Since you mentioned it, I went out and tested the voltage at that wire. Battery voltage is 12.27 with ignition on (12.88v with ignition off), and the wire is at 12.20v, +/- 0.01v (ignition on).
the terminal on the VR you are testing is sometimes called the "Voltage sense" terminal. Meaning that terminal looks at the available voltage when running ( ideally should be about 13.8-14.2) at about 1500- 2K RPM. up to 14.5 or so is acceptable.
The Voltage should be very close to what the battery
AND output stud on the Alternator are. Wiggling things around while testing will sometimes reveal the issue. One can go so far as to hook up a V meter to the regulator placing it inside the truck so you can observe it while it is happening. NOTE: digital meters do not respond as fast as an older analog type and may "mask" the fluctuation.
The VR looks at this voltage and adjusts the field current to the alternator thereby increasing or decreasing the alternators output.
The VR, alternator battery etc all require
good grounds and
Voltage connections for the current to flow and function properly.
A flakey ground or connection will impede the current flow and cause the V to drop. the regulator SENSE's this and cranks up the Field current increasing the Alt's output.I am hoping his simplified explanation will help you find the culprit. As most have said, the bulk head and ammeter connections are common problems but it could be grounds as well. Keep us posted
