Alternator quandary

I’m stalled in the middle of installing a battery isolator to keep the second battery full and prevent draining of the starting battery. Most of the connections make sense, but I’m unsure which terminal on the alternator would be the “S”…matter of fact, the only one I can see a label on (without pulling the alt) is the “B+” that the fat red wire goes to.

My current battery has two connections on the positive terminal: fat black cable and about an 8 gauge red one. My experiments suggest that the red is for starter and the fat is hot line from alt, so I connected the fat one to the “ALT” post on the isolator and left the red one on the starting battery+ terminal. (of course all ground straps are disconnected and taped out of the way while I work)

My Bosch alternator has two connections – the fat red cable at B+, and another blue one that looks to be about 14 gauge. Is this blue one the “S” terminal of which the below diagram speaks? I just can’t see in there and don’t want to pull the alternator just to check that.

With the main battery hooked up to the “battery 1” position on the isolator, the thick cable on the “alt” post, and the smaller red cable still at battery+, accessories work but the car won’t start. From what I’ve read, the alternator is probably stopping it from turning over because there’s a control line missing. Could that blue wire on the alternator be the thing to connect to the isolator/starting bat+?

If I jumper the “alt” and “bat1” terminals on the isolator, the engine starts right up, but there is constant 12v going to the second battery, exactly the thing the isolator is there to prevent. So far, the battery idiot light is doing what it should.

Apparently the information I’m googling for here is so basic that everyone assumes you already know it – ah well, I’m used to that kind of thing, the perils of being self-educated.

Here’s the diagram that came with the isolator:


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