketing term for a late model, low mileage used vehicle that is still under factory warranty. These may come from all the same places that any used vehicles come from (e.g., auctions, trade ins, repossessions, etc.) They should be viewed with the exact same cautions as any other used vehicle.

PSI
An acronym meaning Pounds per Square Inch; a measure of pressure. Also expressed as PSIA meaning pounds per square inch absolute. PSI includes atmospheric pressure which is approximately 14.7 PSI at sea level. NOT the measure of tire pressure. (see also PSIG)

PSIG
An acronym meaning Pounds per Square Inch Gauge; a measure of pressure that a gauge is likely to read. It does not include atmospheric pressure which is approximately 14.7 PSI at sea level. The pressure in a tire is expressed as PSIG. (see also PSI)

Push rod
A tubular rod that connects and transfers motion of the valve lifter, to the engine valves, usually through a rocker arm. The term is also used to refer to an engine that does not have an overhead camshaft and uses push rods to activate the valves.

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Quad-driver
A portion of the power control module (PCM) that receive inputs are sends outputs to other sensors and devices. Similar to the drivers on a home or office computer that control the printer and disk drives. Each quad-driver controls four functions.

Quadricycle
A vintage term for a very early vehicle with four wheels. Henry Ford’s 1896 prototype is referred to as a quadricycle.

Quality
The characteristic of a product or service which makes it the greatest value among the clients choices. Quality is always least expensive, when considered over time. Quality services provide the over all lowest cost.

Quarter bumper
A bumper style, primarily used on early model sports cars, that only protected the corners of the vehicle, leaving the center open. This style was made obsolete by US and other bumper standards.

A rear quarter bumper



Quarter panel
The rear fender of a vehicle, normally welded in place and extending from the rear of the door opening to the rear of the vehicle and attaching to the roof.

A rear quarter panel



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R12
The name of an air conditioning refrigerant also often referred to as Freon. Largely discontinued from automotive use by 1993.

R134A
An automotive air conditioning refrigerant brought into use after the discontinuation of R12.

Rack and pinion steering
A device that attaches the steering wheel to a small gear, known as the pinion. When the gear is rotated by turning the steering wheel, the pinion drives a long gear known as the rack. The rack moves at a 90 degree angle to the steering column. The ratio of the two gears multiplies the steering effort and redirects motion in the proper direction to turn the front wheels in the direction desired.
Basic rack and pinion principle

Rebound
Upward travel of the suspension; suspension returning to normal position after jounce. For more information, please see our Detailed Topic, How to Know When You Need Shocks or Struts. (see also jounce)

Recall
A mandatary correction of a defect in a vehicle, ordered by the National Highway Traffic Safety Administration (NHTSA.) Recalls are for defects that may affect public safety. For information on recalls look in our Links Category.

Refrigerant
A gaseous compound used in the air conditioner system, normally R134A or in older vehicles R12. For more information, please see our Detailed Topic, How Automotive Air Conditioners Work.

Remanufactured
A marketing term used to describe a rebuilt product.

Rich
A condition where the amount of air to fuel is less than the ideal 14.7 to 1. Too much fuel or too little air and can result in fouled spark plugs, poor fuel mileage and dark smoke from the exhaust.

Right side
With the front of the vehicle represented by zero on a scale of 360 degrees, the portion from 0 degrees to 180 degrees is the right side. On US vehicles this is also the passenger side.

Left side of a vehicle

Rotation
Moving front tires to the rear of the vehicle in an attempt to even out the wearing process. Often the left-front is placed on the left-rear and the right-front on the right-rear. Tires may also be crossed to the rear in many cases (e.g., left-front to right-rear and right-front to left-rear. Rotation does not require balance, which is a totally different process. (see also wheel balance)

RPM
An acronym meaning revolutions per minute. The speed at which an object rotates, expressed as the number of times it turns in a minute.

Run out
An error in tolerance, of a rotating object, measuring from the axis of rotation in a given direction. Run out may be radial, such as with an out of round tire. Variation exist, measuring from the center of the wheel to points around the circumference. Lateral run out is movement at an angle to the rotating axis, such as a wobble. A warped brake rotor has lateral run out.

RWD
An acronym meaning Rear Wheel Drive. An arrangement where the rear wheels are linked to the transmission and are used to propel the vehicle. (see also FWD)

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SAE
An acronym meaning Society of Automotive Engineers. One of the organizations that helps set automotive standards such as oil ratings, used in the United States.

