You ought to consider having a Free Alternator test done.

Better, would be to have an Alternator Test done at someplace with a Scope. From the wave form display they can tell you if any of the Diodes are burned.

If there is a poor ground any were from the Alternator to the Battery the increased Resistance can increase the Voltage. That could even mean that where the Alternator Bolts up to the Engine has a poor ground due to Corrosion or the Engine to Chassis Ground Strap contact area is not good.
The Idea of running a Jumper Cable to the Alternator Housing and to the (-) Battery Terminal is to bypass all of the other Ground connections. And, it will do that with the exception of the Ground area between the Voltage Regulator and the Alternator Frame/Housing.

Remove the Voltage Regulator and scrape down to bare metal the area where the Regulator Grounds against the Alternator Frame at one of the Screw Holes.

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84 300D, 82 Volvo 244Gl Diesel