Black wires are ignition positive so they are switched when the ignition is on.
THEY ARE NOT FUSED so DON'T go shorting them to the body or you could burn your wiring out or start a fire
, the reason it stalls is because you draw so much current down a small wire the voltage to the coil/ignition module will drop and that kills the ignition
The wiring for a late DG has four black wires on that side, one larger wire comes from the black box to the coil with a thinner wire also from the black box to the thermoswitch in the thermostat housing, it controls the relay in the black box that switches your manufold heater circuit.
I guess that is the one. go to the thermoswitch and follow the wire back, there is also a temperature sensor in the thermostat housing,
so you can tell them appart the thermoswitch has wire colors black ( probably the broken one) and lilac black the temperture sender has wire colours yellow/red and brown
There is another black wire from the coil to the ignition module and another from the coil to the carb +12V circuit feeding things like the bypass cut off valve but I don't think it will be them as you say it goes to the black box and is small.
I very much doubt this has anything to do with charging, the alternator should connect to the starter motor with a thick wire and the starter motor to the battery with an even thicker wire, if you have 14V on the alternator my guess is the wire from the alternator to the starter motor is duff.
Go easy none of it is fused if you do something wrong you can burn yourself or start a fire. DON'T try touching them to the body to 'see what happens'
