the 12.8 at rest reading tells me the battery is either freshly charged, or brand new. if you did not just take the battery off a charger or a store shelf, the alternator must be charging it.
likely the glow plug relay is still on when you are testing the alternator.
start the car, go for a drive, come back, THEN test the alternator voltages. within the first 3 minutes of starting the car, many items in the car could be pulling more amps than the alternator can handle.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!