First, regulators will be different voltage when cold/ hot. My old '70 manual shows
14.3-15.3 when cold
13.8-14.4 at 80F
13.3-14.0 at 140F
less than 13.8 above 140F
If the regulator hooks up the same, it matters not what amperage alternator you are using. It's probably caused by my old friend "harness voltage drop."
Charge the battery up to "normal" run the engine to simulate "low to med cruise" and check below first with all loads off, then check again with lights/ heater on
Stab one probe of your digi meter directly onto the battery NEG post. Stab the other probe directly onto the regulator CASE. You are looking for a very low reading, the lower the better, and zero would be perfect. MORE than .2V (2 tenths of a volt) is too much, and indicates a ground problem between battery neg. and the regulator.
Now check the hot side. This will show the voltage drop in the wiring from the battery--harness- through the bulkhead--to ign. switch connector - switch contacts -back out connector -back out bulkhead, and wiring between - to regulator IGN terminal.
So rig your voltmeter to the regulator IGN terminal, or to the dark blue IGN supply to the ignition resistor. Hook the other probe directly to the battery positive post.
As before, you want NO MORE than .2V.
Either of these voltage drops being a more will allow the regulator to raise battery charging voltage accordingly.
To try and help with some of this drop, the large frame hi-amp factory stuff used to use a "field loads" relay. I used to have a large-frame unit on my old Landcruiser and never used the relay. If you want to wade through it all, you should be able to look it up in the '72 shop manual that abodyjoe posted.
