My Bentley manual has a electrical diagram for 82-83 diesel.
It could be the same for the 80 diesel in terms of the alternator.

The diagram shows three terminals as you said. Two of them are connected to
the battery. These two are the alternator output. Don't know why. But the
diagram shows two wires connecting to the battery. The remaining one is
the D+ terminal. It serves two purposes. The first is to provide initial
excitation current to the alternator before the engine starts. Once the engine
starts and the alternator generates 14 volt this D+ provides a signal to turn
off the light in the cluster. If this light is on and your battery is not
charged the alternator is not generating electricity. The alternator is most
likely bad.

David

--- Kenneth Lewis <kdlewis@NORTHSTATE.NET> wrote:

> Greetings,
> I am working on a friends early 80s diesel. The alternator light is on as
> the battery is not charging. His alternator has a three conductor plug i.e.
> it is different than the WBX 90a unit. To the best of my recollection the
> regulators look the same. Would it be prudent to test his regulator in my 86
> ?
> TIA,
> Ken Lewis
> http://neksiwel.20m.com/
>

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