Hi there,
You must have gotten hold of a defective regulator. The alternator's output should be at least 13.8v in hot weather with a charged battery. In cold weather it will put out more like 16 volts. Make sure the regulator is grounded well to the case. this is done through one of the screws that hold it in place, and it's possible there could be some corrosion underneath the regulator on the frame of the alternator. Also check for a good ground between the engine and the frame of the vehicle. If you have access to an oscilloscope, look at the output of the alternator, it should show a nice waveform with no missing peaks. You could have a defective new diode pack causing low current output, but the voltage should still be good, just missing a phase or two. My money is on the regulator or regulator ground.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C