academic: this won't fix your tractor (attn Bob@69.178.229.1

The circuit diagram for the internals of the Regulator of the GM 10-SI: analysis would indicate that if the connection to TERM#2 were lost, then TR@ turns off, resulting in TR1 turning on, and the field receives full current, resulting in full alt output to Batt. (as in a VERY dead batt=zero volts).
Whereas, such an open Sense line connection actually results in NO output from alternator.
I suspect that this circuit representation is incomplete & for illustrative purposes only, but there could be more to it than that?
P.S. I have observed this on three different alts. & have not examined this on more, so I can't state that it is universal, but...

Do you know an explanation?

Voila = Some silly French word literally meaning “look there!”

The first transistor, with its base circuit connected to a zener diode and voltage divider to terminal #2, never turns on until the battery voltage rises to a voltage pre-determined by the resistor values in the voltage divider and the zener diode breakover 'on' voltage. The battery charging current is initially set by the two biasing resistors (and a forward-biased diode) in the second transistor's base circuit until the battery voltage at terminal #2 rises enough to turn on the first transistor.
Once the battery voltage at terminal #2 rises enough to switch on the zener diode at the base of the first transistor, its colector-emitter circuit robs (shunts) 'on' biasing current away from the second transistor base-emitter circuit, thus lowering the emitter-collector current through the 2nd transistor in series with the alternator field winding. Lowering the field current lowers the Alt. output voltage/current.

Leaving the #2 terminal disconnected does mean that the first transistor will never turn on and allows the alternator to full-charge continually, but the Alt. output current is still limited by the base-biasing circuits of the 2nd transistor, battery condition (internal resistance and state of charge) and the resistance of the Alt. stator windings (snd the alternator current rating).

The GM-type alternator works in much the same way as older generator charging systems - always on full-charge until rising battery voltage intervenes through some type of a voltage regulator.
The GM-type alternator will not start charging unless:
1) there is enough residual magnetism and rotational speed to generate enough voltage to turn on the second transistor in the VR and apply a small current through the field winding (as in one-wire Alt. setups), OR
2) Terminal #1 receives a start-up voltage/current, typically from the battery through a 'charge lamp', resistor, or diode, to initially furnish a small amount of power to the 2nd transistor in the VR and the Alt. field winding to get charging started. Once started, a 3-diode rectifier connected to the stator windings furnishes the power to the VR, and the start-up circuit becomes idle (charge lamp goes out).

With the original GM regulators, they will "default" to charging about a Volt higher than normal, if the sense lead is lost, APPARENTLY there's more components than shown in the simplified functional schematic diagram, and there's a second voltage feedback source, apparently from the terminal fed by the diode trio.

With the replacment regulators that are around today, from the "land of almost right", I'm sure they are not exact copies of the originals, and may act differently when Voltage Sense is lost.

I still haven't run into any 10SI's that will STOP charging altogether when the "sense" connection is lost, though.

I also attached an old add (for Ford SS Ign) that I was reminded of because of after-market/original parts....for your amusement.

If you ever run across the GM Training materials & if it shows the IC internals, I sure would appreciate seeing it.

Been an interesting thread. Thanks all.

And yes, it goes without saying...I have too much time on my hands! Which is also how I knew what the inside of a Ford SSIgn module looked like.

I don't think this was always the case, as Years earlier I found that loosing battery voltage to the #2 sense wire would cause the alternator to charge flat out, unregulated until it burned itself up.

I discovered this last summer after changing a starter on my 4020 gas tractor. That model has a weak starter and years earlier I added a 10SI Delco alternator with the sense wire routed all the way to the battery connection at the starter. I wanted to make sure that the regulator was seeing actual battery voltage and not inserting any voltage drop from the long output cable run so the already weak starter was not hampered by a less than fully charged battery. All was well with this setup until I replaced that starter this summer. The tractor started fine, but the GEN light would not come on when I turned on the ignition. I grounded the #1 wire at the alt plug and saw that the bulb / wiring was ok, but no light wnen plugged into the alternator. I thought, OK, bad regulator or brushes making an open circuit in the alternator.

I ran the alternator on my test stand and it worked !!??. Thought OK, I have a bad regulator that works on the test stand, but will not excite from that tiny gen light.
I replaced the regulator, verified it worked on the test stand and reinstalled it on the tractor.
Same problem, no gen light lit and no alternator output. Ready to pull out hair now, could I possibly have grabbed another 1 in a million bad regulator ???. I replaced the regulator again with the same result !!??. Then it struck me, I just replaced the starter, could I have missed reconnection a wire at the starter ?? I checked and yes, there was my alternator sense wire dangling behind the starter. Reconnected it and the alternator was back to perfect operation.(Blush) I spent a frustrating afternoon learning that atleast the newer spec Delco internal regulators will not excite the alternator until they see battery voltage at the #2 sense wire.

I'm sure it's safe to say the newer regulators, even if "genuine Delco" are coming from the "land of almost right", and their internals have very little in common with the original US-made OEM GM's.

The originals were SUPPOSED to default to a charging rate about a Volt higher than normal, and hold there, if "voltage Sense" should be lost.

Keith