Sounds like you're making some progress.
Start side is also coil side, so when the key is in run and the points are closed then current will be running through the coil.
With an ECU hooked up instead of points, current will flow through the coil when the key is on.
If you unplugged the ECU's pentagram shaped connector, then no current can flow through the coil.
Lets go over the voltage drop testing.
Key off, nothing turned on, door shut so the dome light is off, no current flows.
Checking for voltage simply shows which wires are hot.
Lets say the battery is fully charged and voltage from positive to ground is 12.8 volts.
View attachment 1715398025
Same situation, turn the key on with the ECU disconnected.
View attachment 1715398027
Why are there voltage drops?
Why might there be a little ammeter movement?
Its because the voltage regulator is seeing less than 14 Volts, so its allowing current to flow through the alternator's rotor.
The ammeter movement will be very small because its only around 2 amps. A little more or less depending on which alternator is installed.
The voltage drop illustrated in this example shows resistance in every push type connection that causes a 0.1 Volt drop for every 2 amps.
Think about that.
Voltage drops where current flows through resistance.
Same drawing with arrows showing flow path.
View attachment 1715398030
Increase the current through a resistance and there will be more voltage loss.
The dash ammeter scale goes from 40 amps discharge to 40 amps charging.
Its good enough to see movement in the needle with 4 or 5 amps flowing through.
Notice it is positioned only to show battery charging or discharging, not alternator output.
