Flickering head lights, what am I missing

A test for that would be to measure votlage difference between alternator out and field terminal feed while the engine is running.
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And more directly focused on the 18 gage wire, if you have enough wire for the leads.
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Mattax,, thanks for the very clear diagrams
but I have a question,
using a multimeter to measure DC 12 V, would I actually get a reading with one probe on the alternator out and one probe on the field terminal indicated?
same question when probing from the field terminal to the disconnect,,,

I thought there had to be a flow potential, for example from a positive to a ground to measure voltage.
or are you saying to measure from the alternator output to ground then the field terminal to ground?
I don't know just asking.
I did similar with the engine not running and got battery voltage at the field terminal alternator connection.
I have a DC clamp amperage meter, tomorrow I can check flow through that wire, or maybe I will just rig up a temporary 12 foot splice the length of the 18 g wire to see if that impacts the flickering.