Yeah, it might be a nerdy gearhead thing, but as enthusiasts we want a lot of information about what our cars are doing. Unfortunately, the manufacturers don't quite see it our way, so cars are equipped with instrument clusters about as informative as a utility company's customer-service recording. We understand--they have to make cars for people who care more about their appearance in the rearview mirror or the latest tweet from the Kardashians. Bummer, because there is way more cool stuff going on under the hood than on reality TV.
If you're saddled with a car bereft of all but the most basic instruments, you're going to want to add some gauges. Most people start with a tachometer, and that's exactly what we did, but how do you hook one up to the car's distributorless ignition system? We're glad you asked.
In the case of the coil on plug ignition in our Crown Victoria, system voltage is fed straight to the coils via a relay near the battery. A single wire from this relay ends in a splice near the top of the engine, from which eight wires branch out to the individual coils,through the primary windings of the coils and out into eight more wires that go to the ECM, which provides the ground path for each coil. With the key on, current is flowing through the relay, the coils, and to the ground path provided by the PCM, energizing the primary windings inside the coils. To cause the spark plugs to fire, the PCM breaks the circuit to each coil individually according to the engine's firing order, stopping the current flow. This causes the energy built up in the primary windings to jump to the secondary windings, which are made with loops of much finer wire. The smaller diameter allows much more wire than that used to make the primary windings. The result is a multiplying effect, as a system voltage of 12 to 14 volts increases to nearly 40,000 volts, or enough to jump the gap of the spark plug. The electronic ignition tach adapter reads the amount of current passing through the system as the engine is running and generates an analog signal readable by most aftermarket tachometers. That signal comes out on the grey wire.
