As for the duty cycles that fall between the cracks - 0% to 4%, 51% to 67%, and from 82.5% to 100% - they are invalid.
However, the FPDM will accept 67.5% to 82% as a valid “off” command. If the PCM wants the fuel pump turned off, it will send a 75% duty cycle to the FPDM. For example, a duty cycle from the PCM of 20% will equal 40% of fuel pump full-on, and a duty cycle of 50% would equal 100% of full-on at the pump.
Whatever the PCM’s duty cycle is, multiply that by “2” and that is the fuel pump’s on percentage. In this range, the fuel pump output is directly proportional to the fuel pump’s “on” time. Between 5% and 50% duty cycle is the normal range of demands from the PCM to the FPDM. To achieve its goal, the PCM communicates a duty cycle to the fuel pump driver module (FPDM). The fuel pump relay is now toggled by the ignition switch. One notable change however, is the PCM no longer controls the fuel pump relay. One Ford-specific component that still remains after more than two decades is the inertia fuel cut-off switch. A couple of things that become optional across other makes that Ford still includes are the fuel pump relay and the in-line fuel filter. This much should remain the same for any electronic returnless system. This system consists of a rotary vane fuel pump (same as before), fuel line (pressure only), fuel rail pressure sensor (that also contains a temperature sensor), fuel pump control module and, of course, the PCM. With an electronic returnless system, the PCM can now raise or lower fuel pressure at will. Also, making it electronically pressure controlled gives the PCM more flexibility over fuel control by adding another option if needed. As an additional benefit, a returnless system, having one less fuel line, reduces the chances of a fuel leak.