Seriously another alternator question.

I have posted on this problem hundreds of times. Here we go in no particular order, except no1

1...VOLTAGE DROP IN THE VR IGN / FIELD CIRCUIT
2...Voltage drop in the VR ground circuit
3...VR that is out of spec for voltage. If it's a solid state replacement, this is rare
4...Bad battery. This is somewhat rare. I believe sulfation can cause this

1....VOLTAGE drop in the VR ign/ field circuit: This is easy to check for and VERY popular as a cause Get a multimeter. Turn ignition switch to "run" with engine stopped. Hook one probe "as close as you can get" to the VR IGN terminal or to the "key side" of the ballast resistor. Stab the other probe directly into the battery POS post. You are hoping to read very little, the less the better. Over .3V (3/10 of one volt) or more start looking. The original path is battery.....starter relay "big stud".....fuse link.....through BULKHEAD CONNECTOR on big red.....to AMMETER....through AMMETER....out on big black to WELDED SPLICE....branch off..... to IGNITION SWITCH CONNECTOR.....through IGNITION SWITCH....back out IGNITION SWITCH CONNECTOR....out on dark blue....back out through BULKHEAD CONNECTOR........to underhood loads, branch off to feed ballast resistor, VR IGN terminal, etc.

WHATEVER YOU READ in this test is ADDED to the normal VR set point, AKA if the VR is correct at nominal 14V, and you measure 1.2V in this test, the battery WILL RUN at 14+1.2= 15.2V

This problem can be caused by ANY terminal or connection in that path. The bulkhead connector, the ignition switch connector terminals, or right in the switch contacts. One way around this is to electrically break the dark blue "ignition run" (IGN1) wire, trigger a relay with the "key" end and feed the engine bay end with the output of the relay. Feed power to the contacts from such as the starter relay "big stud."

ALSO voltage regulators are temp sensitive and temp compensated. Do this test with engine warm

2...CHECKING for V drop in VR ground circuit. Run engine at RPM to simulate low/ medium cruise RPM. Test first with all accessories off, and again with lights, heater, etc powered on.

Stab one probe direct into the top of the NEG battery post. Stab the other into the mounting flange of the VR. As in the first test, the lower the reading the better, zero is perfect. If you read more than just a 10th or so, you need to improve battery--to--body grounding.

I like to use about a 1 foot "starter" (eyelet to eyelet) cable used by Ford. Bolt one end to the rear of the pass side head, the other to the body. The master cylinder mount studs work pretty good