Originally Posted by DerTruck

Sorry, I went by the truck service manual but yours is a van. In any case the terminal circled white is coming from the ASD relay and provides battery power to the field coil. The one circled purple is going to the PCM and the PCM switches the connection to ground rapidly basically creating pulse width modulation. Depending on the on/off ratio the measured voltage changes.
The voltages look good, check them again when charging fails. If you see the same voltage on both terminals that would indicate the PCM has stopped switching the ground connection.