... Our MCI's have all had the gear driven alternators, and all would start charging the instant the engine was running. All would show "Not gen" lights lit up until 100 psi was reached, but a test with a voltmeter would actually show 28-29 volts coming out of them while the light showed "No Gen".
OK, now I am out of my depth, because I don't own an MCI nor do I work on a lot of them, although I have done some charging system diagnosis on a couple. It is quite possible that MCI or some previous operator had wired the light to stay on until air pressure built, even though the field itself was no longer wired through the pressure switch. If that's true, I would speculate that the reason for it was as simple as drivers had gotten used to using the Not Gen tell-tale as the indicator that air was up to operating pressure. IOTW, they made it work the "old" way to avoid confusing coach operators. There is otherwise no real reason for the Not Gen light to be on when the alternator is actually charging.
But the original reason for the air pressure switch was to avoid loading any belt-driven alternator before belt tension was applied.
But on one we had a problem once with the light staying on all the time even after air was built up to 123 psi. It turned out to be a bad relay or switch. (I don't recall and dad is not here to ask, since he does the electrical trouble shooting most of the time)
Well, as I said in my first post here, there are many potential reasons for the Not Gen light to come on even though the alternator may be charging. You've already touched on one, which is that, apparently, the light itself is being illuminated by a pressure switch, separate from the actual charging indicator.
The other is the way the charge indicator works. The alternator has an auxilliary terminal called the "Relay" terminal, which should be labeled "R" or "Relay." This terminal produces half-rectified DC from the stator (output) windings; the voltage varies with alternator speed, but will be roughly half the rated voltage of the alternator. So for the 24-volt units we are talking about, this terminal produces roughly 12 volts, when and only when the alternator is actually producing output.
Somewhere, and on most MCI's I think this is in the rear J-box, there is a 12-volt relay connected to this terminal. When the alternator is charging, this relay closes, and connects the 24-volt blower relay to a 24-volt supply. This is to ensure that the blowers, which use a ton of power, are only on when the alternator is charging. The Not Gen light is connected between +24-volts and the blower relay. When the blower relay is energized, the Not Gen light goes out.
If the 12-volt relay goes bad, and they sometimes do (especially because they are driven by half-rectified voltage), the Not Gen light will come on and stay on (and the blowers, if any, or whatever else might be on that circuit, will shut off), even though the alternator is getting good field and charging properly.
So the first troubleshooting step here is to check the actual output voltage with the engine running on high idle. If the charge voltage is present (27+ volts), then the problem is strictly with the circuit that lights the light. If no charge is present (~24 volts), then there is a regulator, field, or alternator problem.
However, my original question still stands, about an alternate charging source. If there is a battery charger connected, say from a generator, then the charge voltage from that can actually convince the Delco regulator that no charging is needed, and it will then shut off the field. That will cause a Not Gen light, even though the batteries (and everything else) are getting plenty of juice.