SAI
An acronym meaning Steering Axis Inclination. The angle formed by a line drawn through the upper and lower pivot points of the suspension and true vertical. (see also Included angle and camber)

Sedan
A passenger car having four doors and with or without a center post, between the front and rear doors.

Serpentine belt
A flat drive belt, with a series of small grooves, used to drive external accessories on an engine. Because of their great flexibility, they tend to curve around several components, much like the way a serpent is able to coil up. For more information on EPDM belts, please see our Detailed Topic, Symptoms of A Bad Serpentine Belt and EPDM Belts.

SES
An acronym meaning Service Engine Soon. An indicator to the driver that a fault has occurred in the computer management system See our section on Engine Diagnostics. (see also Check Engine Light)

Severe service
As defined by most manufacturers is often the service most folks operate under. For instance, ANY of the following would be considered severe service.
  1. Most daily trips are less than five miles one way.
  2. Most driving is stop and go.
  3. High temperatures, above 90' F.
  4. Towing
If ANY of these conditions describes your use, it is better to follow the severe service schedule. (see also normal service and maintenance) Read more on vehicle maintenance.

Shimmy
A condition where the steering wheel moves side to side when driving at certain speeds. Often a condition of improper wheel balance. (see also wheel balance, dynamic balance, static balance and wheel weight)

Shock absorber
A hydraulic device that resist motion in the suspension of an automobile. They are designed with specific rates of dampening and these rates may vary with the amount of force applied and differ on jounce and rebound. For more information, please see our Detailed Topic, How to Know When You Need Shocks or Struts. (see also jounce, rebound, gas filled shock absorber and McPherson strut)

Shockulla
A derogatory term for a salesperson who sells shock absorbers, struts and other high-profit items to increase their commission check, rather than because of client need.

Short-long arm
A type of independent suspension system, using two arms of different lengths, to attach the spindle to the support members of the vehicle. The short and long arms allow the spindle to remain more perpendicular to the road as the suspension travels through jounce and rebound.

Typical short long arm independent suspension



SIR
An acronym meaning Supplemental Inflatable Restraint. A indicator to the driver that a fault has occurred in the air bag management system See our section on Engine Diagnostics. (see also air bag)

Snap ring
A spring, shaped to fit into a groove and used as a retainer. Snap rings may be internal or external. An internal snap ring is compressed to enter a bore and when installed, snaps or returns to its diameter to fill a groove. An external snap ring is expanded to fit over a larger shaft and snaps into a groove machined to fit its diameter.

internal and external snap rings



Socialized pricing
The practice of averaging repair costs so everyone pays the same for a given service. With socialized pricing people with well-maintained vehicles pay more, subsidizing poorly maintained vehicles that are more difficult to repair. (see also flat rate pricing and non-socialized pricing)

SOHC
An acronym standing for single overhead camshaft. An engine that uses one camshafts per cylinder bank or group. The camshaft is at the top of the cylinder head with this design as opposed to in the engine block as with an engine that uses push rods.

Solenoid
In automotive terms a solenoid is an electromagnetic device used to convert electrical energy into physical movement. Widely used in a many applications from controlling the valves in an automatic transmission to moving the starter drive gear. An electrical coil is energized and draws a magnetic plunger in or out, depending on design. Movement of the plunger is used to open valves, move switches, make or break electrical connections.

cutaway of a simple magnetic solenoid

Spark knock
A rattling, pinging type noise, normally experienced on acceleration and under load. Left unattended engine damage can result. For more information see our Detailed Auto Topics Valve clatter, spark knock, pinging and pre-ignition. (see also detonation, pinging and labor knock)

Spark plug
A replaceable component in the ignition system, with electrodes that provides a gap, for the spark that creates combustion.

Speed rating
A testing method used to determine quality of a tire to resist centrifugal force. Tires are tested by spinning at increasing speed until they are destroyed. A heavier belted tire normally has a higher speed rating and is so noted on the tire sidewall with a letter designation. (see also Load rating and Ply rating)

The speed rating of tires

Please see our Detailed Topic Can I substitute a lower rated tire for more details.

