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NOVEMBER 2020 20 electrical systems and multiple integrated circuits of the 911s after 1990 and the front engine cars after the mid 1980s do not lend them- selves as easily to modification or added equipment, the designers were usually thoughtful enough to provide a few familiar terminals for adding supplemental electrical equipment. The typical major circuit numbers often needed in such projects include: 30: Hot at all times (direct connection to the battery positive post) 31: Universal ground (can be a panel connector or a grounding stud) 15: Hot only with ignition switch on X: Hot with ignition on, but not while starter is running These are handy numbers to remember as they are standard on most Porsches and do not change appreciably over many years (some of the early cars don't have X circuits, but all have the others). The circuit diagram for the specific model will indicate where these circuits are located and help determine the easiest way to connect to them. Note that none of these power sources are fused, so an external fuse should be added if the component being added requires it. In addition to these major circuits, there are several other commonly used circuits, as listed in Figure 3, that have consistently used the same identification number on various Porsche models. Tracing Current Flows Some of the major uses of the diagrams include tracing current flows to a given user component or circuit, identifying interacting circuits, determining wire sizes, and determining ground locations. When tracing a current flow, it is important to remember that when two wires are connected to the same component terminal or fuse input, the terminal may be used just as a junction point and the terminal and its attached component or fuse may not be related to the main current flow being traced. For multiple wire terminal con- nections, all current flow options must be considered. In addition, both the input and output sides of the fuses are important to identify and may not be obvious in all the diagrams. The later complex diagrams can get especially tricky when the circuit leads to another page or several pages before the source is discovered, so the page coordinates have to be carefully followed. The best approach is often to construct a simple sketch on a separate piece of paper that includes the wire terminals and components from all referenced pages. Finding Wiring Diagrams For owners who have not invested in a full workshop manual, but want a factory wiring diagram, there are some options. For the early Porsches up through 1989, there are a number of commercially available publications that have complete wiring diagrams that include some direct copies of the factory diagrams. These are listed in Table 1. A number of the earliest model wiring diagrams up to 1971 on the 914 and 1976 on the 924 were published by Panorama in Volume 5 of Up-Fixin der Porsche. Haynes manuals for the 911 up to 1985 are generally direct copies of the factory diagrams. Haynes manuals are also available for the 944 and are valuable as a general guide but may lack some of the important circuit numbers and component terminal numbers. The recent Bentley Publishers shop manuals have complete wiring diagrams for the 911SCs and 911 Carreras from 1978 to 1989 and are already divided up into individual subsystem areas so may be a little easier to use than the factory multi-page diagrams. On the other hand, the factory diagrams usually have the best identification of terminals and circuit numbers. Another source in models prior to the 1990s are the PCNA new model technical introduction books, which often include complete wiring diagrams on new models in the book for the first year of that model. For Porsches after the late 1980s, the primary specific model diagrams have been the Porsche factory diagrams, either in a new model introduction publication or a workshop manual supplement. Workshop manuals for the 986 Boxster and Cayman or 996 Carrera up through 2004 include wiring diagrams for each model year. Starting in 2005, Porsche no longer offers printed shop manuals (including wiring diagrams). Now all manual data is online to the dealers. On Board Diagnostic Readers Starting in the late 1980s, Porsche introduced a series of electronic system 'readers' that could be plugged into a late series OBD model to read out stored system faults and other conditions in the digital electrical systems. The history and growth of these readers with some pictures is described in Reference 4. By the late 1990s, the digital data systems on the 986 Boxster 996 Carrera models had full OBD systems and the dealer shops had portable PCs that could quickly allow the technicians to read out all the Figure 1. Electrical System Elements. POWER GENERATION AND STORAGE POWER DISTRIBUTION USER CIRCUITS STORAGE BATTERY CENTRAL LOCKING & SAFETY SYSTEMS STARTING SYSTEM FUSES RELAYS BRIDGES ENGINE ALTERNATOR IGN 5W INSTRUMENTS LIGHTS HEATING & AC ACCESSORIES Figure 2. Electrical Power Distribution Evolution.