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insulation, made by wrapping cotton or silk around wire and then coating it with rubber, was easily hardened by heat. The insulation often broke off, leaving bare wire exposed. A common problem in cars that used dry-cell batteries was moisture penetration through the battery’s paper insulation. Current design would flow to ground and the batteries would become discharged. Even washing a car sometimes caused trouble. Water got into the distributor terminals and made the engine hard to start. Some technicians poured melted wax into the space between the plug wires and the distributor cap terminals. For protection from heat, moisture, oil, and grease, wiring was often run through a metal conduit. Armored cable-insulated wire enclosed in a permanent, flexible metal wrapping was also used, especially in a circuit where any voltage drop was critical.
This is an important point to remember. It may be helpful at this time to review the explanations in Chapters 3 and 5 of voltage drops and current flow in various circuits from the source, through all the loads, and back to the source. Every electrical load is attached to the chassis so that current can pass through the ground and back to the grounded battery terminal. Grounding connections must be secure for the circuit to be complete. In older cars where plastics were rarely used, most loads had a direct connection to a metal ground. With the increased use of various plastics, designers have had to add a ground wire from some loads to the nearer metal ground. The ground wires in most circuits are black for easy recognition.
MULTIPLEX CIRCUITS The use of multiplexing, or multiplex circuits, is becoming a necessity in late-model automobiles because of the increasing number of conventional electrical circuits required by electronic control systems. Wiring harnesses used on such vehicles have ballooned in size to 60 or more wires in a single harness, with the use of several harnesses in a vehicle not uncommon. Simply put, there are too many wires and too limited space in which to run them for convenient service. With so many wires in close proximity, they are subject to
Chapter Six
electromagnetic interference (EMI), which you learned about in Chapter 4. To meet the almost endless need for electrical circuitry in the growing and complex design of automotive control systems, engineers are gradually reducing the size and number of wire and wiring harnesses by using a multiplex wiring system. The term multiplexing means different things to different people, but generally it is defined as a means of sending two or more messages simultaneously over the same channel. Different forms of multiplexing are used in automotive circuits. For example, windshield wiper circuits often use multiplex circuits. The wiper and washer functions in such circuit work though a single input circuit by means of different voltage levels. In this type of application, data is sent in parallel form. However, the most common form of multiplexing in automotive applications is serial data transmission, also known as time-division multiplex. In the time-division type of circuit, information is transmitted between computers through a series of digital pulses in a program sequence that can be read and understood by each computer in the system. The three major approaches to a multiplex wiring system presently in use are as follows: • Parallel data transmission • Serial data transmission • Optical data links
We will look at each of these types of system, and then we will discuss the advantages of multiplexing over older systems of wiring.
Parallel Data Transmission The most common parallel data multiplexing circuits use differentiated voltage levels as a means of controlling components. The multiplex wiring circuit used with a Type C General Motors pulse wiper-washer unit is shown in Figure 6-20. The circuit diagram shows several major advantages over other types of pulse wiper circuits, as follows: • Eliminating one terminal at the washer pump
reduces the wiring required between the wiper and control switch. • Using a simple grounding-type control switch eliminates a separate 12-volt circuit to the fuse block. • Eliminating a repeat park cycle when the wash cycle starts with the control switch in the OFF position—in standard circuits, the
