[edwardc]

Hi! Welcome to the forum. You have found the best place on the web for all things A4. The folks here are knowledgable, polite, and eager to help.

Adding a tach is a great idea. Unlike a diesel engine, which requires an alternator feed for a tach, a gas engine like the A4 can run a tach off of the ignition signal present on the "-" terminal of the coil. This is, by far, the commonest way of driving a tach on an A4.

Tachometers of this type usually come with some sort of selection mechanism for the number of cylinders. This tells the tach how many pulses per revolution to expect.

Wiring is pretty easy, consisting of four wires: Ground, +12 power, ignition signal (from the coil "-"), and display illumination.