CHAPTER 4

CHAPTER 4

AUTOMOTIVE ELECTRICAL SYSTEMS AND CIRCUITSLEARNING OUTCOMESUnderstand the electrical systems approachUnderstand electrical wiring, terminals, and switchingUnderstand circuit diagram and symbolsUnderstand vehicle smart wiring systemElectrical Systems Approach System is a word used to describe a collection of related components, which interact as a whole. Modern vehicle is a very complex system. It is split into sub and sub-sub-systems on many level that aid both its design and construction. Vehicle systemMechanical Engine and transmissionElectricalConsidering the electronic control unit (ECU) of the system as the control element, and looking at its inputs and outputs, is the recommended approach.

Vehicle System

Vehicle electrical system is mainly classified into two types:Open loop

Closed loop

InputControlOutputInputControlOutputOpen Loop SystemAn open loop system is designed to give the required output whenever a given input is applied. No feedback is required for the open loop system to operate.As an example of an open loop system would be the electrical circuit of the vehicle headlight.

Close Loop SystemA closed loop system is identified by a feedback loop. It can be described as a system where there is possibility of applying corrective measures if the output is not quite what is desired.The feedback loop is due the fact that the output from the system, i.e., temperature is also an input to the system.The feedback from the system is very important in order to control the temperature of the cabin.As an example vehicle automatic temperature control system.

Electrical Wiring, Terminals, And Switching Cables ConstructionCables used for motor vehicle applications are almost always copper strands insulated with PVC. Copper, beside its very low resistivity of about 1.7 x10-8m, has ideal properties such as ductility and malleability.This makes it the natural choice for most electrical conductors PVC as the insulation is again ideal, as it not only has very high resistance, the order of 1015m, but is also very resistant to petrol, oil, water and other contaminants.

Colour Code and TerminalAs seems to be the case for any standardization, a number of colour code and terminal designation systems are in operation. Ford Motor Company now uses a circuit numbering and wire identification system. The system was developed to assist in vehicle development and production processes. However, it is also very useful to help the technician with fault-finding.

Ford Colour Code

British Standard Colour Codes

European Colour Codes

Terminal Designation

Harness DesignHarnesses are bundles of wires that are grouped together in plastic tubing, wrapped with tape or molded into a flat strip

Types of harnessPVC wound harnessPVC tube and tape harness

When deciding on the layout of a wiring loom within the vehicle, many issues must be considered.Some of these are as follows.Cable runs must be as short as possible.Wire colour and gaugeThe loom must be protected against physical damage.The number of connections should be kept to a minimum.Modular design may be appropriate.Accident damage areas to be considered.Production line techniques should be considered.Access must be possible to main components and sub-assemblies for repair purposes.

Harness Layout

Printed CircuitsThe printed circuit is used almost universally on the rear of the instrument pack and other similar places.

This allows these components to be supplied as complete units and also reduces the amount and complexity of the wiring in what are usually cramped areas.

Fuses and Circuit Breakers FUSEThe simple definition of a fuse is that it is a deliberate weak link in the circuit.If an overload of current occurs then the fuse will melt and disconnect the circuit before any serious damage is caused.Automobile fuses are available in three types:Glass cartridgeCeramic Blade type

CERAMIC TYPE FUSEBLADE TYPE FUSEGLASS TYPE FUSE CIRCUIT BREAKER

Current flows from battery to the other terminal through bi-metal strip

Bi-metal strip made of two different metals which have different coefficients of expansion

When the bi-metal heats up, one of the metal will expand more than the other

The metal that expand more tend to bend upward and disconnect the contacts

Differences between Fuses and Circuit Breakers FUSECIRCUIT BREAKERConstructed of a fine tin conductor inside a glass, plastic or ceramic housingMade of two different metal (bimetallic)Conductor melt and open the circuit if excessive current flows through the fuseBimetallic bends when heated by excessive current result breaking the connectionFuse has to be replaced when it is blowsCircuit breaker can be reset to restore serviceTermination Terminals are used to connect wires together or plug them into components.

Types of terminal:1) Ring2) Fork3) Pin 4) Flat blade5) Hook blade6) Cranked blade7) Butt8) Female push-on9) Male push-on10) Female fully insulated11) Push-on adapter12) Bullet

Types of terminal

SwitchesA simple definition of a switch is a device for breaking and making the conducting path for the current in a circuit.

This means that the switch can be considered in two parts; the contacts, which perform the electrical connection, and the mechanical arrangement, which moves the contacts.

The characteristics the contacts require are simple:Resistance to mechanical and electrical wear.Low contact resistance.No build up of surface films.Low cost.

Types Of Switches

SPST Single Pole Single ThrowSPDT Single Pole Double ThrowDPST Double Pole Single ThrowDPDT Double Pole Double ThrowCircuit Diagram and Symbols

Simple Circuit The electrical pressure at any point in an electrical system is called the potential.

The potential difference (p.d.) is the difference in electrical pressure between two points in an electrical system.

When two points of different p.d. are joined together by an electrical conductor an electric current will flow between them.

Direction Of Current FlowBy convention, electric current flows from high potential (+, positive) to low potential (, negative).

Conventional Circuit DiagramThe conventional type of diagram shows the electrical connections of a circuit but makes no attempt to show the various parts in any particular order or position.

Figure shows an example of this type of diagram.

Layout or Wiring DiagramA layout circuit diagram makes an attempt to show the main electrical components in a position similar to those on the actual vehicle. Owing to the complex circuits and the number of individual wires, some manufacturers now use two diagrams one to show electrical connections and the other to show the actual layout of the wiring harness and components.Citron, amongst others, has started to use this system.An example of this is reproduced in Figure below.

Terminal Diagrams A terminal diagram shows only the connections of the devices and not any of the wiring. The terminal of each device, which can be represented pictorially, is marked with a code. This code indicates the device terminal designation, the destination device code and its terminal designation and, in some cases, the wire colour code. Figure below shows an example of this technique.

Current Flow DiagramCurrent flow diagrams are now very popular.The idea is that the page is laid out such as to show current flow from the top to the bottom. These diagrams often have two supply lines at the top of the page marked 30 (main battery positive supply) and15 (ignition controlled supply). At the bottom of the page is a line marked 31 (earth or chassis connection).Figure 4.33 is a representation of this technique.

vehicle smart wiring system Smart Wiring System/Multiplex technology involves only two cables.One of them is able to carry all the signals in the system at the same time. The other is the electrical cable, which carries the necessary power. These cables run around the entire car and are known as the databus.The information travels in digital packages.All the small network modules are able to recognize their signal for action and do as they are told.Advantages Of Smart Wiring System The benefits of the multiplex system are considerable:

Fewer cables and connections in the car.Improved reliability.Communication between all the components.Easier and improved opportunities for the retro-installation of electrical functionsThe system also has the benefit of self-diagnosis for all functions.Diagnosis is easier, as is servicing. Any information about a fault or malfunction is passed on to the driver by indicator lamps and a message display in the instrument cluster.