Basic Electrical System
Theory and Repairs
Chapter 25
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
Introduction
• Almost every system of the car uses electricity
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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
© 2012 Delmar, Cengage Learning
Electron Flow
• Matter: composed of atoms
– Atoms: composed of protons, neutrons, electrons
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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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
Electron Flow (cont'd.)
• Electricity: flow of electrons from one atom to
another
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning