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NOVEMBER 2020 WWW.PNWR.ORG 19 engine compartment panels to support engine functions. In the late 1990s with the new mid and rear engine sports cars, the range of options increased greatly to include features such as four wheel-drive, a large number of electronic driving aids, and updated automatic transmissions, the electrical systems were expanded to include distributed central processing systems. By the time we got the current range of Porsche models into service, a typical electrical wiring diagram package could be 32 pages long with many oversize pages. Major Wiring Diagram Types Porsche wiring diagram layouts fall into about four major types (Reference 2). The earliest one- and two-page diagrams up into the early 1970s showed the location of major components in roughly the same location relative to other components as they occurred in the car. Starting in 1974, Porsche introduced "current flow" diagrams to more clearly show how the current travels from the battery and power generation section leads at the top of each page down to the user component and then to the ground connections, indicated as a horizontal line at the bottom of the page. Although these current flow diagrams are not as easy to follow as the earlier diagrams that often provide a better picture of where individual components are located, they provide a more obvious trace of current flow on individual component circuits. The current flow diagrams were often no more than two very large fold-out pages with a listing of abbreviations on each. Some accessory options were covered by add-on sheets to the basic diagrams. By the early 1980s, not only were many more accessories being offered, but more were also becoming standard and had to be included in the basic diagrams. At that point, the pure current flow wiring diagrams transitioned to a third type of diagram that featured a combination of equipment groupings on multiple pages. The equipment grouping diagrams use map-like coordinates (letters across the top of a page and numbers down the side) to identify connections when circuits cover more than one page as they often do. In the case of these later diagrams, current may be flowing in any direction and it sometimes becomes a challenge to determine the direction. Often this can be most easily done by tracing backwards from the end-use component (light, horn, etc.) to a fuse or power connection. The fourth type of diagram, starting in the mid to late 1980s (depending upon the model), is similar to the third with a continuation of the map-like coordinates, but includes the addition of large integrated distribution boards usually referred to as Central Electric Systems. Working with Wiring Diagrams Starting with the revised current flow diagrams in 1974, there were initial introductory guides provided in the factory manuals to explain what all the diagram numbers meant, along with the fuse panel layouts showing the input and output sides of the fuse box. These example pages usually occur in the shop manual with or near the first year that type of diagram is used for the model. For example, Figure 3 shows the wiring explanation page for the 1984 911 Carrera (Reference 3), which was the first year for 911 continued on next page 2000 996 Carrera provided by Porsche AG Table 1. Early Porsche Distribution Evolution. multi-page coordinate type diagrams and is typical for the later models except for the central electric panels. Figure 3 shows the X-Y map-like coordinate system for identifying component locations and also identifies the meaning of the symbols, numbers, wire sizes (the numbers are the conductor cross section area in square millimeters), circuit continuation numbers to other pages and color codes. The diagrams also indicate, with connecting double lines, which leads, and connectors are grouped into major wiring harnesses and what the harness part number is. Major ground points and their type are also indicated and there will often be a separate wiring page listing all the ground points and their locations. Many of the basic electrical power distribution and user comp- onent circuits are identified with DIN standard circuit numbers that have not changed much over time. These are useful when checking the diagrams and/or accomplishing repair or addition of additional electrical equipment. Many electrical switches and relays use these common circuit numbers stamped on the terminals to indicate where the connections are to be attached. Although the central