Driving home w/ headlights on last night I noticed my upshift arrow softly flickering - IME an alternator problem. No light in my in-car voltmeter so I hadn't noticed anything amiss. Via the dome light, voltmeter read just < 12V. Usual is ~14V. Battery dead by the time I got home 10 minutes later.

Charged the battery this morning and started. Same thing: < 12V. Alternator-warning light in gauge cluster not working. With car running,

- Grounded excite wire and alt-warning came on (= wire not broken)
- With excite wire removed, put 12V to excite post on alternator. Voltmeter showed ~14V.
- With excite wire back on, test light lit on excite wire but not post. Voltmeter < 12V

1. Replaced the voltage-regular with a used one, voltmerer < 12V
2. Replaced voltage regulator, voltmeter < 12V
3. Replaced unit with used alternator and new voltage regulator, < 12V
4. Pulled a known good alternator from another car, installed it, back to normal ~14V. Alt-warning light now lights when key turned to on.

(History: I'd replaced the alternator ~ 4 months ~2K miles ago with a newish used one because of a bearing squeal.)

OK, so I happened to run into 2 bad alternators - I assume failed diode(s). Why didn't the warning light come on when the car wasn't charging? Why didn't it illuminate when I turned the key to on?

I saw a schematic and notes from Art B (thread below)

https://www.brickboard.com/RWD/volvo/1385621/220/240/260/280/exciter_wire_confirmation.html

..."One particularly obscure trouble comes from the connection to the air conditioning relay. D+ is used there to allow AC only after the alternator is turning for about 10 seconds. Pretty difficult to trace on the maps, but look on the AC page. I think that map Paul linked is for an 85, but the AC worked the same. Here's a catalog of exciter wire connections in this generic view of Volvo 240 charging system. You see depending on year, the ABS, SRS, and A/C make use of that exciter voltage."
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240 drivers / parts cars - JH, Ohio