Spiff
A reward or monetary sum paid to a technician or service advisor in order to influence the sale of a specific product or service. For instance ten dollars for each transmission flush. Express your opinion on flat rate pricing. (see also wallet flushes)

Spiking the job
I derogatory term, used to describe the practice of misleading a client, in order to get the vehicle in the shop and disable it. Often used with low-ball pricing. Once the vehicle is disabled, it is far less likely to be taken out of the shop, once the true price is revealed. (see also menu pricing)

Spray and pray
A derogatory term for cheap rebuilt parts, spray painted and sold, often with a ‘life-time’ warranty. The vendor prays the buyer will get tired of replacing them and go away after the sale.

Staggered wheels
Wheels of different diameter or width on one axle compared to the other. Most often they apply wider wheels and tires on the rear of a rear-wheel drive vehicle. On passenger vehicles, this is primarily for aesthetics. High performance vehicles use the wider rear tire to gain traction, keeping the front smaller for fit.

Starter
The electrically operated motor that causes the engine to rotate when the starter switch is activated.

Static balance
Balancing a wheel in a single plane only (e.g., like a bubble balancer from days gone by) Because wheels have width, they need balance not only from side to side but also from one side of the width to the other. Static imbalance normally manifest as a bouncing vibration. (see also dynamic balance, wheel balance and wheel weight)

Swaptronics
A derogatory term for an unskilled technician’s method of changing parts, especially electronic components hoping to solve a problem and rather than proper testing.

Synthetic oil
A name that refers to certain properties possessed by an oil. Contrary to common belief, it does not specify what the oil is made of and some oils sold as synthetic are actually highly refined petroleum products. Also see our section on Oil, Lubricants and Gasoline for more.

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TCM
An acronym meaning Transmission Control Module. Another name for the computer that controls the transmission. Sometimes transmission control is handled by the power control module, without a separate transmission computer see also PCM

Temporary battery end
A terminal that clamps onto the end of a battery cable, as opposed to a proper factory crimp style terminal. Often causing high resistance and damage to the electrical components of the vehicle. For more information, please see our Detailed Topic, Transient current flow.

Temporary battery cable end



Tensioner
A devise designed to adjust and keep a belt tightly in contact with the drive and driven pulleys on the engine. The tensioner normally has a bearing mounted pulley which presses on the belt to remove slack. Tensioners can be spring loaded, hydraulic or mechanical and are used with most automotive belts. Please see our Detailed Topic Serpentine Belt Tensioners for more details.

TIM
An acronym meaning Technician Induced Malfunction; A derogatory term for an unskilled technician’s who causes further damage while testing or in an attempt to repair.

Timing belt
A flexible drive mechanism, connecting the crankshaft of an engine to the camshafts and having teeth that prevent slipping, so the components remain synchronized to each other. On a four-cycle engine, the crankshaft rotates twice for each revolution of the camshafts and must remain in time to prevent engine damage. Please see our Detailed Topic All About Timing Belts for more details.

Top dead center
The point of fullest extension of a piston in a cylinder, where it is nearest the cylinder head. The furthest point in the compression stroke. top dead center of a piston

Top Tier Gasoline
Six top automakers, Audi, BMW, General Motors, Honda, Toyota and Volkswagen felt the EPA minimum detergent standards were too low. These automakers established the Top Tier standard to address the problem. In order to meet Top Tier, gasoline producers must meet several standards for reduced deposits and detergency in all of their grades. The list of Top Tier providers may serve as a guide to motorist. Some of the major retailers in our area that meet Top Tier include:
  • BP
  • Chevron
  • Conoco
  • Exxon
  • Phillips 66
  • Shell
  • Texaco
You may also inquire with other retailers to see if their products meet top tier.

Torque Converter
On automatic transmissions, the coupling device between the engine and transmission. A fluid-filled turbine that allow the engine to continue to run while the vehicle is stopped. As engine speed increases, torque is transmitted to the transmission. Torque converters serve much the same function as the clutch on a manual transmission.

Torque converter shudder
A vibration that occurs when the lock-up clutch, in the torque converter engages. The shudder normally last a few seconds and is often described as the sensation of running over a cattle guard. For more information see our Detailed Auto Topics What is a torque converter shudder.

Total loss
An insurance term describing a vehicle that is not feasible to repair, due to economic, reliability or safety issues. Damage that is severe enough to make replacement a more sensible alternative to repair. (see also NEFR and non-repairable)

TPMS
An acronym meaning Tire Pressure Monitoring System. A system that monitors tire air pressure and warns of a low tire. Mandated by 2008 but also used on several vehicles much earlier.

