1972 Plymouth scamp charging and grounding issue
But it only happens when the lights are on, if they are off then its dead center. Could it be the lights causing resistance somewhere or is it something else? The battery is 12.68 when idling and 12.93 when brought up to 1700 rpm.
Those ammeter and voltmeter readings are normal for an alternator not putting out properly; understand that the ammeter indcates current going into and out of the battery. The left-of-center (- or Discharge) readings when the lights are on indicates a drain of current out of the battery rather than it coming from the alternator as the light's current should. That might happen at low idle but should NOT happen at fast idle. The lack of right-of-center (+ or Charge) indications without an electrical load says that the battery is not being charged. You should see + or C readings after starting the car, staying that way for a while until the battery is well charged.
The voltage readings you show confirm this. You ought to have more than 12.7v at the battery at low idle, and in the range of 13.7 to 14.3 v at fast idle.
Important question: Did you get the alternator for a 1972 Scamp? That should work right with the same year of VR, but a 68 318 alternator won't work with a 72 Scamp VR. So we all need to know what years of VR and alternator you have. To be right for 1972, the alternator will have 2 small field wire connections on the back (in addition to the large output lug).
If this is the case, then remove the green field wire from the back of the alternator and ground that connection on the alternator (not the wire). This should jump the voltage at the battery up to 15 volts or higher. (Do this only long enough for the test.) If this works, then the alternator is OK and the VR or its wiring are the problem area.
If you replaced the voltage regulator (VR), then it can be bad out of the box. I just went through this with a guy over on the FBBO forum; a lot of tests he ran (like above) showed the circuits to be properly connected and normal. He had replaced the VR twice but still had low charging voltage like you have. He put in a new Wells VR and 'presto', it was all good.
