To start the engine, the starter generator spins one direction to crank for forward and spins in the opposite direction to crank for reverse. In order for the starter to spin in the opposite direction either the field windings (F1 & F2) or armature (A1 & A2) windings have to be reversed, but not both. HDs and Columbias do this by reversing the armature windings with a certain type of switch, which is described later.
All HD and Columbia models have one thing in common, the starter circuit is activated by a starter solenoid which is activated by an ignition ciruit. The ignition circuit contains a circuit breaker, key switch, microswitch, ignition coil, starter solenoid and other components, some of which are not shown in the illustrations to keep things simple. As the years went by, the wiring has changed on where certain components are wired in so your wiring may not exactly match what is shown but the concept remains the same.
TYPES OF SWITCHES
Early HD models have a control box that controls the starters circuit and a regular key switch for the ignition circuit.
Later HD models have a reversing switch (lever) that reverses the armature windings and a regular key switch for the ignition circuit.
Columbia models have a directional key switch that reverses the armature windings within it and also switches the ignition circuit. This is no ordinary type of switch. All of the starters current runs through this key switch. That is why the wires are so big.
89 Columbia models may have a factory 5 solenoid system. 1 starter solenoid with 4 solenoids wired in such a way to reverse the armature windings. They also have a directional key switch that controls the solenoids and the ignition circuit. This keeps the starters current from entering the directional key switch. This type of solenoid system is often used and modified for other models when the directional key switch goes bad instead of replacing the switch, which is often expensive.
This diagram shows how to bench test the starter:
http://www.buggiesgonewild.com/gas-h...generator.html
The attached thumbnail shows how the starter motor is reversed and is an animated gif.