See what I mean. Trust conner.

Ray, sort of. The coil receives plus voltage at the coil + from the ignition switch constantly while on. The coil minus is connected to switched ground. The switch is the power transistor or ignitor in this case in the dizzy. The dizzy uses the ignitor and the reluctor/detector set to emulate an old set of contact points and break the ground path when the timing system calls for spark event. This interrupts the current flow in the coil, and as the magnetic field collapses, generates the high spark secondary voltage in the coil windings.

That takes care of spark. As conner said, if the tach wire is grounded, you get no spark and the truck does not run.

Tach operation works on the same circuit. At 3000rpms you have 50 spark events per second. At 1500 rpms, you have 25 sparks per second. Per unit time, you have twice as much on time in the needle actuating coil in the tach at 3000 as at 1500, so the needle swings twice as far. The tach gets power from the ign switch (via some other stuff, but that's the source) and is grounded via the coil -, so the ignition make/break ground also makes/breaks the tach needle coil ground path at a rate exactly proportional to rpm. There are diodes and transistors and stuff in there so the 'lectrons don't get notional and do stuff you don't want.

All clear?

Once I rethunk this, my original directions will fry your tach. BE WARNED. Disco the tach wire at the coil, and use a piercing jumper wire to ground to test along the tach wire from tach to round gauge connector to cluserl, looking for a single needle jump every time you connect to ground. Make the ground connection quickly, then break it. A case of St Vitus Dance will make a more easily read indication on the tach. Ign on for tests.