TPMS
An acronym meaning tire pressure monitoring system. A system that checks for air pressure inaccuracies in the tires of a vehicle.

TPS
An acronym meaning throttle-position sensor. Also used to refer to the sensor that reads the throttle position. The power control module monitors the position of the throttle to learn request for acceleration. Idle-speed and transmission shift points relate to throttle position

Transfer case
An apparatus that allows power to be transmitted from the transmission to the front and/or rear wheels. (see also FWD, RWD and 4WD)

Transmission flush
Using a machine to run transmission fluid through a transmission, in an attempt to clean it. The filter is not cleaned and sometimes a universal fluid is used, rather than that specified by the manufacturer. (see also wallet flushes) and the section on proper transmission service.

TSB
An acronym meaning Technical Service Bulletin. This is information, released by the manufacturer of a vehicle, subsequent to production. Often it points to an improvement or change to resolve a problem. It is NOT a recall and does not obligate the manufacturer beyond the normal vehicle warranty terms (see also recall)

Tune up
An obsolete and improper term. Often used to describe replacing certain ignition components, with no regard to whether they are needed or will address the client’s problem.

Type F
An automatic transmission fluid used by Ford Motor Company several years ago. Largely out of current use except in classic vehicles. (see also Dexron, Mercon, ATF)

U
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U-joint
An abbreviation for universal joint. A cross with four caps using roller bearings to allow the drive-shaft to operate at a limited range of angles. For more information on drive shafts, please see our Detailed Topic U-joints and drive shaft service.

Typical universal joint



U.S. built vehicle
According to the American Automobile Labeling Act (AALA) A vehicle assembled in the United States, using at least 75% components that are from the United States or Canada. (see also AALA)

Under inflated
A tires that lacks sufficient pressure to fully force the center of the tread into contact with the road and allows excessive flexing of the sidewall. Having less pressure than needed to support the weight that is being carried. An under inflated tire will wear on both sides of the tread, at a rate faster than the center. For more information, please see our Detailed Topic, How much air should I put in my tires.

Uni-body
A vehicle construction technique where structural body panels are welded together to support the vehicle with no separate frame used. Most cars built today are uni-body construction. (see also frame

Unidirectional tread
A tire tread pattern where the grooves follow a single direction. Theoretically better at dissipating water from under the tread while in motion. Care must be taken to insure the tire is mounted properly and most have an arrow showing proper rotation.

Example of a unidirectional tire tread pattern



Up-sell
Additional items or services sold to a client, beyond what they anticipated they needed. Often services which may not be needed, such as fuel injector cleaning, an engine flush or shocks and struts. (See also Wallet flush)

UTS
An acronym standing for unified thread standard and replacing the older United States Standard. A series of specification concerning inch size fasteners, similar to International Standards Organization or ISO with metric fasteners. Please see our Detailed Topic A Guide to Automotive Bolts and Tightening, Pt.1 for more details.

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Vacuum brake booster
A chamber with a diaphragm dividing the two halves. Vacuum is present on both sides until the brake is applied. On application, atmospheric pressure enters the pedal side, forcing the diaphragm towards the master cylinder. This applies the brakes and greatly reduces brake pedal effort.

Valve body
A component of the automatic transmission where valves, solenoids, check balls, passage ways and springs are used to control shifting and other functions of the transmission.

Valve clatter
A rattling, pinging type noise, normally experienced on acceleration and under load. Left unattended engine damage can result. For more information see our Detailed Auto Topics Valve clatter, spark knock, pinging and pre-ignition. (see also detonation, pinging and labor knock)

Variable-ratio power steering
A system that uses a solenoid to control the amount of boost the power steering receives, based on speed and steering input. At low speed the boost is raised for ease of steering. At speed the boost is reduce for firmer control.

VDM
An acronym standing for vehicle dynamics module. The microprocessor that controls the suspension on air-ride equipped Ford products.

VIN
An acronym meaning Vehicle Identification number. A unique 17 digit number that identifies the vehicle and gives specific information. For instance the first digit reveals the country of origin. The eighth digit represents the engine option. The tenth digit is the year model and the final seven digits are the serial number of the vehicle.

Viscosity
In oil, the resistance to flow at a given temperature and often expressed on a numeric scale. On the scale established by the Society of Automotive Engineers (SAE) the lower the number, the less resistance to flow. For instance an oil rated 10 oil flows more quickly than an oil rated 90, at the same temperature. There are also other scales such as the one established by International Standards Organization (ISO).

