Not Bob, but glad you brought this up. I have done my own alternator rebuilding including Delco 10-12SI for 30 + years and just last year noticed this oddity of the Delco regulators, both genuine Delco and aftermarket. If the regulator #2 terminal detects no sense voltage(broken or disconnected wire) The excite voltage from the #1 regulator wire will be blocked in the regulator and the alternator will not excite.
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.
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