Yes, I've been vague about the voltage measurements. Keep in mind I understand what I'm saying, that don't mean others do. Let's clear up the voltage measurements. One voltmeter needed. Battery voltage to be taken at the battery, using both battery posts (terminals may be dirty). If no post is available, connect VM to terminals. As for the ballast resistor voltage, connect one lead to one terminal of the ballast and the other lead to the other terminal of the ballast. Across the ballast. Polarity doesn't matter, what I need is the number.
Voltage drop at the ballast and knowing ballast resistance, one can determine current flow thru the ballast. Since the ballast is in series with the fuel pump and current flow is the same in a series circuits, we now know fuel pump current. Using fuel pump current and battery voltage minus ballast voltage drop, one can conclude fuel pump running resistance. This resistance value is what I'm trying to tie into fuel pump health.
Everyone punch holes into this. I know I'm leaving a bunch of open ended variables like different voltmeters, ambient temp, fuel density, fuel rail pressure (HOrnbrod), altitude, is it raining, etc, etc. As for engine at normal operating temperature, some will be and other won't. But I have one constant, engine running at idle. So any and all value will reflect this, engine running and at idle. Numbers that fall out of this yet undetermined window, could show as a problem.