Voltage-drop test
A diagnostic procedure, measuring the voltage that flows around a circuit under load, to determine if excessive resistance is causing a problem in the circuit. For more details, please see our Detailed Topic, Voltage-drop Tests.

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Wallet flush
Any of a number of services, of questionable worth, often marketed by shops, merely to increase their profit. (see also fuel injection flush, engine flush and transmission flush)

Wheel balance
Placing wheel weights on the outside and inside of a wheel to allow for variations in the weight of the wheel/tire assembly. A properly balanced wheel will have only one weight per-side of the wheel and will not require re-balance under normal service, for the life of the tire. (see also rotation)

Wheel cylinder
A hydraulic cylinder, attached to the backing plate of drum brakes, that applies the brake shoes when the master cylinder actuates it.

Wheel offset
The distance from the mounting face of a wheel to the actual centerline of the wheel. A wheel with a mount face at the actual center would have zero offset. If the mounting surface is inboard (toward vehicle center) of the wheel centerline, the offset is negative. If the mounting surface in outboard (away from vehicle center) of the centerline, the offset is positive.

Negative and positive wheel offset

Wheel weight
Small pieces of metal (usually lead alloy) clipped or adhesively attached to a wheel in order to achieve balance of the wheel/tire assembly. Wheel weights come in a variety of shapes to fit different style wheels as well as several different weight amount. (see also wheel balance)

One of many styles of wheel weights



White box
A derogatory term for cheap aftermarket parts, often sold in a white box with no name. (see also aftermarket part and counterfeit part)

Window regulator
The device that converts rotary motion of the motor or window handle, into a vertical motion to raise or lower the window glass. Please see our Detailed Topic, Diagnosing Power Window Problems for more information.

X
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X-frame
A vehicle frame that has an X-member welded between the frame rails. (see also X-member)

X-member
An X-shaped member welded between the frame rails to strengthen a vehicle frame.

An X section type frame

XCM
An acronym standing for transfer case module and primarily used by Chrysler. The module that controls the servos, performs diagnosis and monitors the electrical part of the transfer case. (see also transfer case)

XL
An acronym standing for extra load, when applied to a tire designation. A slightly heavier tire design than the standard offering of in the same size. Often used with light trucks, vans and SUV type vehicles, to give improved durability and control. (see also Load rating and Ply rating)

Y
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Y-block
An overhead valve, V-8 engine, used by Ford Motor Company, to replace the flat-head V-8. The Y-block was used from 1954 and discontinued in cars in 1962 and trucks in 1964.

Y-pipe
A Y-shaped pipe that connect the two exhaust banks of a V-style engine into one single pipe. Often used to save money routing both banks through a single muffler and catalytic converter.

Yaw sensor
A sensor, used by the traction control system, to detect motion around the central vertical axis of the vehicle. Yaw changes the direction of travel to the left or right of the intended path.

Yaw motion in an automobile



Yield
The point at which a material is permanently changed from its present shape. With fasteners, torque-to-yield means tighten beyond the elastic state or point at which it will return to its pre-tightened shape. Please see our Detailed Topic A Guide to Automotive Bolts and Tightening, Pt.II for more details.

Yoke
The coupling connecting the front drive shaft universal joint to the rear of the transmission output shaft. The Yoke has splines and is designed to move in and out of the transmission as the suspension changes height. For more information on the yoke and on drive shafts, please see our Detailed Topic U-joints and drive shaft service.

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Zener diode
An electronic component that allows current to flow in one direction and block flow in the other, until a specific point. When the avalanche point or breakdown voltage is reached, the zener will allow flow in the previously blocked direction. This makes zeners well suited in voltage regulators for alternators.

Zerk fitting
The small fitting used to attach a hydraulic grease gun to a component in order to apply lubrication (e.g., a ball joint or tie rod.) Zerk fittings come in two primary types for automotive use. The most popular is the nipple type.

A grease zerk fitting on a tie rod

Where space is very limited, such as on some rotating parts, a flush mount zerk fitting may be used. These are most common on universal joints and shafts.

A flush mount zerk fitting on a universal joint

Zero camber
The condition where neither positive nor negative camber exist. A wheel that is perpendicular to the road surface or 90 degrees from a horizontal plane. Please see our Detailed Topic Wheel alignment, Caster and Camber for more details.

Zero toe
Where two wheels on the same axis, are parallel to each other and neither points out or in.