Voltage drop during hard stop

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Hi guys,

I have 2003 4.4 and I am experiencing voltage dropping during hard stop. However due to the voltage dropped I sometime experience transmission slippage/bang from down shift and also upshift. For example while driving at around 40 - 50 mph and need to slow down to take the right turn without complete stop after quick braking then accelerate I sometime get transmission downshift from 4-3 problem. I also notice that the voltage drop bellow 13 v and sometime down to 12 v in the ODB and INPA while I am experiencing the transmission slippage/bang symptom. I also get the error code from transmission indicating battery is at 12.12 volt.

New battery put in a few days ago. At idling I am reading about 13.6 to 13.8 volt in the ODB. Freeway getting 14 v. If I turn on my heater and light it will drop and fluctuate between 13.6 to 13.8.

Autozone and Oreily tested the alternator while on the car, without load test it is good. With all light on, ac on max, audio on max, wiper on, the voltage bouncing around in the upper 12 v and lower 13 v and fail on the voltage regulator test.

I need and suggestions before I replace the alternator as it is water cool.