The battery is grounded to the vehicle frame at the negative terminal. Its positive terminal is connected to a tachometric relay, and to the ignition switch.
A connecting wire runs from the tachometric relay, to the positive terminal on the fuel pump. A fuse in the connecting wire protects the circuit.
The fuel pump circuit is completed by grounding the fuel pump negative terminal to the frame of the vehicle.
When the fuel pump operates, current flows from the battery, to the pump. It flows through the pump to rotate the armature, and completes the circuit by flowing from the negative terminal, through the vehicle frame, to the battery negative terminal.
Similarly for the other components in this simplified system. Positive connections are made to a number of components from the tachometric relay. The relay only operates when the engine is cranking, or running, so no current flows in any connecting wires at other times.
Source: CDX Global
