67 factory cuda tach

When the key turns on it should energize the wire and you should see +12 volts at the wire terminal.

But If you're just testing the tach, as your original post said, just go under the hood and attach the grey wire to negative terminal of coil and spade terminal on the tach, a ground wire to the tach's case and car. attach the cap''d (blue wire) terminal to the positive terminal on the tach. Start the engine, touch the wire to positive terminal of the battery and observe the dial. But remember if you're using a car with electronic ignition, the tach may not show accurate readings. It was designed for breaker point ignition.