AUTOMOTIVE TECHNICAL TERMS C
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CALCULATED LOAD VALUE
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.
CAMBER
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.
CAMBER ROLL
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.
CAMBER WEAR
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.
CAMBER BOLT
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.
CAMSHAFT
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.
CARBON DIOXIDE (CO2)
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.
CARBON MONOXIDE (CO)
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.
CARBURETOR
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. 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 derivability 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.
CARDAN JOINT
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).
CASTER
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.
CASTER SHIMS
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.
CATALYTIC CONVERTER
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.
CENTER BOLT
The bolt that maintains the alignment of the leaves in a leaf spring, and the position of the axle on the springs.
CENTERLINE
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.
CENTER LINK
The center bar or link in a parallelogram steering system that connects the pitman arm and idler arm. Also called a "relay rod."
CENTER OF GRAVITY
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.
CENTRIFUGAL ADVANCE
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.
CHASSIS
The frame or undercarriage of a vehicle. On unibody vehicles, the lower structure to which the suspension is attached.
CFC CERTIFICATION
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.
CHANGE OF STATE
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.
CHARCOAL CANISTER
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.
CHARGE
A specific amount of refrigerant or compressor oil by weight. This is specified by the vehicle manufacturer for individual A/C system applications.
CHARGING SYSTEM
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.
CHECK ENGINE LIGHT
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.
CHECK VALVE
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.
CHOKE
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.
CHLOROFLUOROCARBONS (CFCs)
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.
CIRCUIT BREAKER
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.
CLOSED LOOP
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.
CLUTCH
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.
COIL-ON-PLUG IGNITION (COP)
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.
COIL SPRINGS
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.
COMPLIANCE
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.
COMPRESSION
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.
COMPRESSION RATIO
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.
COMPRESSOR
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.
COMPRESSOR CYCLING SWITCH
See Thermostatic Switch.
COMPRESSOR CUTOFF 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.
COMPRESSOR OIL
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.
COMPUTERIZED ENGINE CONTROLS
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.
CONDENSATION
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.
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.
CONDUCTION
The transmission of energy (heat) through a medium without perceptible motion of that medium (direct contact).
CONSTANT VELOCITY (CV) JOINT
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.
CONTROLLER AREA NETWORK (CAN)
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.
CONTROL ARMS
Suspension components which connect the steering knuckles to the chassis or subframe, and allow the knuckles to move up and down.
CONVECTION
The transfer of heat by the circulation of a liquid or vapor
COOLANT
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.
COOLANT TEMPERATURE SENSOR
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.
COOLING SYSTEM
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.
CRADLE
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.
CRANKCASE EMISSIONS
See blowby, also Measuring Blowby
CRANKSHAFT
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.
CRANKSHAFT POSITION (CKP) SENSOR
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.
CROSS CAMBER
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.
CROSS CASTER
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.
CROSS COUNTS
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.
CROSSMEMBER
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.
CYCLING CLUTCH ORIFICE TUBE (CCOT)
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.
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.
CAMBER
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.
CAMBER ROLL
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.
CAMBER WEAR
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.
CAMBER BOLT
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.
CAMSHAFT
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.
CARBON DIOXIDE (CO2)
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.
CARBON MONOXIDE (CO)
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.
CARBURETOR
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. 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 derivability 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.
CARDAN JOINT
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).
CASTER
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.
CASTER SHIMS
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.
CATALYTIC CONVERTER
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.
CENTER BOLT
The bolt that maintains the alignment of the leaves in a leaf spring, and the position of the axle on the springs.
CENTERLINE
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.
CENTER LINK
The center bar or link in a parallelogram steering system that connects the pitman arm and idler arm. Also called a "relay rod."
CENTER OF GRAVITY
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.
CENTRIFUGAL ADVANCE
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.
CHASSIS
The frame or undercarriage of a vehicle. On unibody vehicles, the lower structure to which the suspension is attached.
CFC CERTIFICATION
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.
CHANGE OF STATE
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.
CHARCOAL CANISTER
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.
CHARGE
A specific amount of refrigerant or compressor oil by weight. This is specified by the vehicle manufacturer for individual A/C system applications.
CHARGING SYSTEM
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.
CHECK ENGINE LIGHT
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.
CHECK VALVE
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.
CHOKE
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.
CHLOROFLUOROCARBONS (CFCs)
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.
CIRCUIT BREAKER
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.
CLOSED LOOP
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.
CLUTCH
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.
COIL-ON-PLUG IGNITION (COP)
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.
COIL SPRINGS
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.
COMPLIANCE
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.
COMPRESSION
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.
COMPRESSION RATIO
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.
COMPRESSOR
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.
COMPRESSOR CYCLING SWITCH
See Thermostatic Switch.
COMPRESSOR CUTOFF 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.
COMPRESSOR OIL
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.
COMPUTERIZED ENGINE CONTROLS
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.
CONDENSATION
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.
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.
CONDUCTION
The transmission of energy (heat) through a medium without perceptible motion of that medium (direct contact).
CONSTANT VELOCITY (CV) JOINT
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.
CONTROLLER AREA NETWORK (CAN)
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.
CONTROL ARMS
Suspension components which connect the steering knuckles to the chassis or subframe, and allow the knuckles to move up and down.
CONVECTION
The transfer of heat by the circulation of a liquid or vapor
COOLANT
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.
COOLANT TEMPERATURE SENSOR
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.
COOLING SYSTEM
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.
CRADLE
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.
CRANKCASE EMISSIONS
See blowby, also Measuring Blowby
CRANKSHAFT
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.
CRANKSHAFT POSITION (CKP) SENSOR
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.
CROSS CAMBER
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.
CROSS CASTER
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.
CROSS COUNTS
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.
CROSSMEMBER
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.
CYCLING CLUTCH ORIFICE TUBE (CCOT)
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.
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