© 2012 Delmar, Cengage Learning Basic Electrical System Theory and Repairs Chapter 25

© 2012 Delmar, Cengage Learning

Basic Electrical System Theory and Repairs

Chapter 25


Objectives• Explain electrical principles• Describe various electrical terms• Understand and compare voltage, current, and

resistance• Use electrical meters and test instruments in a

safe and correct manner


Introduction• Almost every system of the car uses electricity

– Anti-lock brakes

– Engine emission control devices

– Dash warning lights and gauges

– Electronic fuel injection

– Electrically controlled transmissions

• Technicians in every area – Must understand electricity to be successful


Electron Flow• Matter: composed of atoms

– Atoms: composed of protons, neutrons, electrons• Electrons: negatively charged and orbit protons• Protons: positively charged• Neutrons: no charge• Protons and neutrons: located in the nucleus

– Number of protons and neutrons • Determines the element

• Atoms try to remain electrically neutral– Equal number of protons and neutrons



Electron Flow (cont'd.)• Electricity: flow of electrons from one atom to

another


Conductors and Insulators• To remain in balance an atom will shed or

attract electrons from neighboring atoms– Electrons flow between atoms to equalize charge

• Conductors: atoms with free electrons– Good conductors: silver, copper, and aluminum

• Insulators: few or no free electrons– Prevents flow of electrons between conductors

– Good insulators: glass, rubber, and porcelain


Basic Automotive Electrical System

• Electrical circuit– Complete circuit is needed to do work

• Requires a power source, conductor, and load

– Automobiles: ground is provided by the frame

– Electricity takes the path of least resistance• With enough voltage, electricity jumps air gaps

– Most wiring is insulated with polyvinyl chloride

• Control and protection devices– Switches, fuses, and circuit breakers




Electrical Terms• Voltage

– Electromotive force (EMF): force needed to push or pull an electron out of orbit

• Measured in volts using a voltmeter

• Current – Flow of electricity

– Number of electrons flowing per second • Measured in amperes


Electrical Terms (cont'd.)• Direct current

– Electrons flow in one direction

• Alternating current– Oscillation from positive to negative and back

• Cannot be stored in a battery

– Alternators make alternating current• Converted to DC before recharges the battery

• Resistance – Obstruction to electrical flow

• One ohm is the resistance that will allow one ampere to flow when pushed by one volt


Electrical Terms (cont'd.)• Changes in current flow

– Current flow inversely proportional to resistance

– Current draw is the amount of current used to operate a load

– Light, heat or motion energy result when resistance opposes the flow of current

– Resistors are used to make heat or control load intensity

• Variable resistors control speed and intensity of electrical load



Circuitry and Ohm’s Law• Series circuit: current flows equally through all

parts– Resistances of all loads add up

• Parallel circuit: starts from a common point and branches– Total resistance is less than the sum of individual

resistances

• Series-parallel circuits: combine two types • Ohm’s law: voltage, amperage, and resistance

relationship



Magnetic Fields• Magnets have polarity

– Like poles repel each other and unlike poles attract

• Electromagnetism– Magnetic field is created around outside of

conductor

• Electromagnetic induction: – Electricity is produced by moving magnetic field

over a conductor

• Relay: magnetically controlled switch



Capacitors• Store electricity

– Used to absorb voltage changes

• Connected in parallel in a DC circuit– In AC circuit electricity flows through capacitor as

part of the wiring

• Made of two pieces of foil separated by insulator– Ground side connected to one piece of foil

– Positive side connected to the other



Automotive Electronics• Electrical components: use mechanical parts

– Electronic systems: use solid state parts

• Semiconductor: acts as insulator and conductor– Common materials are silicon and germanium

• Diode: allows electricity to flow in only one direction

• Transistor: electronic relay– Resists electrical flow or allows a predetermined

amount of current to flow


Basic Electrical Tests• Meters: permanent magnetic (analog) or digital

– Analog meter has a needle moved by a magnet

– Digital volt-ohmmeters have only a voltmeter and ohmmeter

– Digital multimeters are popular today

• Voltmeter tests:– System voltage at the battery or alternator

• Voltage drop testing

– Difference in voltage between two points

– Excessive voltage drop due to resistance


Basic Electrical Tests (cont'd.)• Ammeter: measures amperage (current flow)

– Must be hooked in series with the load

– Amp draws are tested with system under load

• Current probe: available for low- and high-amps • Ohmmeter: measures resistance

– Must be calibrated• Hand calibrate by connecting its wires together to

read zero resistance

– Never connect an ohmmeter across an energized circuit


Basic Electrical Tests (cont'd.)• Circuit problems

– Open circuit: break in path of electrical flow

– Short circuit: electrical path has been shortened

– Grounded circuit: current flows directly to ground


Basic Electrical Tests (cont'd.)• Jumper wire: simple wire with alligator clips

– Finds open circuits• Never use a jumper lead smaller than the circuit• Never use to bypass a high resistance load• Only use to temporarily bypass a component

• Test lights: tests for open and short circuits when power is disconnected– Self-powered test light: if light glows the circuit or

part has continuity

• Power probe: combines features of test probes

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© 2012 Delmar, Cengage Learning Basic Electrical System Theory and Repairs Chapter 25