Charging System problem with 300D

[kmcmanus10]

I'm having a puzzling problem with my charging system in my 1981 300D. I installed a new BOSCH alternator because the battery getting charged. I checked the alternator after the installation with a voltameter and low and behold it was putting out 13 and 14 volts under a full load. That is plenty to keep the battery charged. However, the problem is when I put the car in drive under a full load (blower, windshield wipers, and lights) it will fall below 12 volts, not enough to keep the battery charged. Does anybody have any idea what the problem is or what I should check?

Thanks for your help.