I have a feeling that each Alternator is going to have a slightly different Voltage that will turn the low charging light on.

This is where one of those $3 Volt/Ohm Meters (Multimeters) from Harbor Freight would come in handy. It would tell you what voltage you light is coming on at.

Then there is the usual suspects:
Belts not tight or oily

Oxidized or Corroded Battery and/or Ground connections

And, the Connector that plugs into the Alternator

The Regulator Brushes worn (easy to check)

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84 300D, 82 Volvo 244Gl Diesel