This is almost always caused by voltage drop in the harness, mostly terminals and connectors, as well as switches. It can also be in the VR ground path to the battery.
RARE cases is a bad VR and sometimes a bad/ sulfated battery.
Part of this is vulnerable because Mopar regulators do not have a dedicated voltage sense terminal. The sensing is on the same terminal as the power terminal.
How it works
Power from BATTERY---to STARTER RELAY STUD--to FUSE LINK--through BULKHEAD CONNECTOR (big red)--through AMMETER--out on BIG BLACK--to WELDED SPLICE--to IGNITION SWITCH connector and switch--out switch on DARK BLUE IGN1 "run" back through BULKHEAD CONNECTOR and to splice / terminals in engine bay, feeding alternator field, VR IGN terminal, ignition system, electric choke if used, and on some late cars, some smog doo dads
ANY VOLTAGE DROP in the ENTIRE CIRCUIT from battery to VR, INCLUDING THE ground circuit, will cause the VR to "sense low" and ramp up the alternator o compensate. If you check, while running, at the ballast resistor terminal coming from the key, you will likely find 14V while running, while the battery is seeing the high reading of 15 or so.
TO CHECK HARNESS
Turn key to "run", with engine stopped. Stab one probe of your meter into the top of the battery POS post. Connect remaining probe to as close to VR IGN terminal as you can get. The ballast key side terminal. You will read some voltage, the lower the better. If you read over .3V or so, look for a reason.
TO CHECK GROUND
Run engine at high idle to simulate low to medium cruise RPM. Make test will all acessories, lights, heater, etc off at first, then again with them powered
Stab one meter probe into the top of the NEG battery post. Stab the remaining probe hard into the VR mounting flange. Again, the less voltage, the better, zero is perfect
WORKAROUND FIX
One way to solve the problem without a bunch of re-wiring, and the ignition switch may be part of the problem, is to add a relay to operate underhood ignition loads. Break the IGN1 "run" wire coming out of the bulkhead connector into the engine bay. Use the bulkhead end of the wire to trigger a Bosch style relay, and feed the relay contacts power from the starter relay stud, and through a fuse/ breaker. Keep the fuse away from engine heat. Connect the engine bay harness end of the cut "run" wire to the switched contact. THIS SHOULD now get the VR "ign" terminal much closer to actual battery voltage.
