charging system trouble

Voltage at the old style field is irrelevent.

You need to measure the voltage drop supplying the ignition voltage under the hood

The current path "in general" is

Battery........fuse link........through the bulkhead.........through the ammeter.......through the ignition switch......back out the bulkhead........to the underhood loads

Orginally these loads in that old car would have been the ballast resistor and the IGN terminal of your VR

Hook one terminal of your meter to the IGN terminal of the VR. Hook the other probe to battery POS. Set the meter to low DC volts, and turn the key to 'run' with engine off

If you measure more than .3V (three tenths of one volt) you have a problem in that path.

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The second problem is VR grounding. Run these checks with engine warm, battery "normalized," and engine running "fast" to simulate low to medium cruise. Run this test first with all accessories off, and again with headlights, heater, etc turned on

Again with your meter on low DC volts, stab one probe into the NEG battery post. Stab the other probe into the metal mounting flange of the regulator. As before you hope to read a very low value, the less the better. Zero would be perfect. More than just .1--.2V or so means you have a grounding problem between the VR and the battery.

These two voltage drops, the drop in the ignition harness, and the drop in the ground, ADD to the VR voltage calibration. That is, if the VR is properly trying to regulate at about 14V, and if you have say, 1 V drop in the ignition harness, your charging voltage will be 15V.