1. Basic Electrical System Theory and Repairs
Chapter 25

2. 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

3. 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

4. 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

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

7. 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

8. 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

11. Electrical Terms Voltage Current
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

12. 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

13. 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

15. 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

17. Magnetic Fields Magnets have polarity Electromagnetism
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

19. Capacitors Store electricity Connected in parallel in a DC circuit
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

21. 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

22. 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

23. 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

24. 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

25. 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