The battery light comes on when the engine gets near 3,000 RPM's and stays on until I back off acceleration. I have tightened the alternator belt twice and sprayed belt dressing on it, so I am pretty sure it is not the belt slipping. Is my alternator dying, do I need a new regulator, or something else? I am pretty sure it is not the idiot light, because the headlights slightly dim at night when this is happening.

1983 240, wiring harness replaced less than a year ago (by a very competent Volvo mechanic)

same thing happened to me. Change the brush pack in the alt as the brushes are probably no longer connected to the wiries that go to the brusholder but are transmitting the juice thru the springs. When you are at higher RMPs the springs can sag and the brushes no longer are passing juice to the regulator and *ding* on comes the idiot light...

10 min swap.

Don't overtighten alt belts else you'll ruin pump/alt bearings. The reason there are two belts is so that you can ease up on tightening as 2 belts give more gripping area.

Your regulator brushes may be short. With the springs extended the brushes may be floating at high revs and making poor contact.
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1980 245 Canadian B21A with SU carb and M46 trans in Brampton, Ont.