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John: There's more than one way for voltage to get from the B+ terminal on the back of the alternator to the B+ terminal on the battery. One way is through a fat red wire from the B+, through a rubber grommet just forward of the engine oil thermostat, to a big ring terminal on the starter, and then through the large black wire from the starter, through the tunnel and up to the Battery.

The other way goes through the warning lamp. Out of the alternator "trio" diodes, into the blue wire, through the 14 pin connector on the back of the electrical console, where a short blue wire branches off to connect to the D+/61 terminal on the VR, then back into the 14-pin connector into the car's main wire harness, then forward to the bulb holder for the warning lamp in the oil pressure gauge. The other side of the bulb holder is connected to a red wire with black stripe that goes to the ignition switch.

Your understanding of the operation is correct: when the ignition is turned on with the engine not turning, the battery voltage (from B+ to ignition switch to bulb holder) is higher than the voltage at D+/61 (which is zero).

When the voltage regulator senses that voltage on D+/61 is less than the "set-point" or desired alternator output voltage, it sends whatever it's getting on D+/61 to the field through the DF wire. So the battery voltage flows out of the battery, into the igntion switch, through the bulb, back to the VR, into DF, into one brush, through the alternator rotor, out through the other brush and to ground. Because current is flowing in this circuit, the bulb lights up.

Now, when current's flowing in the field with the engine turning, a current is induced in the stator which gets rectified by the "trio" diodes and fed OUT of the alternator at D+/61. Let's say it's about 14V for discussion sake. This is enough voltage to "balance" the voltage flowing from the battery and the light goes out. (The potential difference of 1.5V isn't enough to make the bulb glow bright enough that you can see it.)

Now, even though you may be seeing 14V at the battery, that could be due to the fact that the alternator output is reaching the battery via the starter cable route, not via the blue wire. This is most often caused by a high resistance in the blue wire circuit that's dropping the voltage.

How to measure? Well, I would dig out the volt-ohm meter, set it to the 1K scale and measure each segment from the biggest to the smallest. Disconnect the blue wire from the bulb holder, put one probe in there, put the other probe on MALE pin #11 on the electrical console. (The pin numbers are in very small numbers on the FEMALE side of the connector). Shouldn't be more than a few ohms of resistance: 16 gauge copper has .0473 ohms per foot, and I figure there's about ten feet of wire in there, so about half an ohm for the wire, and there are TWO connections in that run, one from the car to the first 14-pin, then the second to the next 14-pin.) If you get a high resistance that means something's amiss. Next, measure from the ring terminal on the back of the alternator to the FEMALE pin #11 in the 14-pin connector. High resistance? And so on.