So found this, not sure if the screenshot is helpfull but thought for completeness.
No word on the voltage output at idle but basically saying that: (if voltage drop test are within spec)if the voltage between idle and 1600rpm is great than 1.0 , to repair or replace the generator. That's it.
Hope they don't jump to replacements too easy: reading up on battery sensor voltage for instance it says that if its out of wack (throwing a code though) and the code persist after deleting the code, to replace the PCM 😳. Sounds a bit drastic. But that's no my problem!
So when the weather clears up, I'll be taking the alt out and see what I can find and clean up.
Read about one more suggestion on another forum.
Sometimes the alt pulleys aren't entirely correct size, esp if they have been reman-ed. If its larger, the belt would spin it at less rpm's and it would make sense it doesn't charge at idle as it should (although it would be weird to first have worked and not anymore now)
Anyway, the weather cleares, I had some time and had my OEM alt still here so compared with the one in there atm: pulley diameter difference about 3 mm. Curious what the influence of the relative "smaller" pulley would be on the voltage output at idle, took the alt out again (getting pretty handy with it by now) swapped the pulleys, put in back in. Alas, it made no difference. Haven't got a super small pulley to try but the theory made sense!
Slowly...but surely... I am dragging myself towards admitting to have to fork out a new alternator. But not because of lack of trying, ha!
Funny you mention the mafia; Cruiser's first response to my return made me think of the scene from The Godfather below (if familiar with it) where he replies to someone asking a favour, who has never invited him for like a drink or dinner LOL. Of course he grants the favour, leaving the person forever in his debt..
Ps: I could never leave 'La Famiglia'. Even if I no longer owned the XJ..
One terminal is connected to the ASD Relay, the other grounded by the PCM. I'm not 100% certain whether the ECM output is on-off or proportional, but you can test the alternator by just grounding it. It"s extremely unlikely that the PCM would work at high RPM but not low RPM, while that is very much the symptom of a failing alternator.
edit: Many other options out there, including OEM parts, but ...
