Don't jump voltage to D+. It's not supposed to work that way. That terminal normally receives only about .15 amps of current, as provided by the draw thru 4 instrument panel bulbs with Key On. It's not meant to get a straight 12 volts with no current restrictions.

If you get 14 volts at D+ (and presumably B+) with the jumper off, the Alternator is OK (although your jumper may have filled in for missing preexitation current at the start—as described in an earlier thread).

With 14V at the alternator and only 11.9V (essentially battery voltage) at the Battery, you have poor contact between the two.

Disconnect the Battery negative—for safety—then check/clean every junction between Alt B+ and Battery Positive—from the B+ terminal, to the Starter teminal, to the Battery.

For good measure, I'd also remove and clean both ends of the Battery and Alternator ground cables.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.