so you converted a 68 single field charging system to the dual field charging system for the 78 amp alt.

one field wire from the new vr, the second field wire from the key on 12 volt power that went to the original VR. with a split to the new vr for sensing.


you need to verify sensing voltage first. Be sure you use the same reference ground.
Battery pos to battery neg on the meter for battery voltage.

sensing line to case of the VR for sensing line voltage. Do not use the battery ground or you may get a false reading.

Alt from the back post to the alt case.

I have had charging issues because of 3 things.
the new connector is bad for the sensing voltage line. causing intermittent over charge conditions.

The sensing voltage has dropped too much passing through the bulkhead 3 times. you can clean the bulkhead connector, or use a relay to give power directly from the battery.

The VR was not grounded properly. I always add a ground cable to the bolt holding the vr to the firewall.

15 volts at the back of the alt is a bit high, but 13.5 at the battery is fine.
amp meter might have issues, your battery could be low and is charging to compensate.
or you are actually over charging and don't know it.