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 brown/yellow comes in to one of these from the ignition warning light, then loops back into the harness and out again to the other, probably to protect the alternator if one of the plugs should become detached from the alternator. There may be a black earth wire on the '-' terminal. 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). However this differs from the MGOC drawing and Peter Mitchell's plug, both below.
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 showing the 'BSH' connection with one large and two standard spades:

Added March 2012: Peter Mitchell's 78 plug also showing one large and two standard spades ...

... and his alternator showing the two standard spades, the lower one with chamfered corners. The thin brown connects to this spade (B+), the brown/yellow to the unchamfered standard spade. Ironically it is probably this chamfering that the Brown and Gammons drawing is trying to depict, but if so is labelled incorrectly!

October 2014: Converting between 4-wire, 2-wire and 3-wire alternators. As shown above 4-wire alternators have two large spades in one plug, the '+' terminal is the output, the '-' terminal is an earth terminal (unused). The other plug has three terminals - two IND terminals side by side and the B+ or battery sensing terminal below them. 2-wire alternators have two large spades and one small, both the large spades are connected together and are outputs, the smaller spade is the IND terminal. 3-wire alternators have one large spade and two small, with one small spade having chamfered corners. The large central spade is the output, the plain small spade is the IND terminal, and the chamfered small terminal is the B+ or battery sensing terminal. When converting from the 4-wire discard the loop of brown/yellow that goes between the two IND terminals. When converting to the 2-wire optionally connect the thinner brown to the second output terminal to reduce volt-drop and improve system voltage. When converting to the 3-wire all three wires must be connected as shown.

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.