Alternator Terminals

Early internally regulated unit, showing the separate plugs for the + and -, and the IND and B+ terminals. There are two IND spades, although one (nearest the designation) is not as clearly visible as the others. The rear cover seems to be a common item for 15, 16 and 17ACR units. Photo courtesy of Craig Locke.

1972 alt plug, showing a single large brown and the smaller brown/yellow (photo David Bolton)

Bee's alternator showing the two large and one small spade. Definitely a replacement unit as the voltage regulator is a 21TR with a 1987 date code.

Looking straight into the connector, the two large spades do seem to be connected together.

The (1973) harness plug, showing the large output wire in the middle, and a smaller gauge brown wire on the other large spade, plus the brown/yellow.

An A115-45 alternator from a Metro, the two large spades are definitely connected together here.

Various 3-pin termination arrangements.

Type A (battery sensed) having a rectangular plug with two large and one standard spades, the centre large spade being the output, the other large spade being battery sensing, and the standard spade for the IND connection.
Type B (machine sensed) being physically the same plug and spade arrangement as for Type B, but the two large spades inside the alternator are connected together, hence either (or both for greater current carrying capacity and less volt-drop) can be used for the output, no battery sensing terminal, and again a standard spade for the IND connection.
Type C (battery sensed) having a 'square' plug, large output spade, medium battery sensing spade, and small IND spade (Brown & Gammons GXE2213 1975-78).
One thing to be aware of is that some sources show the output terminal as B+. If there is no '+' terminal on your alternator, but there is a 'B+', then it may well be the output. But if you have both + and B+ terminals then work to the principle that + is the output and B+ is the battery sensing terminal.

Image from Moss USA showing a 'standard' three-pin socket with two large output spades and the smaller Indicator spade, but also two additional spades which aren't used.

Added November 2009: This from the MGOC catalogue:

V8 (1975) AC-Delco alternator, same 3-pin plug, but this time only two wires (normally - the second output wire is my modification giving a boosted voltage supply to the cooling fan relay for improved performance). Two large spades in the alternator both joined together i.e. both output spades plus the smaller Indicator spade.