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NOVEMBER 2020 18 Photography by Porsche AG Figures 1-3 are provided by Allan Caldwell. NOVEMBER TECH NOTES Allan L. Caldwell WINTER ELECTRICAL PROJECTS With the reduction in fall and winter driving activities due to the Coronavirus pandemic, this year might be just the right time to devote time on some of those troublesome electrical problems on your older Porsche. The main tools required are a good voltmeter and the wiring diagram for your specific model. The official factory wiring diagram of a given model and production year is often the most accurate representation of that model's systems, how they are powered and how they are intended to work. Wiring diagrams also reflect the many changes in Porsche designs since 1965 from the growth of emission controls, safety systems, security systems and the increased use of electrically powered subsystems. An accurate wiring diagram is an invaluable tool in identifying individual subsystem circuits, how they receive their power, and in troubleshooting intermittent operation and/or power sources. However, as the 1990s got underway, Porsches started getting on-board diagnostic systems (OBD) that record electrical problems and have now become an increasingly important part of troubleshooting the later models. Printed wiring diagrams remain an important owner's aid and have been available for most Porsche models up through the 2004 Boxster and Carrera 996 models. Starting in 2005, Porsche put all their wiring diagrams online to the dealer service departments, and the primary source for owner electrical troubleshooting on the later models has become use of the commercial OBD readers that are available from a number of after- market suppliers. Electrical System Architecture All Porsche electrical systems can be generically divided into three basic groups: power generation and storage, power distribution, and user circuits as shown in Figure 1. The general flow of electrical energy is from the battery and alternator system (storage and generation) to a power distribution grid (fuse block or central electrical panel) where current is distributed to the individual user circuits. Each Porsche model has slightly different arrange- ments of these elements, and it is often an important troubleshooting tool for tracing out the current flow from the generation and storage area to the end user component. In the case of intermittent lights or subsystem operation, it is handy to start at the end user and work backwards through a given user circuit to find local circuit shorts or outages. As Porsche electrical equipment became more extensive in the late 1970s and early 1980s, the electrical system power distribution changed from simple fuse panels and distributed components to integrated centralized electrical systems (Reference 1). With the later systems, most of the cars' fuses and relays and many of their interconnections were first consolidated into one large main panel and, as the cars became even more complex, expanded to multiple integrated panels. In these cases, the primary power distribution panel has input power cables going to it from the battery, alternator and ignition switch. Power is distributed from the panel through multi-pin connection plugs on the bottom of the panel to output electrical cable bundles that go to all the various subsystems. Figure 2 illustrates how the power distribution systems evolved from the early Porsche models to the current family. The later systems save space and have some production cost advantages, but often makes rerouting of wires, circuit isolation, or modification more difficult even though many of the circuits aren't much different than they were for earlier models. The 944 was the second model after the 928 to get a full central electrical system, but the 911s didn't receive it until the major model change in 1989 and 1990 to the Carrera C4 and C2. The centralized system makes the most sense for the front engine cars where so many electrical functions and controls are in close proximity. The rear and mid-engine cars still retain one or more 1957 356 Cabriolet provided by Porsche AG NOVEMBER 2020 18